Forages from intensively managed and semi-natural grasslands in the diet of dairy cows

Abstract

Keywords : Intensively managed grass, semi-natural grasslands, forage species, dairy cows, in vivo digestibility, feed degradation, energy metabolism, milk production, ruminant nutrition, rumen fermentation, rumen kinetics, voluntary intake, feed evaluation This thesis focuses on the nutritional value of grass from intensively managed as well as semi-natural grasslands in diets fed to dairy cows. Aims were to explain why performance of dairy cows, fed intensively managed grass, is lower than expected based on their calculated energy intake, and to obtain knowledge on the nutritional value of forages from semi-natural grasslands if fed to dairy cows. In order to understand the reason for the overestimation of the performance of dairy cows, several feeding and respiration trials were collected. From the feeding trials it was concluded that there was a discrepancy between energy input in grass and concentrates and energy output in milk and maintenance. This was due to the composition of grass and diet, but also due to higher maintenance requirements on grass-based diets than currently assumed. In the respiration trials it was observed that maintenance requirements for dairy cows on grass-based diets should be increased by 10%. This increase was attributed to nitrogen excretion and energy required for digestion. The digestibility of forages from semi-natural grasslands is often low, due to a delayed harvesting date, and thus an advanced stage of maturity. However, though the ( in situ and in vitro ) degradation rate of mature grasses was low, the degradation rate of some dicotyledonous species appeared to be high. Also the intake of silage from semi-natural grasslands, with high proportions of dicotyledonous species was observed to be relatively high, especially when compared to the intake of silages from semi-natural grasslands mainly consisting of grasses in an advanced stage of maturity. The milk production of diets containing large proportions of semi-natural silages was low compared to diets containing mostly intensively managed grass. In vivo digestibility of forages from semi-natural grasslands was approximately similar to the in vitro digestibility. Digestibility could not be estimated based on chemical composition. Rumen fermentation of semi-natural forages did not deviate from expectations based on the chemical composition of the diet. However, a study of the rumen kinetics showed that diets containing large proportions of dicotyledonous species had a higher intake rate and passage rate than expected, and diets containing large proportions of mature grasses had a faster particle size reduction than expected. Also on forages from semi-natural grasslands, maintenance requirements should be increased, due to required energy for nitrogen excretion and for chewing and rumination. For intensively managed grass as well as for forages from semi-natural grasslands, a correction of the energy value of grass, by correcting for the surplus of protein per kg grass, is suggested, together with an increase of maintenance requirements by 10% for digestion and rumination. Including forages from semi-natural grasslands in diets of dairy cows is possible, especially if the forages are fed in small amounts. Replacement of intensively managed grass by forages from semi-natural grasslands until a maximum of 30% seems to offer best possibilities.</p