Is the grass always greener? Foraging decisions of dairy cattle (Bos taurus) in a free-choice pasture system

Background/Question/Methods

Energetic trade-offs and variety in forage types may modify large herbivore movements and foraging decisions at a habitat level. Using dairy cows (Bos taurus) as a large herbivore model, we tested whether grazing sequences of consistent forage species or alternating forage species predicted differences in patch visitation frequency (PVF) and patch residence time (PRT) across spatially distinct pastures. We fitted 12 animals with GPS collars, allowing them to graze either consistent or alternating patch sequences in a voluntary grazing and milking system. We predicted that 1) cows would increase PVF and decrease PRT in heterogeneous grazing sequences, but 2) would decrease PVF and increase PRT as distance between pastures increased. Milk yield (MY) and voluntary milking frequency (MF) were also measured as response variables. Analysis of variance and path analysis of standardized regression coefficients (r) were conducted for the relationships among distance, PVF, PRT, MF and MY.

Results/Conclusions

Contrary to our expectation, grazing sequences of alternating forage species had no significant effects on behavior or milk variables (P ≥ 0.42). Consistent with our prediction, PVF decreased with distance, while PRT increased. Our path analysis detected several significant covariate relationships between distance, behavior variables, and milk variables. Increased distance between patches resulted in decreased PVF (r=-0.20; P < 0.001) and increased PRT (r=0.16; P < 0.001). By altering PVF and PRT, distance also indirectly impacted MF and MY. Increases in PVF significantly increased both MF (r=0.36; P < 0.001) and MY (r=0.22; P < 0.001), but increases in PRT significantly reduced MF (r=-0.25; P < 0.001) and increased MY (r=0.16; P < 0.05). Although a strong behavioral trade-off existed between PVF and PRT (r=-0.79; P < 0.001), we conclude that both variables positively influenced MY through changes in MF and/or potential increases in forage intake. These findings suggest that on a habitat scale, spatial variability of forage resources may play a greater role in influencing foraging decisions than heterogeneity in forage resources.