Dairy cow parity affects relationships among nutritional parameters in the blood of dams, umbilical cords, and calves and placental development at calving

Abstract

Heifer growth and milk production in lactating cows may diminish the nutrient supply to the fetus. This study aimed to analyze the characteristics of the nutrient supply to the fetus in primiparous and multiparous cows. We investigated maternal, umbilical cord, and calf blood glucose and amino acid levels, as well as placental development in 28 primiparous (PP) and 30 multiparous (MP) Holstein cows. Although the total cotyledonary weight and surface area showed no significant differences, the MP group exhibited larger individual cotyledons (P < 0.01) and fewer medium-sized cotyledons (P < 0.05). Within the PP group, total cotyledonary weight and surface area positively correlated with blood glucose (r = 0.71–0.77; P < 0.01) and total essential amino acid (r = 0.55; P < 0.05) concentrations in the umbilical veins. However, no significant correlation was observed in the MP group. Blood glucose and amino acid concentrations in the umbilical vein, umbilical artery, and calf were significantly lower in the MP group (P < 0.05), although no difference was observed in the dams between the groups. In conclusion, the nutrient status of primiparous cows can alter fetal nutrient supply. Moreover, multiparous cows have larger individual cotyledons as an adaptive response to increased milk production during pregnancy. However, this adaptive response in multiparous cows did not completely restore nutrient supply to the fetus to the same extent as that in primiparous cows. Therefore, the nutritional management of multiparous cows during pregnancy must be reconsidered.

The placenta, the interface between the dam and the fetus, transfers all the nutrients necessary for fetal growth and development [1]. The capacity for nutrient transfer is affected by the placental size, which alters the surface area available for transport [2]. Numerous animal models have demonstrated a positive correlation between placental and fetal weights at full term [3, 4]. Therefore, placental growth is crucial in fetal growth and directly influences birth weight [3, 5].

Glucose and amino acids are the main nutrients required for fetal growth [6, 7]. The placenta and fetus have limited glucose [2]. Therefore, glucose availability depends on the supply from the maternal circulation [8,9,10,11]. Owing to transport along a concentration gradient, maternal glucose levels higher than fetal levels facilitate net glucose transport toward the fetus [12, 13]. Conversely, blood amino acid concentrations in fetuses are typically higher than in dams, with most amino acids transported against a fetal-maternal concentration gradient through active transport processes [6, 8, 14]. A study of pregnant ewes showed that short-term maternal fasting did not notably alter the supply of amino acids from dams to fetuses [15]. However, long-term maternal nutrient restriction in ewes reduces blood amino acid concentrations in fetuses and umbilical veins [16]. Additionally, maternal malnutrition during pregnancy has been associated with delayed uterine and fetal development and placental overdevelopment in beef cattle [17,18,19,20].

In the dairy industry, heifers are inseminated at 13–15 months of age or 55–65% of their mature weight [21,22,23]. Therefore, during pregnancy, they grow continuously and require nutrients for fetal growth [22, 23]. Furthermore, dairy cows are expected to calve at intervals of no longer than 385–400 days to optimize their lifetime productivity [24], necessitating insemination during early to mid-lactation when milk production is high. Consequently, metabolic priority during lactation results in substantial nutrient loss to the uterus [25], complicating the provision of adequate nutrients for fetal growth.

Therefore, dairy heifer growth and milk production in dairy cows require high nutrient demands during pregnancy, potentially reducing the nutrient availability for placental and fetal growth. Several studies have reported the full-term placental size in primiparous and multiparous dairy cows [26, 27]. However, studies on glucose and amino acid supply to the fetus through the placenta are lacking. Therefore, this study examined serum glucose and plasma amino acid concentrations in dams, umbilical cords (veins and arteries), and calves at calving as well as placental development at full term to determine whether nutrient supply to the fetus is reduced in primiparous and multiparous cows.

