Effects of dietary oil source in pre-starter diets on growth performance, nutrient utilization, and organ weights in broiler chickens

Abstract

After hatching, broiler chicks transition from relying on yolk lipids to dietary nutrients. Providing dietary oils with a fatty acid profile similar to that of the yolk, particularly rich in oleic acid, may facilitate this metabolic shift. This study evaluated the effects of different oil sources in the pre-starter diet on growth performance, nutrient digestibility, and organ development in broiler chickens. A total of 2,016 one-day-old male Ross-308 broilers were allocated to three pre-starter diets (0–4 d) containing 30 g/kg of soybean oil, safflower oil, or sunflower oil (12 replicates of 56 birds). Pancreas, liver, and residual yolk weights were assessed at 3 d. Ileal digesta were collected at 4 d to determine DM, CP, ash, ether extract digestibility, and ileal digestible energy. After pre-starter diets, all birds were fed common diets based on three phases (7-10 d, 11-28 d, and 29-36 d). Growth performance was monitored until 36 d. No significant differences were observed in BW, BW gain, feed intake, or feed conversion ratio among treatments during any growth phase. However, ether extract digestibility differed (P < 0.001), with broilers fed safflower oil exhibiting the highest values, followed by sunflower oil, and the lowest in birds fed soybean oil. Ileal digestible energy differed (P = 0.012), with broilers fed safflower oil exhibiting higher values compared to birds fed sunflower oil. In addition, birds fed soybean oil exhibited greater pancreas weight than those fed safflower oil, with sunflower-fed birds showing intermediate values. Although early dietary lipid source did not affect growth performance, it influenced ether extract digestibility and pancreatic development. Future research should investigate the effect of early lipid nutrition on immune responses and performance under challenging conditions.

Introduction

The egg yolk, the primary nutrient reservoir for the developing chicken embryo, is composed of approximately 51–52 % water, 16–17 % protein, and 31–33 % lipids (Cherian, 2015). Among these, lipids serve as the predominant nutritional component for embryonic growth following water.

During incubation, yolk lipids act as the principal energy reserve (Nehlig et al., 1980; Cherian, 2015), with rapid uptake beginning in the second week of embryogenesis (Yadgary et al., 2013; van der Wagt et al., 2020). After hatch, energy requirements shift to exogenous nutrients, primarily carbohydrates from grains.

The inability of avian species to synthesize polyunsaturated fatty acids (PUFA) due to the lack of desaturase enzymes that insert double bonds beyond the n-9 carbon necessitates dietary inclusion of essential fatty acids, such as linoleic acid and α-linolenic acid (Brenner, 1971). Typically, linoleic acid constitutes over 50 % of the total dietary fatty acids, whereas α-linolenic acid contributes approximately 3–3.5 %, largely due to the widespread inclusion of corn and other lipid sources rich in n-6 fatty acids (Cherian, 2008).

Despite the significance of fatty acids in poultry nutrition, nutrient requirement guidelines for broilers primarily recommend linoleic acid, with dietary inclusion levels ranging from 10 to 12.5 g/kg (NRC, 1994; Aviagen, 2022). However, industry perceptions have traditionally discouraged fat supplementation for newly hatched chicks due to their limited bile salt and lipase production during the early post-hatch period (Lilburn, 1998; Lilburn and Loeffler, 2015).

Egg yolk lipids predominantly contain oleic acid (∼60 %) and palmitic acid (∼20 %) (Noble and Cocchi, 1990; Shibata et al., 2023). Studies have shown that these fatty acids are the primary components absorbed from yolk during embryonic development in ducks (Li et al., 2024). Although dietary modification can slightly increase the deposition of PUFA in egg yolks, oleic and palmitic acids remain the dominant fatty acids (Akbari Moghaddam Kakhki et al., 2020; Thanabalan, 2023).

