Maternal Parity, Age, and Successive Pregnancies: Implications for Colostrum, Milk Production, and Foal Growth in Mares

The Developmental Origins of Health and Disease (DOHaD) framework emphasizes that prenatal and early postnatal environments have long-term consequences on offspring's health. In domestic species, including pigs, sheep, cattle, and humans, maternal parity and age are well-recognized determinants of fetal growth. Horses are no exception. It is well established that first foals are lighter and smaller than subsequent foals^1,2. 

However, for practicing veterinarians, clinical relevance extends beyond birthweight. Maternal age, parity, and nursing status at conception influence colostrum quality, milk composition, milk yield, and ultimately foal growth and resilience during early life. Understanding these interactions allows veterinarians to anticipate risks, guide breeding decisions, and optimize neonatal management. 

Parity, Age, and Foal Growth: Not Just a Birthweight Issue 

Parity in horses is closely correlated with mare age, yet both factors exert independent effects on foal development. Studies separating these variables show that foals born to primiparous, very young, or very old mares remain lighter and smaller than those born to middle-aged multiparous mares, at least until weaning. Importantly, this lack of catch-up growth suggests that postnatal nutrition, particularly milk intake, is compromised^1,3. 

Foal average daily gain (ADG) is strongly correlated with milk intake during the first two months of life. Therefore, lighter foals from primiparous or aged mares are not simply constitutionally small; they are likely receiving less effective nutritional support during early lactation¹. 

Colostrum Composition: Immunity and Fatty Acids 

Immunoglobulins and Passive Transfer 

Colostrum is the foal’s sole source of passive immunity, rich in immunoglobulins, especially IgG . Colostrum typically contains 15–18% protein, while fat and lactose range between 2–4% ^1,4. 

Interestingly, colostrum IgG concentration was higher in primiparous mares than in multiparous mares in the present work, though this contrasts with other studies showing no parity effect. Moreover, parity does not consistently influence the efficiency of IgG absorption in foals. A large epidemiological study in Paso Fino foals reported a lower risk of total failure of passive transfer in foals born to multiparous mares, highlighting the complexity of this relationship¹. 

Clinical takeaway: Veterinarians should not assume superior colostral immunity based solely on parity. Colostrum testing remains essential, especially in primiparous mares, despite occasionally higher IgG concentrations.  

Effects of Age, Parity, and Nursing on Colostrum Fatty Acids 

Maternal age influences colostrum fatty acid (FA) profiles, though findings remain inconsistent. Older Konik mares (>10 years) showed increased PUFA, whereas the present study observed decreased PUFA but increased SFA in older mares. Similar controversies exist in human studies^1,5. 

Parity effects on colostrum FA were minimal and inconsistent with previous findings, likely due to confounding by age, breed, nutrition, and group composition. Notably, mares nursing at conception showed an increased proportion of MUFA, and both young and nursing mares had higher PUFA levels¹. 

In humans, increased long-chain PUFA in colostrum is linked to improved neurodevelopment. Whether similar benefits exist for foals remains unknown, but represents an important research avenue¹. 

Milk Composition During Lactation 

Fatty Acid Profiles 

Milk FA composition varied significantly with parity and, to a lesser extent, age. Primiparous mares produced milk with higher FA concentrations on day 3 (D3) and day 30 (D30). Early lactation milk showed increased long-chain PUFA, while later milk was richer in medium and long-chain SFA and MUFA¹. 

These findings mirror cattle studies where first-lactation milk is richer in unsaturated FA, though human milk appears unaffected by parity^1,6. Mares with very high parity (>5) have previously been shown to produce milk richer in short- and medium-chain SFA and poorer in UFA. 

Age effects were subtle. Young mares produced more short-chain SFA early in lactation (D3), whereas older mares produced milk richer in SFA later (D30 and D90), with a lower PUFA/SFA ratio at D90¹. 

