Resilience to acute underfeeding in dairy sheep diverging in feed efficiency: 2) Blood metabolites

Abstract

In the current scenario of climate change and economic instability, it can be expected that livestock will increasingly face nutrition-related challenges. Resilience (understood as the ability of an animal to revert quickly to high production and health status in response to a perturbation) would therefore be of great importance. However, animal breeding is still focused on higher production or, at best, higher feed efficiency (FE). Yet, it is uncertain whether improving FE could detrimentally affect resilience, particularly in small dairy ruminants. This experiment was carried out to examine the relationship between resilience and FE; namely, to compare the variation in some blood metabolites in dairy ewes phenotypically divergent for FE and subjected to an acute nutritional challenge. First, to estimate FE, we used 40 Assaf ewes fed a TMR ad libitum, and measured individual dry matter intake 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, after selecting two groups of FE ewes: higher (H-FE; n = 9) and lower (L-FE; n = 9) efficient ewes, they were subjected to a nutritional challenge. The trial consisted of three periods: the pre-challenge (when animals were fed TMR ad libitum), the challenge (lasting for 3 days, when the TMR was withdrawn and the ewes were only fed wheat straw) and the post-challenge (lasting for 10 days, when animals were fed again TMR ad libitum). At the end of each period, blood samples were collected (before milking and feed administration) and the serum was analyzed for glucose, non-esterified fatty acids (NEFA) and b-hydroxybutyrate (BHB) concentrations during the trial. The data were subjected to repeated measurements analysis, with animals nested within the group. The results showed that glucose levels tended to be marginally greater in H-FE than L-FE (P = 0.058), and dropped significantly (P < 0.001) due to the underfeeding. Surprisingly, pre-challenge levels were not recovered on day 10 post-challenge. No differences between groups were detected for NEFA concentrations, which sharply increased during the straw challenge and showed the lowest values in the post-challenge period (P < 0.001). Regarding BHB, higher levels due to feed deprivation were only observed in the H-FE group (Group Period interaction, P = 0.022). This could derive from a greater tissue mobilization and might have prevented a stronger drop in glycaemia in these more efficient ewes. Results on blood serum metabolites may suggest that selection for high FE dairy ewes would not negatively influence their resilience, as more (H-FE) and less (L-FE) efficient ewes responded to and recovered from the acute nutritional challenge similarly