Materials and Methods

Experimental animals, feeding and management

The experiments were conducted between July 2020 and January 2023 at the Field Center of Animal Science and Agriculture, Obihiro University of Agriculture and Veterinary Medicine, Japan. It involved 58 pregnant Holstein-Friesian cows carrying similar fetal breeds. Of these, 28 were primiparous (PP) and 30 were multiparous (MP). The MP group had an average parity of 2.80 ± 0.16 at calving. Approximately one month before the expected calving date, they were moved to a free-stall house for dry cows with a paddock and were fed a mixed ration comprising grass silage, corn silage, and concentrate at 1500 h until calving. The animals had free access to hay, minerals, and water. The rectal temperature of the cows was recorded at 1500–1600 h daily one week before the expected calving date. When the rectal temperature decreased by 0.5°C compared to the previous day, the cows were moved to individual calving pens and housed until parturition. After birth, the calves were cleaned and dried with a towel and their umbilical cords were disinfected. They were weighed and housed in individual pens until the first colostrum was provided. The body condition score (BCS) of the cows pre-calving (2–3 weeks before the expected calving date) were measured by the same operator using a scale (1–5) at 0.25 intervals [28]. Maternal age, gestation length, and calving difficulty (score 1, unassisted birth; 2, easy calving with human assistance; 3, difficult calving with few humans; 4, dystocia; 5, surgical treatment or death of the cow [29]) were also recorded for all cows.

All experimental procedures complied with the Guide for the Care and Use of Agricultural Animals of Obihiro University (approval numbers: #20–224, 20–226).

Sampling and measurement of milk production and component

During gestation until dry-off, the daily milk production of MP cows was monitored. Milk samples were collected monthly and stored at 4°C for subsequent analysis. Milk fat analysis was conducted by the Tokachi Agricultural Cooperative Association (Hokkaido, Japan) and quantified within 24 h of collection using an infrared analyzer (MikoScan^TMFT+, FOSS Analytical A/S, Hillerød, Denmark). The 4% fat-corrected milk production (kg) was calculated based on milk production and milk fat ratio using the formula: [0.15 × milk fat (%) + 0.4] × milk production (kg) [30].

Sampling and measurement of blood parameters

Blood samples were collected before (2–3 weeks before the expected calving date) and immediately after (within 1 h) calving via caudal venipuncture. Blood samples from the umbilical cord (umbilical vein, UV; umbilical artery, UA) were collected at calving following the procedure described by Kawashima et al. [31]. Additionally, blood samples were collected from the calves via the jugular vein immediately after birth (within 1 h) before the first colostrum feeding. Three UV samples and six UA samples in the PP group and two UV and 13 UA samples in the MP group were not collected because the umbilical cords were cut during the calving process. These data were excluded from the analysis.

Non-heparinized, silicone-coated 9 ml tubes (Venoject, Autosep, Gel + Clot. Act., VP-AS109K; Terumo Corporation, Tokyo, Japan) were used for glucose measurements, and 5 ml tubes containing ethylenediaminetetraacetic acid (Venoject II, VP-NA050K; Terumo Corporation) were used for amino acid measurements. Blood samples were coagulated for 10 min at 38°C in an incubator to obtain the serum. All tubes were then centrifuged at 2,328 × g for 15 min at 4°C, and the separated serum and plasma samples were stored at –30°C and –80°C, respectively, until further analysis. Serum glucose concentrations were analyzed using an automated clinical chemistry analyzer (TBA120FR; Toshiba Medical Systems Co., Ltd., Tochigi, Japan). Plasma amino acid concentrations were analyzed by NTDS Cooperation (Hokkaido, Japan). Blood plasma samples were shipped on dry ice and subjected to high-performance liquid chromatography at NTDS.

