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.
Keywords: grazing behaviour; high-sugar grass; milk urea; nitrogen use efficiency; pasture management; perennial ryegrass.