Dietary nonstructural carbohydrates influence synthesis of mare milk carbohydrates and yield.

Mare milk is the primary source of nutrients for neonatal foals and contains various carbohydrates. Milk oligosaccharides are complex carbohydrates that are resistant to enzymatic digestion and provide support for the immune system and developing microbiome in the neonate. However, factors influencing the synthesis of milk carbohydrates in the mare are not well understood. This study aimed to determine the effects of dietary nonstructural carbohydrate (NSC) on mare milk composition and yield. Thoroughbred mares were paired by age and last breeding date then randomly assigned to one of two dietary treatments at 319 d of gestation: a high NSC concentrate (43.9% NSC; n = 9; HC) or a low NSC concentrate (17.8% NSC; n = 7; LC). The mares were fed the concentrate feeds in addition to forage. Colostrum and milk samples were collected at the following time points postpartum: at parturition, 12 h, 3 d, 7 d, 14 d, and 21 d. Total milk carbohydrates (lactose and oligosaccharides) were analyzed using an infrared-based milk analyzer (MilkoScan FT+, Foss, Denmark). Milk oligosaccharides were separately quantified following defatting and deproteinization with chloroform and methanol, purification by solid phase extraction, and correction for residual lactose. Daily milk yield was measured at 6 and 7 d postpartum. Mixed model ANOVA with repeated measures was used to evaluate the effect of diet and time on milk components and total milk yield. Mares fed the HC concentrate tended to have higher total milk carbohydrates than mares fed the LC concentrate (P = 0.059). There was no main effect of diet on milk oligosaccharides (P = 0.34); however, this study was the first to report a change over time in total oligosaccharide concentrations in mare milk, which were highest after foaling and decreased over time (P = 0.002). Daily milk yield, on a body weight (BW) basis, was higher in mares fed the HC concentrate (2.30% of BW) compared with the mares fed the LC concentrate (2.04% of BW; P = 0.035). These results demonstrate the importance of the maternal diet to milk composition and production, and ultimately, the nutrients provided to the foal.