405 Supplementation of high-producing dairy cows with a standardised rumen-protected grape extract supports milk production in early lactation during heat stress conditions.

Abstract With the intensification of global warming come negative consequences for animal production, the most visible one being heat stress, which is the struggle of an animal to evacuate the excess heat due to high environmental temperature and/or humidity. This leads to physiological stress and negative impacts on health and production. When combined with other intrinsic stressors such as the transition period in peri-parturient cows, it can be detrimental to milk production. The aim of the present study was to evaluate the potential for a standardised rumen-protected grape extract (RPGE, Nor-Grape® BP-O, Nor-Feed, France) to alleviate the burden of heat stress in dairy cows during the start of lactation. 30 high-producing Prim-Holstein cows from a commercial dairy farm in Western France were selected, and allocated, in the same barn, to a control group (CTL, n=15) or a supplemented group (BPO, n=15, receiving the same diet with an extra 500mg RPGE/head/day through an automatic dispenser located at the milking robot), to have balanced groups. The study took place over a summer period of 66 days and lasted from lactation D1 to D40 for every cow enrolled. Cows’ milk production was recorded daily using the data from the two milking robots of the farm. Temperature and humidity data were recorded using 2 dataloggers placed at cows’ level in 2 locations of the barn and computed to calculate the daily THI value throughout the trial. Average milk output of the first 20 days of lactation and 40 days of lactation, as well as their fat and protein content were compared between groups using Welsh t-test. Overall, BPO cows’ milk output tended to be significantly higher than CTL ones, both on L0-20 (CTL: 33.7kg vs. BPO: 37.5kg, p=0,096) and on L0-40 (CTL: 40.8kg vs. BPO: 44.7kg, p=0.097). No difference was observed in terms of fat or protein content of the milk between groups at both timepoints of the study. THI calculations showed that the intensity of heat stress was overall moderate, with only 13 days of ‘moderate heat stress’ (72< THI< 78) and 4 days of ‘intense heat stress’ (THI >78). The milk production of BPO cows was significantly higher than CTL ones during moderate heat stress days (42.3kg vs. 37.7kg respectively, p< 0.001) and tended to also be higher during intense heat stress days (41.9kg vs. 38.1kg, respectively, p=0.059). These results confirm previous findings evidencing that RGPE supported the metabolism of dairy cows during heat stress events. The results show that efficient dietary intervention with RPGE to support dairy cows in their early lactation helps them face heat stress challenges and results in higher milk output when heat stress intensifies, thus benefiting for the farm productivity, even in challenging conditions.