Histidine requirement of broilers in a necrotic enteritis challenge model: Growth performance and carcass parameters

Abstract

Immune challenge can alter the amino acid (AA) requirements. Histidine (His) is an essential AA for broilers, supports multiple physiological functions and may become limiting in reduced CP diets. However, His requirements in modern broilers have been sparsely investigated, particularly across a full production cycle and under immune challenge conditions. Therefore, a 35-day, 3-phase His requirement study was conducted with 1680 one-day-old male Ross 308 broilers allocated to fourteen treatments (n = 8). Equally divided, a first group, comprised seven treatments without challenge (unchallenged, UC) and a second group comprised seven treatments with immune challenge (challenged, C). The challenge consisted out of an in feed coccidiosis challenge on d6 and d14, followed by an oral bacterial challenge on d21, d22, and d23. In both groups, birds received either a basal diet (BD) containing 0.3 % standardized ileal digestible (SID) His, or a BD supplemented with graded levels of l-His to achieve 0.33, 0.36, 0.39, 0.44, 0.49, 0.54 % SID His in starter, grower, and finisher phases. Histidine requirements were estimated using an exponential asymptotic model. First, the minimum SID His required to achieve 98 % of the maximum performance under immune challenge conditions was determined and used as a reference for comparison between the UC and C group. Over the 35-day period, the estimated SID His requirement for C birds to optimize BW and feed conversion ratio (FCR) was 0.407 % and 0.375 %, respectively. To reach the same performance UC birds only required 0.387 % and 0.325 % SID His, respectively. Second, the UC bird’s requirement was estimated using the same model. Over the 35-day period, the estimated SID His requirement for UC birds to optimize BW, ADG, and FCR was between 0.389 - 0.448 %, whereas the requirement to optimize breast weight and breast yield (relative to live weight) was 0.513 and 0.524 %, respectively.

Introduction

The reduction of dietary CP can deliver environmental benefits and may improve bird health, welfare, and profitability (Woyengo et al., 2023). The commercial availability of non-bound amino acids (AAs) has enabled CP reduction without compromising performance, provided that the AA supply meets the bird’s requirement (Matton et al., 2025). In practical feed formulation, lysine (Lys), methionine (Met), and threonine (Thr) are routinely used, depending on the available ingredients and composition, targeted CP concentration, and digestible (dig) Lys level. Further CP reduction requires access to non-bound valine (Val), arginine (Arg), and isoleucine (Ile) as the next limiting AAs. Additionally, essential AAs such as histidine (His), leucine (Leu), and phenylalanine (Phe) may also become limiting depending on feed ingredients and production targets (e.g. low CP or soybean meal (SBM) free diets). These AAs are often overlooked in commercial broiler feed formulations, and their requirement in modern broilers remain insufficiently defined.

Recent studies have contributed to refining requirements for these potentially limiting AAs but important gaps remain. Siebert and Scharch (2025) determined the optimal standardized ileal digestible (SID) Leu to Lys ratio in male Ross 308 broilers during finishing period (19-33d of age). Franco et al. (2017) determined SID His, Leu and Phe+Tyr to Lys ratio in 8-17d old male Cobb 500 broiler chickens. Nelson et al. (2024a) investigated whether His requirements in female Cobb 500 (22-42d) depends on His supply during the starter period (0-21d). Same group conducted a titration study in 15-35d male Cobb 500 birds, reporting linear response for BW, BW gain (BWG) and feed conversion ratio (FCR). The dig His to Lys ratio for tender yield and breast filet yield was 37 % and 39 %, respectively, using the 95 % quadratic maximum method (Nelson et al., 2024b). However, the authors noted that the analyzed AA values of the test diets were below expected values, which may have limited the His response, additionally the tested range (up to 43 % dig His to Lys) may not have been sufficiently high to clearly define a His requirement (Nelson et al., 2024b). These considerations highlight key methodological requirements for dose–response studies: an adequately deficient basal diet (BD) for the target AA, a sufficiently wide supplementation range, and avoidance of co-limitations by other AAs to ensure robust estimation of requirements.

Histidine is essential and is incorporated into His-containing proteins (e.g., hemoglobin) and dipeptides such as carnosine (Wu, 2021). Such components may act as endogenous reserves and could potentially buffer dietary His supply for protein synthesis (Ousterhout, 1960), thereby potentially influencing dose response outcomes. Nelson et al. (2024a) evaluated this concept by feeding lower vs. higher His during the first 21d and found no influence on His requirement during the subsequent 21d in male Cobb 500 birds (33 % vs. 38 % dig His to Lys).

Beyond growth, His and its metabolites are relevant to physiological functions that may be particularly important under stress. The imidazole ring of His contributes to metal ion chelation, proton buffering, and antioxidant properties as reviewed earlier (Wu, 2021; Holeček, 2020). Histidine also plays a key role in enzyme function as part of the catalytic triad of serine proteases (e.g. trypsin) (Carter and Wells, 1988), and in its decarboxylation form histamine (Li et al., 2007), which is involved in regulation of acetylcholine release in the central nervous system and stimulation of gastric acid secretion (Wu, 2021).

Coccidiosis (Cocci), caused by protozoa such as Eimeria spp., represents a major burden to poultry production, with global costs estimated at ∼ £ 10.36 billion in 2016 (Blake et al., 2020). Under commercial conditions, Eimeria infection is frequently associated with secondary bacterial infections, which can exacerbate clinical symptoms and impair nutrient utilization. Cocci and bacterial infections are therefore important risks for the broiler industry and can negatively affect AA digestibility as reviewed earlier (Chalvon-Demersay et al., 2021). Nutritional strategies to mitigate the clinical challenges remain an active area of research. Increasing dietary His may be one of such strategy, either His availability may be reduced during enteric challenges or because His and its metabolites support functions are relevant to oxidative stress, digestion, and immunity.

In the present study, we hypothesized (section 1) that a combined coccidial-bacterial challenge alters SID His requirement for Ross 308 broilers to achieve performance comparable to unchallenged birds. In section 2, we determined the optimal SID His requirement for male Ross 308 unchallenged birds across a 35d trial.

Material and methods

Ethics

A study approval was granted by the Poulpharm Ethical Commission (trial code P24117-FP- 046). The study was carried out according to “Directive 2010/63/EU of the European Parliament and of the Council of 22 September 2010 on the protection of animals used for scientific purposes” (European Parliament and Council of the European Union, 2010) and according to the EFSA Guidance (EFSA, 2018).