Placental collection and measurement

The complete placenta spontaneously expelled within 12 h of parturition was collected from 15 of the 28 cows in the PP group and 19 of the 30 in the MP group. The entire placenta was weighed immediately after expulsion. The cotyledons were separated from the intercotyledonary membranes by pinching each cotyledon away and cutting with scissors. Cotyledons weighing < 2.0 g were excluded from further analyses [32]. Individual cotyledons were counted, weighed, flattened on a grid sheet, and photographed for surface area analysis using image analysis software (ImageJ; https://imagej.net/software/fiji/). The intercotyledonary membrane was weighed after umbilical cord excision. Total cotyledonary weight and surface area were calculated as the sum of all cotyledonary weights and surface areas, respectively. The time between calving and placental expulsion was recorded.

Statistical analysis

All statistical analyses were performed using SigmaPlot^® 14.5 (Systat Software, Inc., San Jose, CA, USA). The normality of the data was tested using the Shapiro-Wilk test. Differences in maternal BCS, age at calving, gestation length, calving difficulty, calf birth weight, placental measurements, serum glucose concentrations, and plasma amino acid concentrations between the PP and MP groups were analyzed using the Student’s t-test or the Mann–Whitney U test. Furthermore, the calf sex ratio between the groups was analyzed using the chi-squared test. Pearson’s correlation analysis or Spearman’s rank correlation analysis was used to explore the correlations between blood parameters, placental measurements, and milk production variables. To examine the distribution of individual cotyledonary weights and surface areas in the PP and MP groups, all data were categorized into four quartiles from heaviest or largest (first stage: 76–100%; second stage: 51–75%; third stage: 26–50%; fourth stage: 1–25%) [32]. The number of cotyledons at each stage was calculated, and the distribution of individual cotyledonary weights and surface areas between the two groups was compared using a chi-square test. Additionally, the average percentage of each stage between the groups was compared using the Student’s t-test or Mann–Whitney U test. All data were presented as the mean ± standard error of the mean (SEM), and a P-value of < 0.05 was considered statistically significant.

Results

Maternal BCS, gestation length, calving difficulty, calf birth weight, and calf sex

Maternal pre-calving BCS (P < 0.001) and calving difficulty (P < 0.001) were significantly higher in the PP group than in the MP group (Supplementary Table 1). Moreover, cows in the MP group had a significantly longer gestation period (P = 0.048) than those in the PP group. Although calf sex (P = 0.588) was not significantly different between the groups, calf birth weight was significantly higher in the MP group than in the PP group (P < 0.001).

Placental development

Full-term placental development in the PP and MP groups is shown in Table 1. Although the duration of placental expulsion in the PP group was significantly shorter than in the MP group (P = 0.003), the total placental weight, intercotyledonary membrane weight, total cotyledonary weight, total cotyledonary surface area, and the number of cotyledons were not statistically different between the groups. The percentages of the four stages of cotyledonary weight (A) and surface area (B) in the PP and MP groups are shown in Fig. 1. The cotyledonary weight ranges for the first to the fourth stage were 32.41–136.54, 20.29–32.40, 11.35–20.28, and 2.00–11.34 g, respectively. Similarly, the cotyledonary surface area ranges for the first to the fourth stage were 76.91–282.20, 49.76–76.90, 29.79–49.75, and 4.82–29.78 cm², respectively. The distribution of individual cotyledonary weights and surface areas differed significantly among the groups (P < 0.001). However, cotyledonary weight at each stage did not differ significantly between the groups. Comparing the cotyledonary surface area, the percentage of the first stage was higher (P = 0.002), whereas that of the third stage was lower (P = 0.012) in the MP group than in the PP group.

Table 1. Placental development in the PP and MP groups.

	PP group	MP group	P-value
	(n = 15)	(n = 19)
Duration of placenta expulsion after calving (min) 1	294.5 ± 15.7	420.2 ± 32.2	0.003
Total placental weight (kg) 2	5.33 ± 0.4	6.16 ± 0.4	0.119
Inter-cotyledonary membrane weight (kg)	2.62 ± 0.2	3.32 ± 0.3	0.069
Total cotyledonary weight (g) 3	2393.4 ± 134.1	2340.4 ± 80.8	0.725
Total cotyledonary surface area (cm2) 3	5443.2 ± 311.8	6073.7 ± 203.9	0.089
Number of cotyledons (n) 3	111.8 ± 10.8	90.6 ± 5.6	0.074

Data are presented as means ± SEM. ¹ Intervals between calving and placental expulsion. ² Placental weight spontaneously expelled within 12 h of parturition. ³ Excluded less than 2.0 g of cotyledon from the experiment [32]. MP, multiparous; PP, primiparous.

