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

## TL;DR

The study found that milk fatty acid profiles in dairy sheep changed significantly during underfeeding, but these changes did not reliably distinguish between more and less feed-efficient animals.

## Contribution

The study demonstrates that underfeeding induces significant changes in milk fatty acid profiles, though these may not be useful for identifying feed-efficient sheep.

## Key findings

- Underfeeding caused a strong reduction in short- and medium-chain fatty acids in milk.
- Limited differences were observed in fatty acid profiles between high- and low-feed-efficient sheep.
- Changes in milk fatty acids suggest a shift from de novo synthesis to uptake of preformed fatty acids during underfeeding.

## Abstract

This study tested the hypothesis that dairy Assaf sheep with divergent feed efficiency (FE) would respond differently to a nutritional challenge, and this different response would be reflected in their milk fatty acid (FA) composition. Therefore, changes in FA profile may serve as an indicator of FE. To explore this, nine high- and nine low-FE lactating ewes were subjected to a nutritional challenge consisting of feeding them with only wheat straw for 3 days. This feed restriction led to a strong response in milk FA profile (especially, a marked reduction in short- and medium-chain FAs of de novo synthesis). However, limited differences were observed between FE groups, which suggests that, in the current scenario, milk FA composition might not be a suitable marker for clustering more and less feed-efficient dairy ewes. Nonetheless, changes in the milk FA profile due to underfeeding highlight its value for exploring the response to nutritional stress.

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.