This similarity in fatty acid composition suggests that newly hatched chicks may efficiently utilize dietary oils with profiles resembling that of egg yolk, particularly those rich in oleic acid. For instance, soybean oil contains approximately 220 g/kg oleic acid (CVB, 2021), while safflower and sunflower oils vary from 132 to 780 g/kg and 220 to 820 g/kg, respectively, depending on the source (Ben Moumen et al., 2013; Kovaleva et al., 2018; CVB, 2021).

Although, evidence suggests that chicks may possess sufficient lipid digestion capacity early post-hatch (Noy and Sklan, 1995; Batal and Parsons, 2002), few have specifically examined the effects of pre-starter lipid composition, particularly oleic and palmitic acids, on nutrient utilization and digestive organ development during the early post-hatch period.

It was hypothesized that supplemental oils with higher oleic acid content may enhance fat digestibility and support early growth performance in broiler chickens. To address this knowledge gap, the present study compared three pre-starter diets differing only in oil source (soybean, safflower, and sunflower oils), which vary in their oleic and palmitic acid contents, and evaluated their effects on growth performance, nutrient digestibility, and organ development during early life.

Materials and methods

All experimental procedures in this study were approved by the Animal Ethics Committee of the Poultry Research Centre (Trouw Nutrition R&D) and complied with Spanish guidelines for the care and use of animals in research (del Estado, 2013).

Housing and management

A total of 2,016 one-day-old male Ross-308 broilers were housed in collective pens (56 chicks per pen; dimensions 2.5 × 1.6 m), evenly distributed across two identical rooms at the Trouw Nutrition Poultry Research Centre (El Viso de San Juan, Toledo, Spain). Each room was equipped with artificial lighting and an environmentally controlled ventilation and temperature system. The arrangement of pens within each room served as the blocking factor, with 12 established blocks (six blocks per room).

Each pen contained wood shavings litter, a water line with four nipples, and a hopper feeder. Treatments were randomly assigned to pens within each block. Room temperatures were maintained at 32°C for the first 2 d, gradually decreasing with age to reach 20°C by 36 d. Lighting was provided for 24 h during the first 3 d, followed by 16 h of light and 8 h of darkness for the remainder of the experiment. Feed and water were provided ad-libitum throughout the trial.

Experimental design and diets

The study was conducted as a randomized complete block design with three dietary treatments (12 replications each) during the pre-starter phase (micro-pellet, 0–4 d). Diets differed only in the oil source: soybean oil, high-oleic safflower oil, or sunflower oil, each included at 30 g/kg (Table 1). For formulation purposes, the AME of all oil sources was considered equivalent, as the calculated energy contribution at the inclusion level of 3 % differed by no more than 5 kcal/kg by different oil sources (soybean: 251 kcal/kg, safflower: 256 kcal/kg, sunflower: 255 kcal/kg; AME values obtained from CVB, 2021). After the pre-starter phase, all birds were fed a common starter (micro-pellet, 5–10 d), grower (pellet, 11–28 d), and finisher (pellet, 29–36 d) diet containing soybean oil. All diets were formulated to meet or exceed nutrient requirements based on CVB (2018) recommendations. All diets included a non-starch polysaccharide degrading enzyme, added on top of the formulation without accounting for energy uplift. A phytase was included at 500 FTU/kg, and only the available phosphorus values were adjusted to reflect the phytase matrix contribution. Calcium levels were not adjusted for the inclusion of phytase. Titanium dioxide (5 g/kg) was included as an indigestible marker in the pre-starter diets to calculate apparent ileal nutrient digestibility.

Table 1.

Feed formulation and calculated nutritional composition of experimental diets, as-fed basis.