Animals and housing

A total of 1680 one-day-old male Ross 308 birds were obtained from a commercial hatchery (Vervaeke-Belavi, Tielt, Belgium) and housed at the Poulpharm BV test facility (Izegem, Belgium). Birds were vaccinated against Infectious Bronchitis and Newcastle Disease at the hatchery. Upon arrival, clinical healthy birds were randomly allocated to fourteen treatments (T) (8 pen replicates each, 15 birds per replicate, 120 birds per treatment) in a feeding experiment without challenge (T01-T07; unchallenged, UC) or with challenge (T08-T14; challenged, C). Birds were fed in three phases: starter (0-14d), grower (14-28d), and finisher (28-35d). Broilers were housed in 112 floor pens, (1 m²/pen) bedded with 5 cm wood shavings with ad libitum access to water and feed. The stock density was according to the EU council directive 2007/43/EC allowing a max density of 33 kg per m² (Council of the European Union, 2007). Farms that comply with an array of additional environmental and management items can have a derogation to 39 or max 42 kg per m². Each pen was equipped with one commercial pan feeder with a feed reservoir and four nipple drinkers mounted on the pen side; nipple height was adjusted according to the bird size. From d1 to d7, lighting was reduced stepwise from 24 h to 18 h light per day (30-40 lux), and thereafter maintained at 18 h light (10-20 lux) and 6 h dark until d35. Room temperature was 37°C upon placement and gradually reduced to 23°C. From d15 until d35, room temperature exceeded the reference maximum temperature; however, close monitoring revealed no abnormalities or sign of discomfort. Housing units were arranged to ensure comparable light and environmental conditions across all pens. To minimize cross contamination between UC and C groups, pens were allocated with a wide corridor between groups, which was kept throughout the study. Pens facing the corridor were fitted with solid plastic boards as physical barrier. All study events were performed first in the UC pens followed by the C pens. Mortality was recorded daily at individual bird level. All birds found dead or culled for welfare reasons were recorded with date of death and BW. Necropsy was performed on all birds that died or were culled. When immediate examination was not possible, carcass were stored refrigerated (approximately 4°C) until necropsy.

Study design and diets

A His deficient BD containing 0.30 % SID His was formulated for the starter (18.83 % CP, 1.27 % SID Lys, AMEn 3,052 kcal/kg), grower (18.11 % CP, 1.14 % SID Lys, AMEn 3,094 kcal/kg), and finisher (16.95 % CP, 1.02 % SID Lys, AMEn 3,200 kcal/kg) and fed to both C and UC groups (T01 and T08) (Table 1). Graded levels of l-His HCl (72 % l-His; BestAmino, CJ Bio, Seoul, Korea) were added at the expense of alanine and/or proline in a 1 to 1 ratio. Stepwise increase of 0.03 % SID His were applied for treatment T02-T04 and T09-T11, followed by 0.05 % SID His increments for treatments T05-T07 and T12-T14. This resulted in SID His levels of 0.30 %, 0.33 %, 0.36 %, 0.39 %, 0.44 %, 0.49 %, and 0.54 % in both the UC groups (T01-T07) and the C groups (T08-T14), respectively (Table 2). All other nutrients were formulated close to or above breeder recommendations (Aviagen, 2022a) and raw material matrix was based on CVB (2023). Basal diet formulations can be found in Table 1.

Table 1.

Ingredient and nutrient composition of the basal starter diet (0-14d), grower (14-28d) and finisher (28-35d).

	Starter	Grower	Finisher
Ingredient composition (% as-fed)
Corn	27.64	20.22	22.51
Wheat	20.00	30.00	30.00
Soybean meal	8.60	7.50	9.20
Peas	10.00	10.00	10.00
Field beans	10.00	10.00	5.00
Wheat middlings	5.00	5.00	5.00
Corn starch	5.00	4.00	5.00
Soybean oil	3.60	4.48	5.96
Potassium carbonate	-	0.18	0.19
Others1	4.20	3.64	3.22
L-Lysine HCl2 (79 %)	0.85	0.69	0.55
DL-Methionine (99 %)	0.63	0.55	0.45
L-Threonine (98 %)	0.47	0.36	0.27
L-Valine (96.5 %)	0.43	0.37	0.27
L-Isoleucine (90 %)	0.41	0.33	0.25
L-Arginine (99 %)	0.63	0.53	0.42
L-Tryptophane (98.5 %)	0.09	0.06	0.04
L-Glutamic acid	0.60	0.60	0.60
L-Glycine	0.62	0.43	0.29
L-Leucine	0.46	0.34	0.20
L-Phenylalanine	0.50	0.41	0.27
L-Alanine / l-Proline	0.33	0.33	0.33
Calculated nutrient composition (% as-fed)
AMEn (kcal/kg)	3052.00	3094.00	3200.00
Crude protein	18.83	18.11	16.95
Crude fat	5.86	6.49	7.98
Crude Ash	5.42	5.15	4.73
Crude Fiber	3.17	3.20	2.87
Starch	43.39	43.71	44.14
dEB3 (mEq/kg)	180.40	200.60	200.40
Calculated dietary SID4amino acid contents (g/kg as-fed)5
Lys	12.69	11.41	10.19
Met+Cys	9.65	8.87	7.99
Thr	8.52	7.65	6.83
Trp	2.06	1.82	1.62
Arg	14.60	13.09	11.74
Ile	8.28	7.73	7.04
Leu	13.81	12.49	11.20
Val	9.40	8.91	7.96
His	2.99	3.07	3.08
Phe+Tyr	14.18	12.75	11.38
Gly+Ser	16.49	14.79	13.28
Caclulated SID amino acid ratios to Lys
Met+Cys/Lys	0.76	0.78	0.78
Thr/Lys	0.67	0.67	0.67
Trp/Lys	0.16	0.16	0.16
Arg/Lys	1.15	1.15	1.15
Ile/Lys	0.65	0.68	0.69
Leu/Lys	1.09	1.09	1.10
Val/Lys	0.74	0.78	0.78
His/Lys	0.24	0.27	0.30
Phe+Tyr/Lys	1.12	1.12	1.12
Gly + Ser/Lys	1.30	1.30	1.30

¹Contains: premix, limestone, monocalcium phosphate, salt, sodium bicarbonate.

²HCl= hydrochloride.

³dEB= dietary electrolytic balance (Na + K - Cl).

⁴SID = Standardized ileal digestible.

⁵Arg = arginine, Cys = cysteine, Gly = glycine, His = histidine, Ile = isoleucine, Leu = leucine, Lys = Lysine, Phe = phenylalanine, Ser = Serine, Thr = threonine, Trp = tryptophane, Tyr = tyrosine, Val = valine.

Table 2.

Overview of the different treatment groups. Unchallenged (UC) (T01 – T07) Challenged (C) (T08 – T14) with 8 pen replicates per treatments and 15 birds per replicate.

