Milk Fatty Acid Profile in Response to Acute Underfeeding in Dairy Sheep Divergent for Feed Efficiency.

Milk fatty acid (FA) composition derives from de novo synthesis in the mammary gland and from the uptake of preformed FA derived from diet, ruminal biohydrogenation, and body tissue mobilization. Consequently, milk FA profiles have been related to nutritional stress responses and feed efficiency (FE). This study tested whether dairy sheep with divergent FE would show different milk FA responses to a nutritional challenge. After calculating two FE indices in 40 ewes, the 9 most efficient (H-FE) and 9 least efficient (L-FE) animals were selected and subjected to a 3-day challenge replacing their total mixed ration (TMR) with wheat straw, followed by TMR refeeding. Temporal patterns of milk FA variation showed only minor differences between H-FE and L-FE ewes. The L-FE group exhibited higher concentrations of iso- and anteiso-17:0 and 18:0, suggesting differences in rumen fermentation and biohydrogenation. In contrast, underfeeding induced marked changes in milk FA composition, including a strong reduction in short- and medium-chain FAs (≤C16), consistent with a shift from de novo synthesis toward increased uptake of preformed FA. Overall, limited differences suggest that milk FA composition may not be a suitable marker for FE. Nonetheless, underfeeding-induced changes highlight its value for exploring nutritional stress responses.