Our study objective was to determine the effect of a short feed restriction (4 d) and subsequent refeeding (4 d) on the performance and metabolism of beef cows with a different nutritional status by particularly focusing on their milk fatty acid (FA) profile, to consider its potential use as biomarker of metabolic status. Thirty-two Parda de Montaña multiparous lactating beef cows were individually fed a diet based on the average cow's net energy (NE) and metabolizable protein requirements. At 58 d in milk (DIM, day 0), cows underwent a 4 d feed restriction (55% requirements, restriction period). Before and after the restriction, diets met 100% of their requirements (basal and refeeding periods). Cow performance, milk yield and composition, and plasma metabolites, were determined on day -2, 1, 3, 5, 6, and 8. Cows were classified into two status clusters according to their pre-challenge performance and energy balance (EB) (Balanced vs. Imbalanced). All traits were statistically analyzed considering the fixed effect of status cluster and feeding period or day, with cow as a random effect. Imbalanced cows were heavier and had a more negative EB (P < 0.001), but similar milk yield, milk composition, and circulating metabolites (except for greater urea) than Balanced cows (P > 0.10). Milk contents of C18:1 cis-9, monounsaturated FA (MUFA), and mobilization FA were greater (P < 0.05), whereas saturated FA (SFA) and de novo FA were lesser in Imbalanced than Balanced cows (P < 0.05). Restriction decreased body weight (BW), milk yield, and milk protein compared to the basal period, but increased milk urea and plasma nonesterified fatty acids (NEFA) (P < 0.001). Milk contents of SFA, de novo, and mixed FA decreased immediately during the restriction, while MUFA, polyunsaturated FA and mobilization FA increased (P < 0.001). Basal milk FA contents were recovered on day 2 of refeeding, and all their changes strongly correlated with differences in EB and NEFA (P < 0.05). The general lack of interactions between status clusters and feeding periods implied that the response mechanisms to diet changes did not differ between cows with a different pre-challenge nutritional status.
Keywords: beef cows; induced feed restriction; metabolites; milk fatty acid profile; refeeding.
Lactating cows can undergo periods with a negative energy balance due to feed shortages, which trigger metabolic adaptations to support cow maintenance and milk yield. We explored beef cows’ response to a short feed restriction (4 d, 55% of their energy and protein requirements) and subsequent refeeding (4 d, 100% of their energy and protein requirements) in the second month of lactation. We analyzed the effect on their performance and metabolism by placing special emphasis on milk production and milk fatty acid composition in two beef cow groups with a different nutritional status before the challenge. When cows faced a food restriction, both groups had similar changes in productive and metabolic traits. These changes are similar to those occurring in restricted dairy cows, but of lesser magnitude due to the lower milk yield and associated metabolic load of beef cows. The milk fatty acid profile, rarely analyzed in beef cows, proved to be an accurate indicator of their metabolic status.
© The Author(s) 2023. Published by Oxford University Press on behalf of the American Society of Animal Science.