	Treatment	Diet description	SID HIS1 (%) content (calculated)
	T01	BD2 deficient in histidine (0.3 % SID His)	0.30
	T02	BD + 0.03 % SID His	0.33
	T03	BD + 0.06 % SID His	0.36
UC3	T04	BD + 0.09 % SID His	0.39
	T05	BD + 0.14 % SID His	0.44
	T06	BD + 0.19 % SID His	0.49
	T07	BD + 0.24 % SID His	0.54
	T08	Basal diet (BD) deficient in histidine (0.3 % SID His) + Challenge	0.30
	T09	BD + 0.03 % SID His + challenge	0.33
	T10	BD + 0.06 % SID His + challenge	0.36
C4	T11	BD + 0.09 % SID His + challenge	0.39
	T12	BD + 0.14 % SID His + challenge	0.44
	T13	BD + 0.19 % SID His + challenge	0.49
	T14	BD + 0.24 % SID His + challenge	0.54

¹SID His: standardized ileal digestible histidine.

²BD: basal diet.

³UC: unchallenged.

⁴C: challenged (in feed challenge of Eimeria acervuline, Eimeria maxima and Eimeria tenella on d6 and d14 followed by an oral gavage on d21, 22, 23 with a mixture of Escherichia coli, Ligilactobacillus salvarius, Lactobacillus crispatus, Clostridium perfringens and Enterococcus spp.).

For the challenge groups (T08-T14), a combined coccidial-bacterial challenge model was used. A mild Cocci infection, consisting of a mixture of sporulated oocysts (Eimeria acervuline, Eimeria maxima and Eimeria tenella), was applied to the feed on d6 and d14 by spraying the inoculum onto the feed present in the feeding towers. Subsequently, bacterial enteritis was induced using a mixture of Escherichia coli, Ligilactobacillus salivarius, Lactobacillus crispatus, Clostridium perfringens, and Enterococcus faecalis by oral gavage on d21, d22, and d23. Experimental diets were manufactured by Research Diet Services BV (Wijk bij Duurstede, The Netherlands) and pelleted (2.5 mm for starter; 3 mm for grower and finisher). Prior to formulation, raw materials (wheat, corn, soybean meal, wheat middlings, peas, faba beans) were analyzed and composed feeds as well by Agrolab Lufa GmbH (Kiel, Germany) for proximate composition (moisture VO(EG) 152/2009, III, A: 2009-01 (mod.); crude ash REG(EG) 152/2009, III, M: 2009-01; crude protein (N*6.25) REG(EG) 152/2009, III, C: 2009-01; crude fat REG(EG) 152/2009, III, H, method B: 2009-01; crude fiber REG(EG) 152/2009, III, I: 2009-01) and AA analysis according to REG(EC) 152/2009, III, F: 2009-01 (mod.). The analyzed levels of His were in line with the calculated values (Table 3).

Table 3.

Calculated (Calc.) vs. analyzed (An.) histidine (His) levels of the different starter (0-14d), grower (14-28d) and finisher (28-35d) diets.

	T01/T08	T02/T09	T03/T10	T04/T11	T05/T12	T06/T13	T07/T14
Starter
Calc. total His (g/kg)	3.54	3.84	4.14	4.44	4.94	5.44	5.94
An. total His (g/kg)	3.70	4.00	4.30	4.60	5.00	5.40	5.80
Grower
Cal. total His (g/kg)	3.65	3.95	4.25	4.55	5.05	5.55	6.05
An. total His (g/kg)	3.40	3.80	4.00	4.30	4.70	5.30	6.20
Finisher
Calc. total His (g/kg)	3.63	3.93	4.23	4.53	5.03	5.53	6.03
An. total His (g/kg)	3.30	3.90	4.00	4.40	4.70	5.20	5.70

Measurements

Body weight and feed intake (FI) were determined on pen level on d0, d6, d14, d21, d28 and d35, and mortality was recorded daily. The ADG, ADFI and FCR were calculated per pen and corrected for mortality. By the end of d35, feed was removed from the pens and four birds per pen, with BW close to the pen mean, were individually weighed and identified by a colored tag. For logistical reasons, two of these birds were sacrificed on d35, stored at 2-8°C, and used for slaughter yield assessment on d36. On d36, the remaining two tagged birds were slaughtered and carcass weight, leg weight, and breast meat weight were determined. Breast and leg yields were calculated relative to live weight on d35 and carcass weight.

Litter quality was assessed on d14, d21, d28, and d35 by scoring six different locations per pen (two near the drinker, two near the feeder, and two random locations). The mean of the six scores was used as the pen litter score. Litter scores ranged from 0 to 4, where 0 indicated dry and/or dusty, clean, and loose shavings and 4 indicated wet/watery litter with a caked, muddy surface and dirty birds. Footpad lesions (FPL) were scored in four birds per pen on d21, d28, and d35 according to the Bristol scoring system (Rushen et al., 2011), with scores ranging from 0 (no evidence of pododermatitis) to 4 (severe pododermatitis). Both feet were evaluated and, when scores differed, the higher (worse) score was recorded.

Statistics

Data were analyzed in R (version 3.2.5). Mortality was analyzed using a cox proportional hazards model with treatment as a fixed effect. Post-hoc pairwise comparisons were performed using Tukey’s adjustment for multiple comparisons within each block.

Body weight, ADG, ADFI and FCR were analyzed using linear regression models with treatment as a fixed effect (procedure lm of the core package). Slaughter parameters were analyzed using linear mixed regression models with treatment as a fixed effect and pen as a random effect (procedure lmer of the package lme4). Litter and FPL scores were analyzed using ordinal regression models (procedure clm of the package ordinal) with treatment as a fixed effect and pen as random effect for FPL. Post-hoc pairwise comparisons were performed using Tukey’s adjustment for multiple comparisons within each block. Statistical significance was declared at P ≤ 0.05. Analysis were conducted in two blocks: UC treatments (T01–T07) and C treatments (T08–T14). Subsequently, four different models were tested and the best fitting model i.e. an exponential asymptotic model was fitted for BW, ADG, ADFI, FCR, and slaughter parameters.

Exponential asymptotic model:

$$y=c+\left(d-c\right)\left(1-\exp \left(-X/e\right)\right)+error$$

y = outcome variable, X = % SID His, c = the lower limit (performance at 0), d = the upper limit (= maximum performance), e = parameter determining the steepness of the increase in performance by % SID His.

The model was fit with the procedure drm (function AR.3()) of the package drc and was done separately for the UC and C group.

In section 1, the minimum % SID His required under C conditions to achieve 98 % of the maximum response was determined and used as reference for comparison between both the UC and C groups. For FCR, the requirement was defined as % SID His needed to reach 102 % of the minimum FCR under C conditions. In section 2, the SID His requirement for the UC birds was determined using the same approach, the minimum % SID His required to obtain 98 % of the maximum response or 102 % of the minimum FCR.

Results

Performance parameters

Histidine is an essential amino acid in broilers thus His cannot be synthetized endogenously. A shortage of His will generally leads to a reduction in growth performance. Additionally, a coccidial-bacterial challenge can cause damage to the intestinal integrity, alter nutrient absorption and triggers different immune responses leading to a reduction in performance.