Items	Pre-starter (1-4 d)			Starter (5-10 d)1	Grower (11-28 d)	Finisher (29-36 d)
	Soybean oil	Safflower oil	Sunflower oil
Ingredients, g/kg
Corn	331	331	331	331	87	50
Wheat	315	315	315	315	500	650
Soybean meal	152	152	152	152	252	186
Sunflower meal						30.0
Barley					27.3
Potato protein	46.5	46.5	46.5	46.5
Concentrated soy protein	50.0	50.0	50.0	50.0
Safflower oil		30.0
Soybean oil	30.0			30.0	29.0	28.4
Sunflower oil			30.0
Corn DDGS					70.0	23.6
Straw barley	25.0	25.0	25.0	25.0
Mono calcium phosphate	13.7	13.7	13.7	13.7	6.30	3.40
Calcium carbonate	12.5	12.5	12.5	12.5	13.2	12.0
Mineral and vitamin premix2	5.00	5.00	5.00	5.00	5.00	5.00
Sodium bicarbonate	4.10	4.10	4.10	4.10	1.00	2.70
DL-Methionine	2.20	2.20	2.20	2.20	2.20	2.30
Sepiolite				5.00
Titanium oxide	5.00	5.00	5.00
L-Lysine	2.00	2.00	2.00	2.00	2.50	3.20
Sodium chloride	1.00	1.00	1.00	1.00	2.20	1.40
NSPase, Axtra XB3	1.00	1.00	1.00	1.00	1.00	1.00
Phytase, Phyzyme XP4	1.00	1.00	1.00	1.00	1.00	1.00
Coccidiostats, Maxiban	0.600	0.600	0.600	0.600
Coccidiostats, Elancoban					0.500
Nutrient, g/kg unless stated
DM	904	904	904	904	906	910
Ash	57.9	57.9	57.9	57.7	50.7	44.4
Crude protein	210	210	210	210	208	183
Ether extract	53.1	53.1	53.1	53.1	51.6	45.8
AME, Kcal/kg	2,850	2,850	2,850	2,850	2,950	3,150
Digestible amino acids
Lys	11.5	11.5	11.5	11.5	9.9	8.9
Met	5.20	5.20	5.20	5.50	4.50	4.30
Met + Cys	8.10	8.10	8.10	8.40	7.50	7.00
Thr	7.00	7.00	7.00	7.10	6.20	5.50
Trp	2.20	2.20	2.20	2.20	2.20	2.00
Arg	11.7	11.7	11.7	11.7	10.7	9.53
His	4.70	4.70	4.70	4.70	4.31	3.77
Ile	7.97	7.97	7.97	7.97	6.90	6.04
Leu	15.5	15.5	15.5	15.5	13.0	11.0
Phe	9.63	9.63	9.63	9.63	8.38	7.36
Val	8.72	8.72	8.72	8.72	7.59	6.71
Calcium	9.50	9.50	9.50	9.50	8.60	7.50
Available phosphorous	4.10	4.10	4.10	4.10	4.00	3.20
Palmitic acid (C16:0)	6.40	5.60	5.00	6.40	6.00	5.50
Oleic acid (C18:1)	10.6	26.6	30	10.6	10.1	8.7
Linoleic acid (C18:2)	25.4	15.2	12.0	25.4	28.0	24.8
Saturated fat	7.3	3.0	3.0	7.3	7.5	6.8
Unsaturated fat	39.1	40.4	40.2	39.1	41.6	36.8

¹After the d 4, all birds were fed the starter diet contained soybean oil for the remainder of the starter phase, followed by soybean oil-based diets during the grower and finisher periods.

²Added per kg of final feed: 10,000 IU, vitamin A (trans-retinyl acetate); 2,500 IU, vitamin D3 (cholecalciferol); 50 IU, vitamin E (all-rac-tocopheryl-acetate); 2.0 mg, vitamin B₁ (thiamine mononitrate); 6 mg, vitamin B₂ (riboflavin); 40 mg, vitamin B₃ (niacin); 4.0 mg, vitamin B₆ (pyridoxine HCl); 25 µg, vitamin B₁₂ (cyanocobalamin); 2.0 mg, vitamin K₃ (bisulfate menadione complex); 10 mg, pantothenic acid (d-Ca pantothenate); 1.0 mg, folic acid; 300 mg, choline (choline chloride); 150 mcg, d-biotin; 0.25 mg, Se (Na₂SeO₃);1.0 mg, I (KI); 15 mg, Cu (CuSO₄·5H₂O); 65 mg, Fe (FeCO₃); 90 mg, Mn (MnO₂); 80 mg, Zn (ZnO); 2.25 mg/kg, butylated hydroxyanisole; 11.25 mg/kg, butylated hydroxytoluene.

