Could bite composition of grazing dairy cows inform on animal performance, milk quality or enteric methane emission?

Grazing selection is a driver of the quality and quantity of ingested herbage and could thus be related to performance. During summer 2023, we compared the composition of selected herbage, estimated through simulated bites (SB), of two groups of 14 dairy cows grazing two paddocks differing for plant diversity level (DIV+/DIV-). We used ANCOVA and MANOVA to test the effect of the proportion of grass leaves/stems, legumes and forbs in the SB on dairy performance, milk fatty acid (FA) profile, urea excretion and enteric methane (CH4) emission. Grass leave SB proportion was higher in DIV- (65.9% vs 22.8%) while forbs’ one was higher in DIV+ (25.5% vs 6.5%). Within paddocks, differences in diet composition were driven by individual selection. In DIV-, milk yield (MY) was positively related to the SB proportion of legumes while protein content was negatively linked to the proportion of forbs. In DIV+, MY increased with grass stems proportion while fat and protein content decreased with grass stems and forbs proportions in SB. In DIV-, the level of saturated FA decreased with legumes proportion, while the level of polyunsaturated FA increased. Grass leave proportion in SB was determinant in DIV+, acting in the opposite way. Urea excretion in DIV- increased in milk and urine with legumes proportion in SB (reflecting SB higher nitrogen content), but decreased in DIV+ urine with grass leave proportion. The CH4 emissions did not differ between paddocks, even though DIV+ cows had a higher proportion of fibrous grass stems in SB which should have led to higher CH4 emissions. However, the high fibre content could have limited the dry matter intake. Furthermore, DIV+ cows also had a higher SB proportion of forbs, rich in tannins that may contribute to a potential mitigation effect. Within paddocks, the SB composition was not related to CH4 emissions. Further investigations are ongoing to explore if the spatio-temporal distribution of grazing activities recorded through sensors, can provide insights into SB composition, individual performance and environmental impacts