Review: Role of dietary carbohydrates in the metabolic and hormonal control of nutrient partitioning in lactating dairy sheep and dairy goats.

Sheep and goats are both small ruminants but differ to some extent in their feeding behaviour and selectivity, milk production, nutrient utilisation, and metabolism. Several studies have classified goats as intermediate feeders and sheep as grass and roughage eaters. Sheep, in general, are less productive and have shorter lactation periods. They also produce milk with higher fat and protein concentrations than goats. Moreover, in sheep, high-starch diets (above 20% of starch, DM basis) promote milk production in early lactation and body fat deposition in mid-lactation. Dairy goats, however, appear to benefit from high-starch diets throughout lactation. These differences may result from species-specific hormone levels involved in energy partitioning. This review explores how dietary carbohydrates affect the metabolic and hormonal control of nutrient partitioning in lactating dairy sheep and dairy goats, and the biological reasons underlying differences in carbohydrate utilisation, focusing on starch and highly digestible fibre. Overall, it appears that specialised dairy goats have a hormonal status (high growth hormone and low insulin concentrations) that promotes energy partitioning towards milk production. In contrast, dairy sheep exhibit low growth hormone and high insulin levels. This hormonal profile promotes the partitioning of dietary energy towards body reserve accumulation, especially during mid and late lactation. The findings suggest that to support milk production, dietary starch levels should be reduced in dairy sheep during mid-lactation by incorporating highly digestible fibre sources. In dairy goats, however, it is important to maintain a high-starch diet even during mid-lactation. The metabolic evidence discussed in this review is primarily based on studies conducted on Saanen goats and Sarda sheep under Mediterranean conditions, whereas the milk production evidence is also available for East Friesian and Assaf dairy sheep and for Murciano-Granadina and Sarda dairy goats. Thus, further studies should be considered to confirm the general applicability of these findings and the mechanism suggested in different sheep and goat breeds.