³Danisco Animal Nutrition, Marlborough, UK. 1,220 U endo-1,4 beta-xylanase and 152 U endo-1,3(4)-beta-glucanase. Axtra XB was included on top of the formulation without energy contribution.

⁴Elanco, Indianapolis, Indiana.500 FTU. Phytase matrix values were used to adjust available phosphorus, but not calcium.

Sampling Procedure

BW and feed disappearance were determined collectively per pen at 4, 10, 28, and 36 d, corresponding to the endpoints of the pre-starter, starter, grower, and finisher phases, respectively. BW gain, feed intake, and feed conversion ratio (FCR) were calculated for each phase and cumulatively for the overall period. Mortality-corrected data were used in performance calculations. At 3 d, one bird per pen was humanely euthanized by cervical dislocation to weight the residual yolk, liver, and pancreas.

At 4 d, 15 birds per pen were randomly selected and euthanized to collect ileal digesta for nutrient digestibility analysis. The lower half of the ileum (defined as the section of the small intestine extending from the vitelline Meckel’s diverticulum to 4 cm anterior to the ileocecal junction) was excised and the contents removed, by gently flushing with distilled water into plastic containers. Then, the ileal digesta of all the birds of each pen, were pooled, frozen at −20°C.

Sample analysis

Ileal digesta samples were freeze-dried and ground to pass through a 0.5-mm screen before analysis. DM, ash, nitrogen, ether extract (EE), and titanium dioxide contents were determined to calculate the coefficient of apparent ileal digestibility for DM, ash, nitrogen, and EE. Ileal digestible energy was calculated as the product of respective coefficients and the gross energy (Kong and Adeola, 2016).

Experimental diets, and ileal digesta underwent comprehensive analyses following AOAC International (2005). The analyses included determining DM content by oven-drying (method 934.01), total ash (method 942.05), nitrogen by combustion (method 990.03) using a LECO analyzer, and EE (method 960.39). Titanium dioxide was determined as describe by Short et al. (1996). The gross energy using an adiabatic bomb calorimeter (Model 6100, Parr Instrument Company, Moline, IL, USA).

For fatty acid analysis, feed samples were ground and analyzed at MasterLab Trouw Nutrition, Spain. Briefly, samples were digested using water, methanol, and potassium hydroxide, followed by iso-octane extraction to concentrate fatty acid methyl esters in the organic layer. Individual fatty acid methyl esters were identified and quantified using a gas chromatograph (Hewlett Packard, HP 6890 Series, Wilmington, Delaware, USA) with a flame ionization detector and automatic injector. Separation of fatty acid methyl esters was achieved using a Supelcowax™ semi-capillary column (30 m, 0.53 mm I.D., 1.00 µm film thickness, Supelco, Sigma-Aldrich, Pennsylvania, USA).

Calculations and statistical analysis

The apparent ileal digestibility of nutrient was calculated from the ratio of nutrient to titanium dioxide in the diet relative to the corresponding ratio in the ileal digesta.

$$\text{apparent}\text{ileal}\text{digestibility}(\%)=\frac{(\text{nutrient}/\text{titanium}\text{dioxide})\text{diet}-(\text{nutrient}/\text{titanium}\text{dioxide})\text{ileal}}{(\text{nutrient}/\text{titanium}\text{dioxide})\text{diet}}\times 100$$

Where (nutrient/titanium dioxide)_diet = ratio of a nutrient (DM, ash, CP, EE) to titanium dioxide in the diet, and (nutrient /titanium dioxide)_ileal = ratio of the same dietary components to titanium dioxide in the ileal digesta.

