Dairy cows with different milk urea nitrogen breeding values display different grazing behaviours

Abstract

The objective of this study was to describe the diurnal pattern of grazing behaviour of multi-parous dairy cows divergent for milk urea N breeding values (MUNBV) and the consistency of these differences across different sward compositions (perennial ryegrass [RG; Lolium perenne L.] or ryegrass with plantain [Plantago lanceolata L.; RGPL]) and stages of lactation (early and late lactation). Jaw movement recorders were fitted to 24 animals in early lactation and 16 cows in late lactation, in early lactation 12 cows were assigned to the ryegrass diet and 12 cows assigned to the plantain diet, in late lactation 8 cows were assigned to ryegrass with the remaining 8 cows assigned to the plantain diet. For each diet half of the animals during both stages of lactation were classified as high for MUNBV and the other half as low for MUNBV. Low MUNBV animals had more mastications per bite over the day (P < 0.01) with a one-unit decrease in MUNBV resulting in 0.07 more bites per mastication during the first two grazing bouts, as well as differences in the temporal distribution of grazing bouts compared with high MUNBV cows. A one-unit decrease in MUNBV resulted in a 0.11 ± 0.02 increase in mastications per bite during the first grazing bout across both stages of lactation and sward composition. Ingestive behaviour has a large impact on the physical features of ingesta and thereby rumen function. The results of this study indicate that dairy cows divergent for MUNBV grazing the same forage apply different grazing strategies in terms of oral processing of ingesta and diurnal meal pattern. These results present potential explanatory variables for phenotypical differences observed in dairy cows divergent for MUNBV.