Abstract

Simple Summary Improving the performance of grazing systems and reducing their environmental impact are crucial in the current and future world contexts. In temperate grazing dairy systems, perennial ryegrass is widely used. Its nutritional composition can by modified via cultivar selection and management. We tested two swards: (1) a high-sugar cultivar submitted to fertilisation and defoliation regimes aimed at increasing sugar content and sugar-to-protein ratio and (2) a standard cultivar submitted to fertilisation and defoliation regimes aimed at decreasing sugar content and sugar-to-protein ratio. We tested these two swards with mid-lactation dairy cows grazing in daily strips for nine days in autumn and measured the amount of forage offered and consumed, the milk production and composition, and the grazing behaviour between morning and afternoon milkings. We found that the amount of residual herbage was greater in the standard cultivar sward, but the herbage consumed per cow was similar. Cows spent less time grazing in the high-sugar sward, but no difference was observed in the rumination time. Milk production and composition were similar between the two groups. This could imply that good-quality pastures would require a greater difference in nutritional composition to have an impact on animal performance. Abstract Grazing ruminant systems can be sustainably intensified by improving efficiency while reducing their environmental impact. The objective of the present study was to examine the potential of pastures differing in water-soluble carbohydrates (WSC) and crude protein (CP) contents to affect milk production and composition as well as the behaviour of cows grazing perennial ryegrass (PRG) swards. By modifying the nitrogen (N) fertilisation rate (83 and 250 kg/ha per year) and the defoliation frequency (two or three leaves per tiller) in combination with cultivar selection (high-sugar vs. standard cultivars), we obtained two swards differing in WSC and CP contents. The two contrasting swards were each grazed by six dairy cows in nine daily strips in autumn. Pasture samples were collected to determine herbage mass and quality. Cow behaviour was recorded by direct observation. Herbage offered and apparently consumed were similar between swards (averaging 37.3 and 18.2 kg/cow, respectively), although the residual was lower in the high-sugar sward (1735 vs. 2143 kg/ha). Cows spent less time grazing in the high-sugar sward (66.9% v. 71.6%), but the rumination times was similar (14.6%). Milk production and composition were similar between groups, suggesting that high-quality pastures would require a greater difference in nutritional composition to affect animal performance.