Data normality was assessed using the PROC UNIVARIATE procedure (Shapiro–Wilk test) in SAS 9.4 (SAS Institute Inc., Cary, NC). Outliers (mean ± 3.0 SD) were identified using the INFLUENCE option in the MIXED procedure. Performance, digestibility, and organ weight data were analyzed using the GLIMMIX procedure of SAS, assuming a normal distribution. Mortality was analyzed using the GLIMMIX procedure with a binomial distribution. Treatment means were compared using Tukey’s multiple comparison test. Statistical significance was declared at P ≤ 0.05, and trends were considered for 0.05 < P ≤ 0.10.

Results

Analyzed composition of the experimental diets

The analyzed composition of the experimental diets is presented in Table 2. EE content increased progressively from the soybean oil diet (51.5 g/kg) to the sunflower oil diet (65.0 g/kg), with all analyzed values exceeding the calculated value of 53.1 g/kg. Notable differences in fatty acid profile were observed among the experimental diets. Palmitic acid (C16:0) content was similar across diets, ranging from 6.2 g/kg to 6.6 g/kg. Oleic acid (C18:1) was highest in the sunflower oil diet (32.0 g/kg), followed by the safflower oil diet (27.2 g/kg) and the soybean oil diet (13.3 g/kg). Linoleic acid (C18:2) content was highest in the soybean oil diet (26.8 g/kg), followed by safflower (23.4 g/kg) and sunflower oil (21.8 g/kg). Concentrations of other fatty acids were all below 5 % of total fatty acids.

Table 2.

Analyzed nutritional composition of oil sources and experimental diets, as-fed basis.

Items	Soybean oil	Safflower oil	Sunflower oil
Pre/Starter feed
DM, g/kg	921	920	922
Ash, g/kg	58.7	60.5	60.0
Crude protein, g/kg	205	203	200
Ether extract, g/kg	51.5	63.0	65.0
Fatty acid profile, 1
g/kg of feed
Palmitic acid (C16:0)	6.20	6.60	6.20
Oleic acid (C18:1)	13.3	27.2	32.0
Linoleic acid (C18:2)	26.8	23.4	21.8
Saturated fatty acids	8.90	9.70	8.80
Unsaturated fatty acids	42.4	52.8	55.4
UFA/SFA ratio	4.80	5.40	6.30
Monounsaturated fatty acids	13.5	27.6	32.3
Polyunsaturated fatty acids	28.8	25.2	23.1
Total n-3 fatty acids	2.00	1.80	1.30
Total n-6 fatty acids	26.8	23.4	21.8
% of total fat
Palmitic acid (C16:0)	12.1	10.5	9.5
Oleic acid (C18:1)	25.8	43.2	49.2
Linoleic acid (C18:2)	52.1	37.2	33.5
Saturated fatty acids	17.3	15.4	13.6
Unsaturated fatty acids	82.3	83.8	85.2
Monounsaturated fatty acids	26.2	43.8	49.7
Polyunsaturated fatty acids	56.0	40.0	35.5
Total n-3 fatty acids	3.90	2.80	2.00
Total n-6 fatty acids	52.1	37.2	33.5
Grower feed
DM, g/kg	916
Ash, g/kg	49.8
Crude protein, g/kg	203
Finisher feed
DM, g/kg	903
Ash, g/kg	41.5
Crude protein, g/kg	177

¹Fatty acids with concentrations less than 5 % were not presented, including: caprylic acid (C8:0), capric acid (C10:0), lauric acid (C12:0), myristic acid (C14:0), myristoleic acid (C14:1), pentadecanoic acid (C15:0), palmitoleic acid (C16:1), margaric acid (C17:0), stearic acid (C18:0), heptadecenoic acid (C17:1), alpha-linolenic acid (C18:3), arachidic acid (C20:0), gadoleic acid (C20:1), eicosadienoic acid (C20:2), behenic acid (C22:0), docosenoic acids (C22:1), and lignoceric acid (C24:0).