Fig. 1.

Percentages of four stages of cotyledonary weight (A) and surface area (B) in PP (n = 15) and MP (n = 19) groups. Data are presented as means ± SEM. The number in the bracket indicates the number of cotyledons. The sums of the cotyledon number of all stages in the PP and MP group were 1,677 and 1,722, respectively. ^※ indicates statistical differences in the distribution of cotyledonary weight and surface area between the groups (P < 0.001). ^ab different letters indicate statistical differences in the percentage of each stage between the groups (P < 0.05). MP, multiparous; PP, primiparous.

Relationship between umbilical vein blood parameters, placental size, and milk production

Table 2 shows the relationships among the UV blood parameters, placental size, and milk production. In the PP group, positive correlations were observed between the total cotyledonary weight and serum glucose (r = 0.766; P = 0.001) and plasma total essential amino acid (r = 0.554; P = 0.040) concentrations of UV. Similarly, the cotyledonary surface area was positively correlated with UV serum glucose (r = 0.706; P = 0.005) and plasma total essential amino acid (r = 0.553; P = 0.040) concentrations. Meanwhile, negative correlations were observed between the cotyledonary surface area in the second stage and UV serum glucose (r = −0.545; P = 0.044). However, in the MP group, no relationship was observed between total cotyledonary size and UV serum glucose or plasma amino acid concentrations. For individual cotyledons, the cotyledonary surface area in the second stage was negatively correlated with UV total amino acid (r = −0.523; P = 0.030) and total essential amino acid (r = −0.503; P = 0.038) concentrations. Additionally, total milk production during pregnancy in the MP group was positively correlated with the percentage of cotyledonary weight in the first stage (r = 0.471; P = 0.042) and negatively correlated with the percentage of cotyledonary weight (r = −0.532; P = 0.019) and surface area (r = −0.464; P = 0.046) in the second stage.

Table 2. Coefficient of correlation between UV blood glucose and amino acid concentrations, total milk production during pregnancy, and placental development in the PP and MP groups.

Variable		Cotyledonary weight					Cotyledonary surface area
		Total (g)	First stage (%)	Second stage (%)	Third stage (%)	Fourth stage (%)	Total (cm2)	First stage (%)	Second stage (%)	Third stage (%)	Fourth stage (%)
PP group 1
	Glucose (mg/dl)	0.766 **	−0.300	−0.239	0.364	0.375	0.706 **	−0.301	−0.545 *	0.438	0.474
	TAA (µmol/l)	0.487	0.244	−0.464	−0.074	0.004	0.508	0.189	−0.165	−0.055	−0.067
	TEAA (µmol/l)	0.554 *	0.106	−0.441	0.068	0.097	0.553 *	0.023	−0.234	0.111	0.067
	TNEAA (µmol/l)	0.444	0.301	−0.464	−0.138	−0.039	0.474	0.261	−0.129	−0.130	−0.127
MP group 2
	Glucose (mg/dl)	0.414	−0.116	−0.030	0.043	0.200	0.344	−0.150	−0.144	0.108	0.273
	TAA (µmol/l)	0.011	0.038	−0.265	0.093	0.210	0.251	0.138	−0.523 *	0.118	0.081
	TEAA (µmol/l)	0.030	0.049	−0.398	0.163	0.133	0.044	0.036	−0.503 *	0.187	0.210
	TNEAA (µmol/l)	0.350 e-3	0.026	−0.156	0.043	0.060	0.296	0.164	−0.445	0.065	0.005
Total milk production during pregnancy (kg)		0.010	0.471 *	−0.532 *	−0.302	−0.120	−0.179	0.398	−0.464 *	−0.322	−0.027

^* P < 0.05; ^** P < 0.01. ¹ n = 15. ² n = 19. MP, multiparous; PP, primiparous. TAA, total amino acid; TEAA, total essential amino acid; TNEAA, total non-essential amino acid; UV, umbilical vein.

