Exploration of robustness indicators using adaptive responses to short-term feed restriction in suckling primiparous beef cows.

Animal robustness is a complex trait of importance for livestock production systems and genetic selection. Phenotyping is essential for evaluation of the adaptation of different genotypes to changing environments. This study tested an experimental framework to induce marked deviations in the adaptive responses of suckling beef cows and to identify relevant indicators of responses to characterise individual differences in the robustness of cows. The production and metabolic responses of primiparous suckling Charolais cows to two periods of feed restriction (FR, 50% of their net energy requirements) of different durations were monitored. After calving, 13 cows (aged 39 ± 2 months, BW of 680 ± 42 kg at calving) had ad libitum access to a diet composed of hay and supplemented with concentrate to meet their energy and protein requirements. Starting at 54 ± 6 days postcalving, the cows underwent two periods of FR: 4 days of FR (FR4), which was followed by 17 days of ad libitum intake to study the recovery from FR4, and 10 days of FR (FR10), which was followed by 18 days of ad libitum intake to study the recovery from FR10. The milk yield (MY), BW, body condition score and plasma non-esterified fatty acid (NEFA), β-hydroxybutyrate, glucose and urea concentrations were measured before, during and after each FR. Among all measured variables, the MY and NEFA concentrations showed the most significant changes in response to FR. A functional data analysis approach was applied to the MY and NEFA data to model the adaptive responses and extract quantifiable indicators of deviation and recovery. Linear correlations (P < 0.03-0.07) between FR4 and FR10 were found for some indicators describing MY and NEFA levels before and after FR. The overall repeatability of MY and NEFA responses between both FR accounted for 46% based on quartile analysis performed on average responses. Moreover, the variance in both the MY and NEFA variables did not differ significantly between FR4 and FR10, despite a trend for higher variances in FR10. Altogether, (1) the calculated variables derived from the functional data analysis of the time patterns of the MY and NEFA accounted for the differences in the cow responses to FR, and (2) the animal responses appeared to show concordance between FR4 and FR10. In conclusion, short-term FR is a relevant framework for studying productive and metabolic adaptive responses in suckling cows and allows the identification of potential robustness indicators.