Saturated fatty acid content was higher in the safflower oil diet (9.7 g/kg) than in the soybean and sunflower oil diets (8.9 and 8.8 g/kg, respectively). In contrast, unsaturated fatty acid content increased progressively from the soybean oil diet (42.4 g/kg) to the sunflower oil diet (55.4 g/kg).

Monounsaturated fatty acids (MUFA) were highest in the sunflower oil diet (32.3 g/kg), followed by safflower (27.6 g/kg) and soybean oil diet (13.5 g/kg). PUFA decreased from the soybean oil diet (28.8 g/kg) to the sunflower oil diet (23.1 g/kg).

Total n-3 fatty acid content decreased across diets, with the highest concentration in the soybean oil diet (2.0 g/kg) and the lowest in the sunflower oil diet (1.3 g/kg). Similarly, total n-6 fatty acids were highest in the soybean oil diet (26.8 g/kg) and lowest in the sunflower oil diet (21.8 g/kg).

Growth performance

No differences (P > 0.05) were observed among treatments for BW, BW gain, feed intake, FCR, or mortality across the pre-starter (1–4 d), starter (5–10 d), grower (11–28 d), finisher (29–36 d), or overall (1–36 d) periods (Table 3). A two-way analysis of variance (Supplementary Figure 1) revealed no significant interaction between oil source and growth period (P > 0.10), nor a main effect of oil source on any performance variable (BW, BW gain, feed intake, or FCR).

Table 3.

Effect of oil sources in the pre-starter diet (1–4 d) on growth performance of male broilers.

Items	Soybean oil	Safflower oil	Sunflower oil	SEM	P-value
Pre-starter (1-4 d)
Body weight at 4 d, g1	90.6	89.5	89.6	0.902	0.664
Body weight gain, g	55.0	53.9	54.0	0.962	0.635
Feed intake, g	48.2	47.5	47.9	1.022	0.852
Feed conversion ratio	0.875	0.882	0.888	0.012	0.764
Mortality, %	0.298	0.595	0.745	0.285	0.672
Stater (5-10 d)
Body weight at 10 d, g	257	257	257	2.45	0.998
Body weight gain, g	167	168	168	1.74	0.837
Feed intake, g	191	192	190	2.18	0.805
Feed conversion ratio	1.14	1.14	1.13	0.005	0.271
Grower (11-28 d)
Body weight at 28 d, g	1,623	1,593	1,601	15.5	0.263
Body weight gain, g	1,366	1,336	1,343	14.4	0.203
Feed intake, g	1,903	1,870	1,880	16.4	0.267
Feed conversion ratio	1.39	1.40	1.40	0.006	0.660
Finisher (29-36 d)
Body weight at 36 d, g	2,414	2,380	2,386	16.2	0.288
Body weight gain, g	791	787	785	15.3	0.931
Feed intake, g	1,460	1,448	1,439	14.2	0.595
Feed conversion ratio	1.85	1.85	1.84	0.023	0.839
Overall (1-36 d)
Body weight gain, g	2,378	2,344	2,350	16.3	0.288
Feed intake, g	3,602	3,558	3,557	24.3	0.341
Feed conversion ratio	1.49	1.49	1.49	0.005	0.859
Mortality, %	2.68	2.68	2.68	0.620	0.999

Data are least-squares means of twelve replications per treatment.

¹Body weight at d-1 was used a covariate.

Residual yolk, liver, and pancreas weights

No differences (P > 0.05) were observed among dietary treatments for BW, residual yolk weight (g and % BW), or liver weight (g and % BW) at 3 d (Table 4). However, significant differences were observed for pancreas in absolute (g) and relative (% BW) weights. Broilers fed soybean oil diet had a higher pancreas absolute (P = 0.029) and relative (P = 0.023) weights compared to those fed safflower oil, whereas birds fed sunflower oil exhibited intermediate values, not significantly different from either group.

Table 4.

Effect of oil sources in the pre-starter diet on organ weights in 3-d-old male broilers.