Growth performance results for the UC and C groups are shown in Table 4, Table 5, respectively. Increasing the SID His from the deficient basal level (BD; T01/T08, 0.30 % SID His) to the highest level tested (T07/T14, 0.54 % SID His) increased BW on d6 in both the UC and C groups (P ≤ 0.05).

Table 4.

Overview of the least square mean and the standard error of performance and carcass parameters multiple comparisons between the treatments of the unchallenged group with Tukey’s adjustment for multiple comparisons. Significant differences (P ≤ 0.05) are demonstrated with different letters.

Treatments	T01	T02	T03	T04	T05	T06	T07	SE
SID His2	0.30 %	0.33 %	0.36 %	0.39 %	0.44 %	0.49 %	0.54 %
Period d0-21d
BW d0	45.8	46.0	45.6	45.6	45.9	46.0	45.8	0.18
BW d6	147a	156b	161bc	161bc	167cd	167cd	172d	2.05
BW d14	421a	505b	547c	554cd	568cde	579e	576de	5.55
BW d21	847a	1044b	1127c	1144cd	1175cd	1186d	1165cd	11.5
ADG (g/d/bird)	38.0a	47.1b	51.3c	52.2c	52.5c	53.7c	53.2c	0.70
ADFI (g/d/bird)	50.9a	61.2b	64.0bc	64.1bc	64.0bc	65.4c	64.6bc	0.83
FCR2 (g/g)	1.338c	1.299b	1.248a	1.228a	1.220a	1.218a	1.216a	0.009
Period d0-28d
BW d28	1454a	1744b	1836bc	1832bc	1914c	1935c	1910c	29.7
ADG (g/d/bird)	49.9a	60.2b	63.8bc	63.2bc	65.1bc	65.9c	66.3c	1.27
ADFI (g/d/bird)	69.0a	81.6b	84.0b	82.3b	83.0b	84.2b	84.6b	1.39
FCR (g/g)	1.384c	1.355bc	1.319ab	1.304a	1.277a	1.279a	1.276a	0.010
Period d0-35d
BW d35	2048a	2345b	2466bc	2437bc	2484bc	2508c	2501c	34.0
ADG (g/d/bird)	56.5a	65.3b	68.8b	67.5b	67.6b	68.4b	67.8b	1.25
ADFI (g/d/bird)	85.4a	98.0b	99.5b	95.0b	96.0b	97.8b	97.1b	1.79
FCR (g/g)	1.512c	1.502bc	1.446abc	1.407a	1.420a	1.432ab	1.433ab	0.017
Mortality (%)	5.8	2.5	2.5	5.8	12.5	10.8	10.8	0.0316
LW3 (g)	1842	2285	2338	2337	2401	2489	2506	215
CW4 (g)	1294	1652	1725	1745	1819	1886	1896	349
Slaughter yield (%)	70.0	72.1	73.7	74.6	75.6	75.7	75.7	3.6
Breast weight (g)	225	361	411	457	492	528	527	256
Breast yield (% LW)	12.0	15.6	17.5	19.4	20.4	21.1	21.0	3.2
Breast yield (% CW)	17.1	21.7	23.7	26.1	27.0	27.8	27.7	2.7
Leg weight (g)	431	508	507	503	505	511	522	31
Leg yield (% LW)	23.4	22.2	21.7	21.5	21.0	20.6	20.8	0.8
Leg yield (% CW)	33.5	30.9	29.5	28.8	27.8	27.2	27.5	1.5

¹SID His = standardized ileal digestible histidine.

²FCR = feed conversion ratio.

³LW = live weight on d35 (4 birds/pen).

⁴CW = carcass weight on d36 (4 birds/pen).

^a,b,c Values with different superscript in the same row differ significantly (P ≤ 0.05) – Tukey test.

Table 5.

Overview of the least square mean and the standard error of performance and carcass parameters multiple comparisons between the treatments of the challenged¹ group with Tukey’s adjustment for multiple comparisons. Significant differences (P ≤ 0.05) are demonstrated with different letters.

Treatments	T08	T09	T10	T11	T12	T13	T14	SE
SID His2	0.30 %	0.33 %	0.36 %	0.39 %	0.44 %	0.49 %	0.54 %
Period d0-21d
BW d0	45.0	44.9	44.9	44.9	44.3	44.8	44.5	0.49
BW d6	150a	164b	168b	163b	166b	167b	170b	2.61
BW d14	399a	481b	507bc	518c	534c	527c	527c	7.39
BW d21	764a	931b	1013c	1035c	1045c	1047c	1024c	17.6
ADG (g/d/bird)	33.9a	42.1b	44.1b	44.8b	47.0b	44.1b	42.0b	1.52
ADFI (g/d/bird)	49.7a	58.3b	57.0b	58.5b	59.6b	56.8b	55.0ab	1.62
FCR3 (g/g)	1.464c	1.384bc	1.300ab	1.306ab	1.270a	1.293a	1.313ab	0.020
Period d0-28d
BW d28	1282a	1567b	1647bc	1683bc	1747c	1741c	1731c	27.1
ADG (g/d/bird)	42.4a	53.5b	53.4b	55.6b	58.0b	55.0b	53.1b	1.90
ADFI (g/d/bird)	63.8a	77.2b	74.3ab	75.9ab	77.5b	73.2ab	77.2b	2.80
FCR (g/g)	1.504	1.443	1.394	1.366	1.336	1.334	1.501	0.071
Period d0-35d
BW d35	1923a	2275b	2352bc	2395bc	2466bc	2392bc	2519c	54.8
ADG (g/d/bird)	51.3a	62.4b	61.3b	63.3b	65.3b	60.6b	61.4b	2.14
ADFI (g/d/bird)	80.0a	95.1b	91.0ab	91.7ab	93.0b	88.1ab	90.6ab	2.83
FCR (g/g)	1.560	1.524	1.485	1.449	1.424	1.467	1.525	0.065
Mortality (%)	12.5a	6.7a	19.2ab	18.3ab	17.5ab	27.5b	31.7b	0.0658
LW4 (g)	1815a	2226b	2264b	2333b	2327b	2282b	2273b	64
CW5 (g)	1312	1610	1647	1739	1771	1700	1731	153
Slaughter yield (%)	72.3	72.3	72.8	74.5	76.1	74.3	76.0	2.8
Breast weight (g)	230	338	384	456	480	469	477	222
Breast yield (% LW)	12.6a	15.1ab	16.9bc	19.5cd	20.5cd	20.4cd	20.8d	0.9
Breast yield (% CW)	17.4	20.8	23.1	26.1	27.0	27.5	27.2	3
Leg weight (g)	433	496	491	490	484	459	466	24
Leg yield (% LW)	23.9	22.3	21.7	21.0	20.8	20.1	20.5	3.1
Leg yield (% CW)	33.1	30.9	29.9	28.2	27.4	27.1	27.0	4

¹Challenge: in feed challenge of Eimeria acervuline, Eimeria maxima and Eimeria tenella on d6 and d14 followed by an oral gavage on d21, 22, 23 with a mixture of Escherichia coli, Ligilactobacillus salvarius, Lactobacillus crispatus, Clostridium perfringens and Enterococcus spp.).