Blood parameters of dams, umbilical cords, and calves at calving

Supplementary Table 2 presents the serum glucose and plasma amino acid concentrations of the precalving cows in the two groups. Serum glucose concentrations were significantly higher in the PP group than in the MP group (P = 0.002). However, no differences in plasma total, total essential, or total non-essential amino acid concentrations were observed between the groups. For individual amino acids, cows in the MP group had significantly lower plasma glutamine (P = 0.029) and glutamic acid (P = 0.004) concentrations and higher plasma lysine (P = 0.012) and arginine (P = 0.032) concentrations than cows in the PP group. Figure 2 illustrates the serum glucose and plasma amino acid concentrations of dams immediately after calving, umbilical cords at calving, calves immediately after birth, and their first colostrum feeding in both the PP and MP groups. Although serum glucose and plasma total, total essential, and total non-essential amino acid concentrations of the dams did not differ significantly between the groups, some individual amino acids showed statistical differences. Specifically, dams in the MP group exhibited lower plasma concentrations of phenylalanine (P = 0.041), glutamic acid (P < 0.001), and alanine (P = 0.022) and higher concentrations of serine (P = 0.016) than those in the PP group. The MP group had significantly lower UV serum glucose (P = 0.003), plasma total amino acid (P = 0.003), total essential amino acid (P < 0.001), total non-essential amino acid (P = 0.008), isoleucine (P < 0.001), valine (P < 0.001), leucine (P < 0.001), histidine (P = 0.028), asparagine (P = 0.037), aspartic acid (P = 0.026), glutamic acid (P = 0.035), alanine (P = 0.001), proline (P = 0.004), and tyrosine (P = 0.005) levels than those in the PP group. Furthermore, calves in the MP group had significantly lower plasma total amino acid (P < 0.001), total essential amino acid (P < 0.001), total non-essential amino acid (P < 0.001), isoleucine (P < 0.001), valine (P < 0.001), leucine (P < 0.001), histidine (P = 0.015), lysine (P = 0.006), threonine (P = 0.006), tryptophan (P = 0.030), methionine (P = 0.010), asparagine (P = 0.004), alanine (P < 0.001), proline (P < 0.001), and tyrosine (P = 0.002) concentrations than those in the PP group. The MP group had significantly lower UA serum glucose (P = 0.002), plasma total amino acid (P = 0.012), total essential amino acid (P = 0.009), total non-essential amino acid (P = 0.019), isoleucine (P = 0.001), valine (P < 0.001), leucine (P = 0.002), asparagine (P = 0.037), aspartic acid (P = 0.037), glutamic acid (P = 0.008), proline (P = 0.031), tyrosine (P = 0.028) concentrations than those in the PP group. Tables 3 and 4 present the relationships between the blood parameters of dams, UVs, and calves. In the PP group, the serum glucose concentrations of dams before and after calving were positively correlated with UV (r = 0.524; P = 0.007) and calf (r = 0.545; P = 0.003) concentrations, respectively. Additionally, plasma total non-essential amino acid concentrations in dams before and after calving were positively correlated with plasma total, total essential, and total non-essential amino acid concentrations in UV (r = 0.337–0.449; P < 0.050) and calves (r = 0.382–0.383; P < 0.050). In the MP group, maternal glucose concentrations before and after calving were positively correlated with UV (r = 0.721; P < 0.001) and calf (r = 0.420; P = 0.021) concentrations.

Fig. 2.