Items	Soybean oil	Safflower oil	Sunflower oil	SEM	P-value
Body weight, g	72.3	70.7	69.4	0.979	0.192
Residual yolk, g	0.476	0.554	0.469	0.081	0.745
Residual yolk, % BW	0.655	0.784	0.676	0.114	0.718
Liver, g	3.34	3.09	3.16	0.130	0.437
Liver, % BW	4.60	4.37	4.54	0.139	0.473
Pancreas, g	0.367a	0.311b	0.322ab	0.015	0.029
Pancreas, % BW	0.506a	0.439b	0.461ab	0.017	0.023

Data are least-squares means of twelve birds per treatment.

^a,bMeans within a row not sharing a common superscript are different at P < 0.05.

Ileal nutrient digestibility

No differences (P > 0.05) were observed among treatments for DM, ash, or CP digestibility (Table 5). However, significant differences were found for digestible energy (P = 0.012) and EE digestibility (P < 0.001). Broilers fed the safflower oil diet exhibited the highest digestible energy, followed by those fed the soybean oil diet and the sunflower oil diet. For EE digestibility, broilers fed safflower oil had the highest value, followed by sunflower oil, with the lowest value observed in broilers fed soybean oil diet.

Table 5.

Effect of oil sources in the pre-starter diet (1–4 d) on apparent ileal digestibility in 4-d-old male broilers.

Items	Soybean oil	Safflower oil	Sunflower oil	SEM	P-value
Dry matter, %	70.0	71.0	70.3	0.474	0.241
Digestible energy, kcal/kg	3,332a,b	3,367a	3,279b	22.9	0.012
Ether extract, %	94.9c	97.1a	96.5b	0.137	<0.001
Crude protein, %	76.4	76.6	75.9	1.18	0.803
Ash, %	43.8	43.1	44.1	0.874	0.518

Data are least-squares means of twelve replications per treatment.

^a,b,cMeans within a row not sharing a common superscript are different at P < 0.05.

Discussion

The first two weeks post-hatch are critical for chicks due to rapid gastrointestinal maturation and increased secretion of digestive enzymes, with considerable variation among individual enzymes (Marchaim and Kulka, 1967). Lipid digestion improves markedly with age, particularly for PUFA (Krogdahl, 1985; Sell et al., 1986). For example, Carew Jr et al. (1972) reported that fat absorption from a diet containing 20 % corn oil increased from 83.7 % at 2–7 d to 95.2 % at 8–15 d of age, based on excreta digestibility measurements.

In the present study, despite clear differences in the fatty acid profiles of the pre-starter diets, especially in oleic acid concentration, no significant differences in growth performance parameters were observed. This suggests that newly hatched broilers are metabolically capable of adapting to dietary fats with varying degrees of saturation and unsaturation, consistent with previous findings showing metabolic plasticity in lipid utilization once yolk reserves are depleted (Lilburn and Loeffler, 2015; Krogdahl, 1985; Noy and Sklan, 1995).

One possible explanation for the absence of performance differences is the rapid maturation of digestive enzyme activity during the early post-hatch period. Although neonatal chicks initially exhibit limited bile salt and pancreatic lipase production (Lilburn, 1998), the dietary lipid inclusion rate of 30 g/kg may not have challenged their digestive capacity sufficiently. Furthermore, feeding a common diet after 4 d of age could have masked any early transient effects of the oil source.

Fat digestibility exceeded 85 % by 4 d post-hatch in previous studies (Noy and Sklan, 1995), and high AME values were observed even immediately post-hatch when chicks were fed simple diets (Batal and Parsons, 2002). These findings suggest that under appropriate conditions, young broilers have a more advanced capacity for lipid digestion than traditionally assumed. Factors such as the fatty acid profile, degree of saturation, and presence of emulsifiers may all contribute. In this study, the very high EE digestibility values (>94 %) further support the emerging understanding that young broilers can efficiently utilize selected lipid sources.