²SID His = standardized ileal digestible histidine.

³FCR = feed conversion ratio.

⁴LW = live weight on d35 (4 birds/pen).

⁵CW = carcass weight on d36 (4 birds/pen).

^a,b,c Values with different superscript in the same row differ significantly (P ≤ 0.05) – Tukey test.

In the UC group, BW on d14 and d21 was lower in T01 (BD; 0.30 % SID His) and T02 (0.33 % SID His) than in the other treatments (P ≤ 0.05). On both d14 and d21, the numerically highest BW was observed in T06 (0.49 % SID His). On these days, BW in T06 was higher than in T03 (0.36 % SID His) (P ≤ 0.05), but did not differ from the remaining treatments, except for T04 (0.39 % SID His) on d14.

In the C group, the first weighing after the initial Cocci challenge (d14) shows a maximum BW of 534 g in T12 (0.44 % SID His), which was higher than T08 (BD; 0.30 % SID His) and T09 (0.33 % SID His) (P ≤ 0.05) but did not differ from the other treatments in the C group. Similar results were observed on d21 (7d after the second Cocci challenge and prior to the bacterial challenge), where BW peaked at 1,047 g in T13 (0.49 % SID His), differing from T08 (BD; 0.30 % SID His) and T09 (0.33 % SID His) (P ≤ 0.05) but not from the other treatments in the C group. These results indicate the importance of meeting His demand for maximizing BW and the fact that birds stay behind in their growth if an important essential amino acid requirement is not met.

Across the period 0-21d (i.e., the phase following the Cocci challenge), FCR in the C group was lowest in T12 (0.44 % SID His; 1.270) and T13 (0.49 % SID His; 1.293), compared with T08 (BD; 0.30 % SID His; 1.464) and T09 (0.33 % SID His; 1.384) (P ≤ 0.05), with no differences versus the remaining C treatments. In the UC group, the best FCR during the period 0-21d was observed in T07 (0.54 % SID His; 1.216), which differed (P ≤ 0.05) only from T01 (BD; 0.30 % SID His) and T02 (0.33 % SID His). The FCR results mirror the BW response to the lack of His in feed. Thus, lack of an essential amino acid such as His also lead to poor feed efficiency.

On d28 and d35, BW in the UC group was numerically highest in T06 (0.49 % SID His). The lowest FCR was observed in T07 (0.54 % SID His) on d28 and in T04 (0.39 % SID His) on d35. For BW and FCR at these time points, the higher SID His treatments were superior to T01 (BD; 0.30 % SID His) and T02 (0.33 % SID His) (P ≤ 0.05), but generally did not differ from the remaining treatments in the UC group.

In the C group on d28 (five days after the final bacterial gavage), the highest BW (1,747 g) was observed in T12 (0.44 % SID His), which was higher than T08 (BD; 0.30 % SID His; 1,282 g) and T09 (0.33 % SID His; 1,567 g) (P ≤ 0.05) but not different from the other treatments in the C group. On d35, the highest BW (2,519 g) occurred at the highest SID His level (T14; 0.54 % SID His) exceeding T08 (BD; 0.30 % SID His; 1,923 g) and T09 (0.33 % SID His; 2275 g) (P ≤ 0.05), while differences versus the other treatments within the C group were only numerical.

Mortality differed by treatment in the C group. Higher SID His levels were associated with increased mortality, reaching 27.5 % in T13 (0.49 % SID His) and 31.7 % in T14 (0.54 % SID His), compared with 12.5 % in T08 (BD; 0.30 % SID His) and 6.7 % in T09 (0.33 % SID His) (P ≤ 0.05). In the UC group, mortality differed only numerically among treatments. Checking individual mortality reasons did not revealed a reason to link His to a particular element increasing mortality rate under challenge conditions. The higher mortalities observed in the UC and C group compared to commercial practice can affect the precision and robustness of the results.

Carcass traits

The effects of increasing dietary His supplementation on broiler carcass characteristics are presented in this section. No differences were detected among UC groups although breast and slaughter yields were numerically lowest at the lowest SID His levels. In the C group, breast yield (relative to live weight) was highest in T14 (0.54 % SID His; 20.8 %) and differed from the three lowest SID His treatments (T08, T09, and T10; P ≤ 0.05). In the UC group, slaughter yield, breast weight and breast yield (relative to carcass weight) increased numerically with increasing SID His, whereas leg yield showed the opposite numerical trend (Table 4, Table 5). These results indicate the first signs of His demand being linked to the health status of birds. Eventually a higher His demand in C birds caused a better or higher magnitude of response to His turning it to a significant difference knowing that His is a precursor for histamine synthesis and that histamine is an important mediator in the initiation of the inflammatory response.

Litter score and footpad lesions scores

No difference in litter score was observed within either block at d14, d21, d28, and d35 (data not shown) which was unexpected for the C group due to the challenge given. No FPL were observed on d21 and d28 in any treatment, On d35, differences were detected only within the C group (P ≤ 0.05): T09 (0.33 % SID His) had the highest FPL score (0.8), whereas T08 (BD; 0.30 % SID His), T13 (0.49 % SID His), and T14 (0.54 % SID His) had the lowest FPL score (0.1) (Table 6).

Table 6.

Overview of the mean footpad lesions (FDL) at d35 for every treatment. Multiple comparisons were performed between all groups within each block with Tukey’s correction and significant differences (P ≤ 0.05) are demonstrated with different letters.

	SID His1	0.30 %	0.33 %	0.36 %	0.39 %	0.44 %	0.49 %	0.54 %
UC2	Treatments	T01	T02	T03	T04	T05	T06	T07
	Mean FDL score	0.1a	0.4a	0.4a	0.2a	0.2a	0.4a	0.4a
C3	Treatments	T08	T09	T10	T11	T12	T13	T14
	Mean FDL score	0.1a	0.8b	0.5ab	0.4ab	0.3ab	0.1a	0.1a

¹SID His = standardized ileal digestible histidine.

²UC: unchallenged.

³C: challenged (in feed challenge of Eimeria acervuline, Eimeria maxima and Eimeria tenella on d6 and d14 followed by an oral gavage on d21, 22, 23 with a mixture of Escherichia coli, Ligilactobacillus salvarius, Lactobacillus crispatus, Clostridium perfringens and Enterococcus spp.)

^a,b,c Values with different superscript in the same row differ significantly (P ≤ 0.05) – Tukey test.