Serum glucose and plasma amino acid concentrations of dams immediately after calving, umbilical cords at calving, and calves immediately after birth before the first colostrum feeding in PP and MP groups. Data are presented as means ± SEM. * P < 0.05; ** P < 0.01; *** P < 0.001. The number of dams, UVs, calves, and UAs in the PP group were 28, 25, 28, and 22, respectively. The number of dams, UVs, calves, and UAs in the MP group were 30, 28, 30, and 17, respectively. MP, multiparous; PP, primiparous; UA, umbilical artery; UV, umbilical vein.

Table 3. Coefficient of correlation between UV or calf blood glucose and amino acid concentrations and those of dams in the PP group.

Variable		Dam 2–3 weeks before expected calving				Dam immediately after calving
		Glucose (mg/dl)	TAA (µmol/l)	TEAA (µmol/l)	TNEAA (µmol/l)	Glucose (mg/dl)	TAA (µmol/l)	TEAA (µmol/l)	TNEAA (µmol/l)
Calf birth weight (kg) 1		0.040	0.093	0.008	0.173	−0.114	−0.081	−0.017	−0.239
UV 2
	Glucose (mg/dl)	0.302	0.312	0.326	0.204	0.524 **	0.149	0.223	0.246
	TAA (µmol/l)	0.108	0.351	0.221	0.424 *	0.399 *	0.165	0.328	0.402 *
	TEAA (µmol/l)	−0.016	0.212	0.086	0.322	0.261	0.053	0.205	0.337 *
	TNEAA (µmol/l)	0.154	0.394	0.268	0.449 *	0.439 *	0.191	0.340	0.414 *
Calf 1
	Glucose (mg/dl)	0.545 **	0.363	0.261	0.373	0.316	0.362	0.282	0.364
	TAA (µmol/l)	0.069	0.227	0.048	0.382 *	0.314	0.020	0.213	0.320
	TEAA (µmol/l)	0.022	0.198	0.028	0.351	0.252	−0.022	0.172	0.301
	TNEAA (µmol/l)	0.085	0.232	0.054	0.383 *	0.329	0.066	0.234	0.318

^* P < 0.05; ^** P < 0.01. ¹ n = 28, ² n = 25. PP, primiparous; TAA, total amino acid; TEAA, total essential amino acid; TNEAA, total non-essential amino acid; UV, umbilical vein.

Table 4. Coefficient of correlation between UV or calf blood glucose and amino acid concentrations and those of dams in the MP group.

Variable		Dam 2–3 weeks before expected calving				Dam immediately after calving
		Glucose (mg/dl)	TAA (µmol/l)	TEAA (µmol/l)	TNEAA (µmol/l)	Glucose (mg/dl)	TAA (µmol/l)	TEAA (µmol/l)	TNEAA (µmol/l)
Calf birth weight (kg) 1		−0.211	0.150	0.108	0.147	−0.050	−0.060	−0.180	0.065
UV 2
	Glucose (mg/dl)	0.364	0.321	0.403 *	0.120	0.721 ***	0.124	0.277	0.006
	TAA (µmol/l)	−0.113	−0.105	−0.263	0.203	−0.056	0.374	0.211	0.274
	TEAA (µmol/l)	0.153	−0.132	−0.194	−0.072	0.316	0.376	0.289	0.262
	TNEAA (µmol/l)	−0.206	−0.058	−0.247	0.291	−0.187	0.365	0.162	0.297
Calf 1
	Glucose (mg/dl)	0.420 *	0.187	0.073	0.319	0.314	0.157	0.390	−0.046
	TAA (µmol/l)	−0.021	−0.180	−0.283	0.311	−0.078	0.334	0.222	0.318
	TEAA (µmol/l)	0.116	−0.084	−0.155	0.072	0.121	0.470	0.483	0.312
	TNEAA (µmol/l)	0.864 ***	−0.232	−0.288	0.345	−0.223	0.215	0.095	0.220

^* P < 0.05; ^*** P < 0.001. ¹ n = 30, ² n = 28. MP, multiparous; TAA, total amino acid; TEAA, total essential amino acid; TNEAA, total non-essential amino acid; UV, umbilical vein.

