Feed efficiency (FE) is a ratio of intake to performance. More efficient animals are either those with higher production for the same dry matter intake (DMI) or those with lower DMI for the same production. Preceding studies suggest that more efficient lactating ewes seem to be those with higher milk yield, which has been associated to loss of rusticity. Therefore, we questioned whether selection for higher FE might negatively affect resilience. Resilience is understood in this context as the ability of an animal to revert quickly to previous performance after a perturbation. This study was conducted to examine the relationship between FE and resilience in high-yielding dairy ewes phenotypically divergent in FE. First, to estimate FE, we used 40 Assaf ewes fed a TMR ad libitum, and measured individual DMI and milk yield daily over 3 weeks. Feed efficiency was calculated as the difference between the actual and predicted intake estimated through net energy requirements for maintenance, production and weight change. Then, higher (H-FE; n = 9) and lower (L-FE; n = 9) efficient ewes were subjected to an acute nutritional challenge by withdrawing the TMR and feeding them only wheat straw for 3 days. After this challenge, all animals were fed again the TMR ad libitum. The milk yield profile was described using a piecewise model with 5 parameters: V1, a constant representing the pre-challenge stage; V2, the linear slope of the response during the 3-d challenge; V3 and V4, the linear and quadratic components of the recovery period (post-challenge); and V5, a constant representing the stabilization period (post-challenge), which started on day 5 of refeeding and lasted until day 10. Orthogonal contrasts were used to test differences between pre-challenge and stabilization periods. As expected, milk yield during the pre-challenge period was greater in more efficient (H-FE) than in less efficient (L-FE) ewes (V1; P 0.10). The drop (V2) caused by the challenge was also stronger in H-FE (P 0.10). In the stabilization period, milk yield (V5) was again higher in H-FE (P < 0.001). However, pre-challenge values (V1) were not fully recovered in either H-FE or L-FE (0.05 < P < 0.10 for V1 vs V5), which might be explained by a persistent detrimental effect of the challenge, at least in the short-term. Overall, the temporal pattern of variation in milk yield seem to be linked to the pre-challenge milk yield level and not to the FE. In any case, results suggest that improving FE would not detrimentally affect resilience.

Trabajo presentado al: 11th International Symposium on the Nutrition of Herbivores (ISNH), P57. Florianópolis (Brasil). 4-8 de junio.

Projects PID2020-113441RB-I00 (MCIN/AEI/Spain) and SMARTER (H2020#772787/European Union), and grant PRE2021-098235 (MCIN/ AEI/Spain).

Peer reviewed