Comparison of Histidine requirement between unchallenged and challenged birds

The comparison of the estimated SID His requirements between UC and the C birds, defined as the minimum % SID His needed to reach 98 % of the maximum performance (or 102 % of the minimum performance for FCR) of the C group, is shown in Table 7. The largest differences between UC and C requirements for BW were observed at time points closest to the applied challenges (d14, d21, and d28; +0.056 to +0.086 percentage points SID His), whereas the difference was smaller at d35 (+0.02 percentage points). A challenge could increase the demand of His either because of a poorer digestion of absorption capacity as a result of the induced gastrointestinal challenge or because of a higher demand for His due to inflammatory response and the need of histamine synthesis.

Table 7.

Comparing the requirements (% SID His)¹ of the unchallenged (UC) and challenged (C) group on different time points, based on the exponential asymptotic model, to reach 98 % of the maximum performance and carcass parameters, or 102 % of the minimum performance for FCR of the C group.

	98 % of max performance of C group2	SID His (%)
Period 0-21d		UC3	C4
BW d14 (g)	526	0.350	0.424
BW d21 (g)	1026	0.336	0.392
ADG (g/day/bird)	44.3	0.324	0.403
ADFI (g/day/bird)	58.8	0.331	0.645*
FCR5 (g/g)	1.301	0.326	0.419
Period 0-28d		UC	C
BW d28 (g)	1725	0.344	0.430
ADG (g/day/bird)	55.1	0.319	0.403
ADFI (g/day/bird)	75.7	0.322	0.411
FCR (g/g)	1.403	0.300	0.385
Period 0-35d		UC	C
BW d35	2424	0.387	0.407
ADG (g/day/bird)	62.2	0.327	0.390
ADFI (g/day/bird)	92.8	0.330	0.622*
FCR (g/g)	1.489	0.325	0.375
Carcass parameters
LW6 (g)	2279	0.359	0.381
CW7 (g)	1720	0.369	0.408
Slaughter yield (%)	75	0.428	0.472
Breast weight (g)	478	0.413	0.490
Breast yield (% LW)	21	0.480	0.523
Breast yield (% CW)	28	0.482	0.509
Leg weight (g)	472	0.327	0.352
Leg yield (% LW)	24	infinite	0.300
Leg yield (% CW)	33	0.300	0.300

¹SID His = standardized ileal digestible histidine.

²Maximum performance under challenge conditions, 98 % of this value or for FCR, the minimum performance under challenge conditions, 102 % of this value is shown, based on an exponential asymptotic model.

³UC: unchallenged.

⁴C: challenged (in feed challenge of Eimeria acervuline, Eimeria maxima and Eimeria tenella on d6 and d14 followed by an oral gavage on d21, 22, 23 with a mixture of Escherichia coli, Ligilactobacillus salvarius, Lactobacillus crispatus, Clostridium perfringens and Enterococcus spp.).

⁵FCR = feed conversion ratio.

⁶LW = live weight on d35 (4 birds/pen).

⁷CW = carcass weight on d36 (4 birds/pen).

^⁎Poor visual model fit obtained.

For ADG, estimated requirement were similar across periods within each group. Due to poor model fit for ADFI in the C group for periods 0-21d and 0-35d, only the 0-28d period was considered, the C group required 0.089 percentage points higher SID His compared with UC group. For FCR, the challenge effect was evident throughout the trial, with consistently higher SID His requirements in C group vs. UC group. In the C group, the estimated SID requirement decreased over longer evaluation periods (0-21d: 0.419 %; 0-28 d: 0.385 %; 0-35 d: 0.375 %), while UC requirements were more stable (0.300 to 0.326 %) to reach 98 % of the C group maximum performance.

For the carcass traits, the same overall pattern was observed: UC group require less SID His to reach 98 % of the C group maximum response. Leg yields (relative to live weight) showed the lowest estimated requirements (0.30 % SID His). In contrast, breast yield (relative to live weight and carcass weight) required the highest SID His levels within each group and compared with performance parameters overall. Estimated SID His requirements for breast yield (relative to live weight and carcass weight) were > 0.50 % in C group and ≥ 0.480 % in UC group.

Determining the histidine requirement in unchallenged birds

Increasing the His concentration in a dose-response study typically results in changes across multiple response criteria until a maximum response is reached. Estimated SID His requirements for UC birds based on 98 % of the maximum response from the exponential asymptotic model are shown in Table 8. The SID His requirement for BW increased slightly with age: 0.421 % at d14, 0.434 % at d28, and 0.448 % at d35. For ADG and ADFI, estimated requirements decreased when comparing early to later age. A similar pattern was observed for FCR: over the total period (0-35d), 0.389 % SID His optimized FCR, whereas requirements were higher in the earlier periods (0-21d: 0.436 %; 0-28d: 0.430 %). Slaughter yield was maximized at 0.391 % SID His. Breast meat weight and yield (relative to live weight and carcass weight) were optimized at 0.513 %, 0.524 % and 0.486 % SID His, respectively. Leg yield was optimized at the lowest SID His level (0.30 %).

Table 8.

The % SID His¹ requirement of unchallenged (UC) broilers in different time periods (0-21d), (0-28d) and (0-35d) based on 98 % of the maximum performance and carcass parameters, or 102 % of the minimum performance for FCR of the exponential asymptotic model.

	98 % of max performance2	SID His (%)
Period 0-21d
BW d14 (g)	568	0.421
BW d21 (g)	1169	0.429
ADG (g/day/bird)	52.7	0.415
ADFI (g/day/bird)	64.1	0.393
FCR3 (g/g)	1.225	0.436
Period 0-28d
BW d28 (g)	1904	0.434
ADG (g/day/bird)	64.9	0.408
ADFI (g/day/bird)	83.2	0.388
FCR (g/g)	1.291	0.430
Period 0-35d
BW d35	2499	0.448
ADG (g/day/bird)	67.5	0.390
ADFI (g/day/bird)	96.1	0.365
FCR (g/g)	1.443	0.389
Carcass parameters
LW4 (g)	2445	0.435
CW5 (g)	1852	0.449
Slaughter yield (%)	74	0.391
Breast weight (g)	522	0.513
Breast yield (% LW)	21	0.524
Breast yield (% CW)	28	0.486
Leg weight (g)	510	0.407
Leg yield (% LW)	23	0.300
Leg yield (% CW)	33	0.300

¹SID His = standardized ileal digestible histidine.

²Maximum performance, 98 % of this value or for FCR 102 % of this value is shown based on an exponential asymptotic model.

³FCR = feed conversion ratio.

⁴LW = live weight on d35 (4 birds/pen).

⁵CW = carcass weight on d36 (4 birds/pen).