Discussion

In the present study, PP and MP total cotyledonary weight and surface area did not differ significantly. This contrasts with the results of Van Eetvelde et al. [26] and Kamal et al. [27], who indicated that MP dairy cows had a greater total cotyledonary weight and surface area than PP dairy cows. Furthermore, compared to MP cows reported by Kamal et al. [27], the MP cows in our study displayed significantly higher total milk production during pregnancy (7,333 kg vs. 6,043 kg) and lighter total cotyledonary weight (2.3 kg vs. 2.7 kg). During pregnancy, the mammary glands require more nutrients than the uterus [33]. Consequently, lactating cows may experience greater nutrient loss to the placenta and fetus owing to the metabolic priority of lactation [25]. This results in a lighter placenta and fetus during early pregnancy in lactating cows versus non-lactating cows [34]. Additionally, substantial tissue mobilization and nutrient utilization for milk production may reduce nutrient availability for placental and fetal growth in high-lactating cows compared to low-lactating cows [32, 35]. Therefore, the cows in the MP group had high nutrient requirements for the mammary glands during pregnancy, which may have resulted in inadequate placental development.

Differences in individual cotyledonary surface areas were observed. Cows in the MP group had larger cotyledons and fewer medium-sized cotyledons than those in the PP group. Rapid fetal growth occurs in late pregnancy [36,37,38]. Morphologically, the bovine placenta is fully developed by mid- to late pregnancy [39]; however, the microvilli of the cotyledon continue to grow by branching and rearranging until full term, increasing the cotyledonary surface area in response to fetal nutritional requirements [26, 39, 40]. Maternal underfeeding during early to mid-pregnancy in beef cows decreases placental and fetal weights [20, 41, 42]. However, restoring adequate nutrient levels in late pregnancy results in compensatory growth, with fetal weights similar to beef cows fed a nutritionally sufficient diet [19, 20, 41, 42]. This compensatory growth is associated with an increased placental vascular density [42] and individual placental surface area [20]. Increased placental vascularity is related to increased placental blood flow, which continues to increase with fetal growth by increasing the placental transport capacity [43]. In addition, studies on pregnant ewes have suggested that individual placental size may be an indicator of vascular function [44] and, ultimately, placental blood flow and nutrient exchange capacity [45]. In dairy cows, late pregnancy coincides with the dry period, halting the mammary glands’ demand for nutrients and directing more resources toward the placenta and fetus. Consequently, cows can meet elevated fetal nutrient requirements during this period by expanding their cotyledon size [43]. Mashimo et al. [32] suggested that nutrient availability to the placenta and fetus might decrease in high-lactating cows, which require more energy for lactation during early to mid-pregnancy. However, the enlargement of individual cotyledons during late pregnancy could potentially augment the nutrient supply to the fetus to levels similar to those in low-lactating cows. Similar to a previous study [32], this study identified a positive correlation between total milk production during pregnancy and the proportion of large cotyledons in the MP group. The enlargement of individual cotyledon size following lactation cessation in late pregnancy in the MP group may enhance placental vascular function and nutrient exchange between the dam and fetus as an adaptive response to high milk production during pregnancy. However, this study did not eliminate the effects of parity in the MP group. Yoon et al. [46] reported that milk production increased until parity. To the best of our knowledge, few studies have investigated the effects of parity on placental characteristics, including individual cotyledonary distribution, in MP dairy cows. Therefore, further studies are required to determine the effects of differences in milk production during pregnancy on placental development.