The observed increase in pancreas weight in absolute and relative (% BW) terms in broilers fed soybean oil diet suggests a physiological adaptation to digestive demands. Dror et al. (1976) reported that pancreatic lipase activity in chicks fed 15 % soybean oil was approximately 30 % higher than in those fed fat-free diets. Similarly, Krogdahl (1985) and Dänicke et al. (2000) found that dietary fat type can modulate pancreatic enzyme secretion, with less digestible fats inducing higher secretory output. Poorghasemi et al. (2013) also demonstrated that 42-d old broilers fed a diet with 4 % tallow had greater pancreas weight compared to those fed a diet with 4 % canola or sunflower oil, reinforcing that the pancreas adapts based on the digestive challenge imposed by the lipid source.

Soybean oil, being richer in PUFA, may be less efficiently digested by young chicks compared to oils higher in MUFA, such as safflower and sunflower oils. This interpretation is supported by the ileal digestibility data showing lower EE digestibility in birds receiving soybean oil. Reduced fat digestibility likely imposed greater secretory demands on the pancreas, leading to hypertrophy (Krogdahl, 1985).

The differences observed in the apparent ileal digestibility of EE among birds fed safflower oil diet (97.1 %), sunflower oil diet (96.5 %), and soybean oil diet (94.9 %) can be attributed to differences in their fatty acid profiles. Sunflower and safflower oils were rich in MUFA, primarily oleic acid, while soybean oil contained a higher proportion of PUFA, especially linoleic acid.

The role of MUFA in enhancing fat digestibility has not been extensively studied. While there is abundant research on dietary fat effects in broilers (Tancharoenrat et al., 2013; Kamran et al., 2020), fewer studies have isolated the specific contribution of oleic acid. Elbaz et al. (2023) demonstrated that broilers fed olive oil-based diets, richer in MUFA, showed improved growth performance than broilers fed soy, corn and fish oil-bases diets under heat stress conditions, although fat digestibility was not significantly affected.

Oleic acid appears to positively influence lipid absorption. Young and Garrett (1963) showed that oleic acid facilitated the absorption of saturated fatty acids, with higher oleic-to-palmitic acid ratios improving palmitic acid absorption.

Some discrepancies between calculated and analyzed fatty acid profiles must be considered when interpreting these results. The calculated difference in oleic acid content between soybean and sunflower oil diets was 19.4 g/kg, closely matching the analyzed difference (18.7 g/kg). However, the differences for linoleic acid and palmitic acid were smaller than expected, potentially limiting the fatty acid contrasts between diets.

Moreover, the degree of saturation has been shown to exert a greater impact on nutrient utilization than free fatty acid content (Rodriguez-Sanchez et al., 2019; Palomar et al., 2023). Nevertheless, the absence of detailed information regarding free fatty acid content represents a limitation of this study. High free fatty acid levels are known to reduce fat digestibility and energy utilization, particularly in young broilers, by promoting soap formation and oxidative degradation (Wiseman and Salvador, 1991; Rodriguez-Sanchez et al., 2019).

In conclusion, this study demonstrated that four-day-old broilers possessed a high capacity for lipid digestion, with apparent ileal EE digestibility values exceeding 94 % across all dietary treatments. Although early dietary lipid source did not affect growth performance, differences in ileal digestibility and pancreatic development suggest that dietary oil type influences digestive physiology. Oils richer in MUFA, such as safflower and sunflower oil with higher oleic acid content, promoted greater EE digestibility compared to soybean oil. Future research should investigate the impact of early dietary fatty acid profiles on immune responses and performance under challenging environmental or disease conditions.

CRediT authorship contribution statement

Reza Akbari Moghaddam Kakhki: Data curation, Formal analysis, Validation, Visualization, Writing – original draft, Writing – review & editing. Cibele Araujo Torres: Conceptualization, Investigation, Methodology. Lewis Alfonso Aguirre Toribio: Formal analysis, Methodology, Writing – review & editing. Alejandro Saiz Del Barrio: Data curation, Formal analysis, Validation. Ana Isabel García-Ruiz: Funding acquisition, Project administration, Resources, Supervision.

Disclosures

The authors declare that they have no conflicts of interest regarding the publication of this manuscript.