Discussion

This study was designed as a dose response experiment using SID His (%) rather than SID His to Lys ratio, as applied in several recent studies (Franco et al., 2017; Nelson et al., 2024b). Ratio based approaches assume that Lys is the second limiting amino acid; if this assumption is not met, the derived ratio may underestimate the true requirement because another factor might co-limit the performance (Simongiovanni et al., 2012). In the present study Lys was fed according to breeder requirements (Aviagen, 2022a). In addition, SID Lys concentrations differed across feeding phases, meaning that the ratio to Lys differs within one treatment across feeding phases. For these reasons, estimating requirements using graded SID His levels was considered the most appropriate approach.

Another important consideration is the potential confounding effect of endogenous His reserves. Histidine present in body proteins and His-containing dipeptides may buffer short-term dietary changes and complicate interpretation of requirement studies. To minimize this risk, we used the same birds throughout the entire study and sustained low His exposure. While this approach limits the ability to derive phase specific requirements, it allowed to estimate a whole cycle requirement (0-35d) and, importantly, to compare SID His needs under unchallenged versus immune challenged conditions. To our knowledge, this is the first study in modern broiler genetics to evaluate His requirements across a full 35d production cycle while applying a combined coccidial-bacterial challenge.

Overall, mortality did only numerically increase in the UC birds with higher SID His. Nelson et al. (2024b) reported no differences in mortality. In their study, the highest tested level corresponded to 0.51 % dig His in Cobb 500 males 15-35d post hatch. In our trial, 0.49 % and 0.54 % SID His were similarly high, but were followed from 0 to 35d. Lackner et al. (2021) observed higher mortality (about 8 %) when birds were fed higher SID His to Lys ratios (0.44 vs. 0.54 or 0.64) after 53d of age. In the same study, low mortalities (max 2.6 %) were observed, with no difference between treatments after 33d. In contrast, the same author observed in another study no mortality in starter and grower I phases when testing five SID His to Lys ratios (0.41 to 0.57). Mortality was limited in grower II (maximum 2.27 %) and was absent in the finisher phase except at a SID His to Lys ratio of 0.53 (1.14 %) (Lackner et al., 2022). The latter finding aligns with Kralik et al. (2015), who reported no mortality when a BD was supplemented with up to 0.3 % l-His from d22 to d42 of age. In our study, mortality was higher in the C group receiving the two highest SID His treatments, T13 and T14 (27.5 % and 31.7 % mortality, respectively) compared to the BD T08 and T09 (12.5 % and 6.7 %, respectively). The reasons for the increased mortality within the C birds remain unclear. However, higher mortality in the C group compared with the UC group is expectable due to the applied microbial challenge. Gickel et al. (2025) indicated that for their study, focusing on the effect of pathogen induced diseases on the carbon footprint of broiler chickens, that close to 80 % of the trials considered did not publish results on mortality following an Eimeria spp or E. coli infection. When mortality was reported, an increase was observed with the except of one paper. In infection experiments with different Eimeria ssp with or without a C. perfringens challenge, mortality of 21.9 % has been reported for E. maxima in combination with a C. perfringens challenge (Nicholds et al., 2021), which is comparable to the highest mortality observed here. In the present study, the challenge consisted of mixed Eimeria spp., (E. acervuline, E maxima, and E. tenella) followed by a bacterial inoculum including, among others, E. coli and C. perfringens. Therefore, a high variability in broiler response is expectable and might be the reason for the different mortality in the groups rather than a causal effect of His per se. Consistent with this interpretation, Holeček (2020) concluded (in humans) that His supplementation has not been associated with impaired immune function, allergic reactions, or peptic ulcerations, suggesting that a direct toxicological mechanism is unlikely. Because histamine concentrations were not measured, we cannot evaluate whether His driven changes in histamine metabolism contributed to mortality. Although higher mortalities are to be expected in the C group the overall mortalities in the UC and C group compared to average commercial mortality can affect the precision and robustness of the results.

Footpad lesion scores were largely constant across treatments; only on d35 a difference observed in mean score between T09 and T08, T13, T14. Because these comparisons include both the lowest and the highest SID His levels, the data does not suggest a relationship between His supply and FDL occurrence in this trial. Moreover, the overall mean score remained below 1, indicating generally low lesions despite the applied challenge.

During the pre-challenge phase (0-6d), a His deficiency resulted in lower BW in the UC and C groups. In both groups (UC and C), this pattern was persistent across later growth phases: the BD (T01 and T08; 0.30 % SID His) had the lowest BW compared with the other treatments. With increasing age, highest numerical BW tended to occur in treatments with the highest SID His inclusion. Comparable responses have been reported by Nelson et al. (2024b), who observed a linear increase in BW at d35 when testing 0.28 to 0.43 SID His to Lys ratio from 15d post hatch until 35d. In addition, a linear and quadratic response for BW was reported when the same SID His to Lys ratios were tested in a 22d to 42d period (Nelson et al., 2024a). In contrast, Lackner et al. (2021, 2022) did not observe BW differences with increasing SID His supply. Notably, the Lackner studies evaluated His additions on BD already containing relatively high SID His concentrations (0.56, 0.48, 0.45, and 0.42 % in starter, grower, finisher I, II, respectively (Lackner et al., 2021), and 0.53, 0.46, 0.42, and 0.39 % SID His in starter grower I, II and finisher, respectively (Lackner et al., 2022). These values are in the range of T04 (0.39 %) to T06 (0.54 %) in the present experiment, showing no differences in BW, suggesting that birds were most likely fed on or above requirement at these SID His levels.

In the C group, FCR over the entire growth period (0-35d) did not differ among treatments. However, when evaluated closer to the applied challenge (0-21d and 0-28d), FCR tended to be highest with the lower SID His. Nelson et al. (2024b) reported a linear improvement in FCR with increasing SID His supply. The same authors reported (Nelson et al., 2024a) linear and quadratic improvements in FCR with increasing SID His levels from 22d to 42d. Similar quadratic responses were reported by Franco at al. (2017) during 8d to 17d. Again, the absence of FCR effects in Lackner et al. (2021, 2022), may be explained by the relatively high SID His levels in BD (i.e., supplementation largely above presumed requirement).

Regarding carcass traits, breast yield (relative to live weight) differed significantly only in the C group in the current study. Nonetheless, patterns largely aligned with earlier work: Franco et al. (2017) reported quadratic responses in breast (fillet) weight and yield (although that study ended at 17d of age). In contrast, the studies by Nelson et al. (2024a, 2024b) lasted until 42d and 35d post-hatch respectively and observed a quadratic response in breast yield when increasing dig. His to Lys ratio from 0.28 to 0.43 (Nelson et al., 2024a). Carcass yield showed a linear response, while quadratic and linear responses were observed for breast and tender yields in Nelson et al. (2024b), using the same BD and upper level of SID His to Lys ratio as in Nelson et al. (2024a), but testing from 15 to 35d post hatch.

