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Milk fatty acid profiles of beef cows in response to a short feed restriction during lactation.

I. Casasús



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Milk fatty acid profiles of beef cows in response to a short feed restriction during lactation.
I. Casasús*, J. R. Bertolín, K. Orquera, J. Ferrer, M. Blanco. Ctr Invest y Tecnol Agroal Aragon (CITA), IA2 (CITA-Universidad de Zaragoza) Zaragoza, Spain.

The relationship between energy balance and the milk fatty acid (FA) profile is well established in dairy cows but has received little attention in beef cattle. We analyzed the milk fatty acid profile of 16 Parda de Montaña beef cows 2 mo post-calving in response to a 4-d (d) dietary restriction (55% of energy requirements, 6.2 kg dry matter (DM) hay/d), as compared with a previous basal and an 8-d refeeding period (100% of requirements; 7.0 kg DM/d hay + 2.7 kg DM/d concentrate). With d0 as the start of restriction, milk was sampled on days d-2 (basal), d1, d3 (restriction) and d5, d6, d8 (refeeding). Individual FA were identified by gas chromatography, and sums of FA were calculated (saturated (SFA), monounsaturated (MUFA), polyunsaturated (PUFA), cis-MUFA, trans-MUFA, C4-C15 de novo synthesis FA and C16-C24 mobilization FA). These sums and the 4 major FA (C16:0, C18:1—9c, C18:0, C14:0) were analyzed using mixed models, with day as fixed and cow as random effects. All the results presented here were significant at P < 0.001. The milk FA profile responded immediately to changes in the energy balance and/or the diet. On d1 of restriction, the concentrations of SFA decreased, mainly due to a reduction in the de novo synthesis FA and C16. A concomitant increase in MUFA (associated with that of C18:1—9c, predominant in body fat) was observed. These changes, along with the increments in C16-C24 FA, indicate an enhanced fat mobilization from the adipose tissue. During the restriction, C18:0 and trans-MUFA decreased while cis-MUFA and PUFA increased, as a result of both the mobilization and the change in diet composition. The opposite occurred in the refeeding phase. On d5, MUFA decreased (due to the reduction in C18:1—9c) and SFA increased because of the rise in the de novo synthesis FAs and C16:0, reflecting the reversion of fat mobilization. At the end of refeeding (d8), the individual FA returned to basal concentrations, but the sum of C16-C24 mobilization FAs was even lower and that of C4-C15 de novo synthesis FAs was higher than basal values, indicating a possible “rebound effect” after restriction and refeeding.

Keywords: beef cows, nutritional challenge, milk fatty acid profile.