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For estimating herbage intake during grazing, the traditional sward cutting technique was compared in grazing experiments in 2002 and 2003 with the recently developed n-alkanes technique and with the net energy method. The first method estimates herbage intake by the difference between the herbage mass before and after grazing and the regrowth between the 2 points in time. The second technique estimates herbage intake by the ratio of a dosed even-chain synthetic n-alkane (C32) and a naturally occurring odd-chain n-alkane (C31 or C33) in the herbage and feces. The third technique calculated the intake from the animal's energy requirements for milk production and maintenance. The sward cutting technique estimated herbage intake with the highest coefficient of variation and had different results in the 2 experimental years. The n-alkanes method yielded less variable results, whereas the net energy method gave the least variable results. In 2002, the estimates of the alkane ratio C32:C33 were best related with estimations of the net energy method. In 2003, the estimates of the alkane ratio C32:C31 were best related. The estimate based on the alkane ratio C32:C33 had a lower coefficient of variation than the one based on the alkane ratio C32:C31. Therefore, the C32:C33 alkane method was considered to be a better direct estimator for herbage intake by grazing lactating dairy cows

Keywords: Dairy cows, Perennial ryegrass, Rumen fill, Clearance, Degradation, Grazing behaviour.In temperate environments, perennial ryegrass is the most widely used species for feeding dairy cows. That is because of its high productivity, palatability, digestibility and nutritive value. However, perennial ryegrass suffers from two main disadvantages: firstly, its unbalanced nutrient content in terms of crude protein (CP) and energy availability at rumen level and secondly, its low dry matter intake (DMI) by high producing dairy cows. In ruminant animals, DMI is a function of the balance between eating motivation, which is strongly related to palatability, on one hand and rumen capacity on the other. This thesis was divided into three parts: the first part aimed to investigate the possibility of improving pasture DMI by increasing rumen capacity. Rumen capacity is related to the rate of clearance (Kcl) of material from the rumen, which is the summation of both rates of degradation (kd) and passage (kp). Six varieties of perennial ryegrass were screened for their clearance and degradation rates using in vitro , in vivo and in situ techniques. The results suggested that there is a narrow range for selection with respect to clearance and degradation rate. The second part of this thesis aimed at investigating the possibility of improving pasture DMI by improving its palatability. Palatability is mainly a function of flavour and taste, which arise from certain compounds in the grass, especially water-soluble carbohydrates (WSC). High sugar grass varieties were fed to dairy cows in mid lactation. The results showed that feeding high sugar grass did not lead to an improved DMI and milk yield under both stall-feeding and grazing conditions. Moreover, milk composition in terms of fat, protein and lactose, rumen pH, fibre clearance and degradation rates were not influenced. However, rumen ammonia and milk urea concentrations were significantly reduced when feeding high sugar