Applying a challenge model to broilers has been shown to increase the requirement for several amino acids. A recent meta-analysis by Izadi Yazdanabadi et al. (2025) concluded that higher dosages of Thr and Arg are advised during Cocci challenge. In addition, Jespersen et al. (2024) observed an increased requirement of Met during a Cocci challenge. To our knowledge, the present study is the first to directly compare SID His requirement between UC and C birds.

The comparison of the SID His requirements between UC and the C birds was defined as the minimum % SID His needed to reach 98 % of the maximum performance (or 102 % of the minimum performance for FCR) of the C group using an asymptotic exponential model. Across life phases and performance outcomes, C birds generally required higher SID His than UC birds. In periods closer to the in-feed challenge, a higher difference between the UC and C groups for the His requirement were observed (0.056 percentage point to 0.086 percentage point SID His) compared to BW on d35 where only a 0.020 percentage point SID His difference was observed. A similar pattern was observed for FCR, where the difference decreases from 0.093 percentage point during 0-21d to 0.05 percentage point (d0-d35). This reduction may reflect a transition from the acute challenge phase into recovery phase and might explain the reduction in requirement between the UC and C group. For carcass parameters, C birds required higher SID His than UC birds. Notably, the requirements to reach the max performance of the C birds were generally higher than the requirements to optimize performance parameters especially when looking to optimize breast weight and breast yield.

Studies determining His requirement in modern broilers are scarce. Available experiments differ in age, genotype, gender, and are frequently executed as ratio studies than absolute requirement trials (Franco et al., 2017; Nelson et al., 2024b). In addition, several studies evaluating the relationship between His supply and performance either contain not enough dietary SID His levels to be considered a true dose response study, or in the BD, SID His levels are not deficient enough to precisely extract a requirement (Nelson et al., 2024b; Lackner et al., 2021, 2022).

The present study was performed as a requirement study and included a BD containing 0.3 % SID His across all growth phases. This SID His value is equal to the BD used by Franco et al. (2017), assumed to be below requirement. Based on the Brazilian tables for poultry and swine, SID His requirement guidelines for high performance male broilers ranges from 0.506 % to 0.398 % between 0 and 49d of age, respectively (Rostagno et al., 2024).

The obtained performance for BW and FCR in the UC birds (based on the 98 % of the maximum performance of an asymptotic exponential model) over the periods 0-21d, 0-28d, and 0-35d exceeded the performance objectives of male Ross 308 birds (Aviagen, 2022b). In the first period (0-21d), SID His requirement to maximize BW and FCR at d14 and d21 were similar (0.421 % to 0.436 % SID His, respectively). A comparable convergence was observed over 0-28d, with both outcomes optimized at approximately 0.43 % SID His. Across the full period (0-35d), estimated SID His for FCR and BW ranged from 0.389 % to 0.448 % SID His.

Direct comparison with the Brazilian tables for poultry and swine tables is limited because the life phases are not aligned. Nevertheless, when benchmarking other studies, our estimates are broadly consistent. Franco et al. (2017) reported 0.38 SID His to Lys ratio as requirement for male Cobb 500 birds during an 8 to 17 days post hatch experiment. Recalculated based on 1.05 % SID Lys in feed, 0.38 SID His to Lys ratio corresponds to 0.40 % SID His. Likewise, but executed in female, Cobb 500 birds during 22 to 42d post hatch with only 4 levels of dig His tested by Nelson et al. (2024a) suggested an optimal dig His to Lys ratio between 0.35 and 0.38, the later corresponding to a recalculated level between 0.385 % and 0.418 % dig His when applying 1.10 % dig Lys in feed. This indicates that the obtained results in this research are close to the sparce literature.

To optimize carcass traits in UC birds, estimated requirements were close to or above 0.50 % SID His and thus higher than the requirements estimated from growth performance parameters. The requirement observed by Nelson et al. (2024b) to maximize breast filet yield and tender yield were 0.39 and 0.37 dig His to Lys ratio respectively, whereas performance traits (BW, BWG, FCR) responded linearly across the tested range. Likewise, in another study by the same authors (Nelson et al., 2024a), a lower requirement for breast yield (0.34 dig His to Lys ratio) was estimated, compared to performance traits such as BWG and FCR, 0.35 dig His to Lys ratio for both parameters. In that study, 0.34 dig His to Lys ratio corresponds to approximately 0.374 % dig His which is markedly lower than the carcass-related requirement estimates obtained in present study.

The present experiment had its limitations because it was executed using equal % SID His in all life stages within a treatment. Because there is an endogenous reserve of His in the body, it can potentially be used to supply free His (Ousterhout, 1960), we opted using low SID His across life stages to limit the potential influence of endogenous reserve. Nelson et al. (2024a) indicated that the early life His supply does not influence the later requirement based on a starter (0-21d) diet containing 0.41 % dig His (equivalent to a 33 % dig Lys to His ratio). In our experiment, T01-T03 remained below this level, suggesting that endogenous His reserves may have been depleted to a greater extent. Additionally, our study does not enable to accurately estimate separate SID His requirement for each life phase, because the same broilers were followed throughout the trial than introducing new birds at start of each phase. Nonetheless, given the scarcity of SID His requirement data in modern broilers and the novel aspect of directly comparing requirements under UC versus C conditions, these results contribute to improving our understanding of SID His requirements in contemporary broiler production.

In conclusion, His is an essential amino acid with multiple physiological roles beyond muscle protein accretion. Endogenous His reserves may potentially confound requirement estimates and should be considered when interpreting requirement results. In addition to the scarcity of available SID His requirement studies in modern broilers, to our knowledge no literature has evaluated SID His requirement over a 35d period under a combined coccidial-bacterial challenge. Our results demonstrate that over the 35-day period, the estimated SID His requirement for C birds to optimize BW and FCR was 0.407 % and 0.375 %, respectively. To reach the same performance UC birds only required 0.387 % and 0.325 % SID His. To optimize breast weight and breast yield (relative to live weight) in the C birds 0.490 % and 0.523 % SID His was required, respectively. Unchallenged birds only required 0.413 % and 0.480 % SID His to have the same weight and yield. The requirement over a 35d lifespan for optimal BW, ADG, and FCR in UC birds ranged between 0.389 and 0.448 % SID His, whereas optimizing breast weight and breast yield (relative to live weight) required 0.513 to 0.524 % SID His.

Authorship contribution

Bart Matton: Writing original draft, Visualization, review & editing. Adriaan Verhelle: Data curation, Investigation, Methodology, Visualization, reviewing. Diana Siebert: review & editing.Behnam Saremi: Funding acquisition, Feed formulation, Conceptualization, Writing research proposal, Monitoring data, Experimental design, review & editing.

Disclosures

B. Matton, D. Siebert and B. Saremi are currently employed by CJ Europe GmbH, Frankfurt am Main, Germany. A. Verhelle has no conflict of interest to declare.