This study showed that for both the PP and MP groups, the blood glucose concentrations of dams appeared to be positively correlated with UV or calf concentrations. In contrast, the blood amino acid concentrations of dams did not correlate with UV or calf concentrations. Paolini et al. [47] noted that the rates of amino acid transport from dams to fetuses differed among ewes; isoleucine, valine, leucine, methionine, and phenylalanine were transported most rapidly, whereas tryptophan, threonine, histidine, and lysine were transported more slowly [47]. In this study, the rationale regarding the lack of a correlation between the blood amino acid concentrations of dams and UV or calf concentrations remains unclear. However, similar to the findings in ewes, different rates of amino acid transport from the dam to the fetus may have influenced these results. Total cotyledon size and UV glucose and amino acid levels were positively correlated in the PP group, supporting the hypothesis that placental size in livestock serves as an indicator of nutrient transfer to the fetus [45]. PP cows require nutrients for growth, thus inadequate dietary intake may adversely affect placental and fetal growth. Maternal undernutrition during gestation reduces the placental and fetal weights of livestock animals [6]. Moreover, maternal undernutrition in sheep decreases glucose and amino acid concentrations in the UV and fetus [16, 48]. The maternal nutrient status in PP cows can alter the nutrient supply to the fetus, emphasizing the importance of proper nutrient management during pregnancy to ensure optimal fetal growth. Conversely, total cotyledon size and UV glucose or amino acid concentrations were not correlated in the MP group. This could be attributed to the inclusion of cows with varying milk production levels during pregnancy in the MP group, resulting in diverse adaptive responses of the placenta to lactation.

Precalving BCS, gestation length, and calving difficulty differed between the groups, consistent with previous studies [27, 49, 50]. Pre-calving, cows in the PP group had higher glucose concentrations than those in the MP group. Shaffer et al. [51] and Brscic et al. [52] indicated that the difference in pre-calving blood glucose concentrations is related to the difference in metabolic rate and glucose activity, with PP cows having a higher metabolic rate and requiring an adequate amount of energy for continued growth. To our knowledge, this study is the first to investigate the glucose and amino acid supply from the dam to the fetus in PP and MP dairy cows. Although the concentrations of serum glucose, plasma total, total essential, and total non-essential amino acids in the dams immediately after calving were not significantly different, those in the UV, calf, and UA in the MP group were lower than those in the PP group. An increase in individual cotyledons as a compensatory response for milk production in the MP group did not restore the supply of glucose and amino acids to the fetus to the same level as that in the PP group. However, calf birth weight in the MP group was higher than in the PP group. Approximately 90% of the fetal weight is gained during late pregnancy [3], with the fetal growth rate averaging 300–400 g/day during the last month of pregnancy [53]. Consequently, calf birth weight and gestation length are correlated, as longer gestation periods lead to heavier calf birth weights [54, 55]. The present study revealed that the MP group had a longer gestation period than the PP group, thus potentially influencing calf birth weight. Moreover, insulin-like growth factors (IGFs) are crucial in skeletal muscle development in mammals [56]. IGF-1 regulates protein synthesis and inhibits protein degradation, contributing to muscle fiber hypertrophy during late pregnancy and postnatal growth [56]. Plasma IGF-I concentrations in beef calves at birth are positively correlated with calf birth weight [57], and average postnatal serum IGF-1 concentrations are positively correlated with both average daily gain and linear growth [58]. Moreover, fetuses/calves born to undernourished beef cows exhibit higher muscle mRNA levels of IGF-1 and its receptor [59, 60], suggesting greater nutrient availability for growth in these calves than those born to cows with adequate nutritional status. Therefore, the higher calf birth weight in the MP group may also be attributed to the elevated expression of calf muscle IGFs, facilitating efficient growth even with a limited nutrient supply from the dam. Further research is required to determine the growth traits of calves of both PP and MP cows.

In conclusion, blood glucose and amino acid levels in the UVs of PP cows were positively correlated with those of the dams and total cotyledon size, implying that maternal nutrient status can influence nutrient supply to the fetus. Moreover, MP cows had larger individual cotyledons than PP cows, suggesting an adaptive response to high milk production during pregnancy. However, the placental adaptive response in MP cows cannot restore the nutrient supply to the fetus to the same level as that in PP cows. Consequently, the nutritional management of MP cows must be reevaluated, considering milk production during pregnancy.

Conflict of interests

The authors declare that they have no conflict of interest.