Mechanisms: Energy Balance and Fat Mobilization 

Milk FA originate from dietary intake, de novo synthesis, and body fat mobilization. Since all mares were managed identically, dietary differences cannot explain the observed variations. Instead, differences in energy balance and fat mobilization likely play a role. 

Young and primiparous mares may experience a greater negative energy balance, leading to increased mobilization of body fat and altered milk FA profiles. Similar mechanisms are well documented in ruminants during early lactation¹. 

Milk Yield (M2) and Mammary Gland Integrity 

Milk yield was consistently lower in primiparous mares, regardless of age. Age alone did not significantly affect milk production, aligning with previous studies showing peak production around 7 years¹. 

A key clinical insight is the impact of successive pregnancies. Mares nursing at conception produced less milk at D90 and D180, suggesting that overlapping lactation and gestation may impair mammary capacity. 

The observed negative relationship between Na⁺:K⁺ ratio and milk yield mirrors findings in goats and may reflect compromised mammary epithelial integrity. Although weak, this association suggests that nursing mares with successive pregnancies may experience delayed mammary recovery, particularly in the early postpartum period¹. 

Energy Content of Milk and Foal Growth 

Milk energy content was higher in primiparous and older mares, particularly on D3. In primiparous mares, this increase was driven by higher FA concentrations, suggesting a compensatory mechanism for lower milk volume. 

However, this compensation was insufficient. Foals born to primiparous mares had lower ADG between D3 and D30, consistent with previous reports that these foals fail to catch up postnatally ^1,3. 

In older multiparous mares, increased milk energy, primarily due to higher protein content, did not translate into improved foal growth. This reinforces the concept that milk volume, not just composition, is critical for optimal foal development. 

Practical Implications for Veterinary Practice 

• Primiparous mares: Expect lower milk yield despite higher milk energy; closely monitor foal growth and consider early nutritional support. 

• Nursing at conception: Be cautious with breeding decisions; successive pregnancies may reduce lactational performance. 

• Older mares: Higher milk energy does not guarantee better foal growth; routine weight monitoring remains essential. 

• Colostrum management: Always assess quality directly, parity and age alone are unreliable predictors. 

Conclusion 

Maternal parity, age, and nursing status exert distinct and clinically meaningful effects on colostrum composition, milk yield, and foal growth. While primiparous mares may partially compensate through richer milk, reduced milk volume remains a limiting factor for foal development. Successive pregnancies appear to compromise lactational capacity, underscoring the need for strategic breeding and proactive neonatal management. 

For veterinarians, these findings highlight the importance of individualized mare-foal monitoring, rather than relying on generalized assumptions about age or parity. 

 References 

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2. Zotti E, Resmini FA, Schutz LG, Volz N, Milani RP, Bridi AM, Alfieri AA, Silva CA. Impact of piglet birthweight and sow parity on mortality rates, growth performance, and carcass traits in pigs. Revista Brasileira de Zootecnia. 2017 Nov;46(11):856-62.  https://www.scielo.br/j/rbz/a/p9dF7Jmp3rQpBQDKyHrkH8m/?format=pdf&lang=en 

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4. Barreto ÍM, Urbano SA, Oliveira CA, Macêdo CS, Borba LH, Chags BM, Rangel AH. Chemical composition and lipid profile of mare colostrum and milk of the quarter horse breed. PLoS One. 2020 Sep 14;15(9):e0238921. https://journals.plos.org/plosone/article/file?id=10.1371/journal.pone.0238921&type=printable 

5. Sinanoglou VJ, Cavouras D, Boutsikou T, Briana DD, Lantzouraki DZ, Paliatsiou S, Volaki P, Bratakos S, Malamitsi-Puchner A, Zoumpoulakis P. Factors affecting human colostrum fatty acid profile: A case study. PLoS One. 2017 Apr 14;12(4):e0175817. https://journals.plos.org/plosone/article/file?id=10.1371/journal.pone.0175817&type=printable 

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