Perennial ryegrass for dairy cows: Intake, milk production and nitrogen utilization

Keywords: perennial ryegrass, dairy cows, intake, digestibility milk production, nitrogen utilisation.In the Netherlands, grass is one of the main roughages in the diet of high productive dairy cows. Grass is associated with two main problems: the limited dry matter intake (DMI) and the low N utilisation by dairy cows. Grassland is renovated every 3- 15 years and is usually resown with a mixture of mainly perennial ryegrass cultivars. The aim of the research described in this thesis was to evaluate the effects of perennial ryegrass cultivars on 1) degradation of herbage in the rumen of dairy cows, 2) the DMI, digestibility and milk production by dairy cows, and 3) the N utilisation by dairy cows. These effects were then related to sward characteristics and chemical composition of these cultivars, in order to identify new selection criteria that may be used in grass breeding programmes. Therefore, the effects of eight cultivars in a stall feeding experiment and four cultivars in a grazing experiment on intake, grazing behaviour, digestibility, milk production, milk composition and N utilisation were determined. The perennial ryegrass cultivars consistently differed in their water soluble carbohydrate (WSC) content. Among cultivars, the largest differences were found in WSC content, whereas the differences in crude protein and neutral detergent fibre content were small. An increased WSC content was expected to increase the palatability, and this may increase the intake. However, there was no effect of cultivar on intake in three of the four years of experiments. In one year, the lower intake of two cultivars was associated with a lower WSC content, but also with a severe crown rust infestation. However, the grazing behaviour did not differ among cultivars. The milk production and milk composition were almost not affected by cultivar, with the exception of the year with differences in intake. The cultivars differed only slightly in the degradation characteristics in the rumen. The high crude protein content and the relatively high fractional degradation rate, in comparison with organic matter and neutral detergent fibre, resulted in a high supply of N to rumen microbes. This was expected to be balanced by a higher supply of energy from an increased WSC content. However, the milk urea N content and the N utilisation by dairy cows did almost not differ among cultivars.