Perennial ryegrass for dairy cows: Grazing behaviour, intake, rumen function and performance

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 grass. The third part of this thesis aimed to investigate the role rumen fill and fermentation end products may play in signalling the termination of a grazing bout, especially the dusk grazing bout of dairy cows under continuous stocking. It was found that dairy cows interrupted the first two main grazing bouts at the morning and afternoon long before their maximal rumen capacity was reached. Moreover, rumen fill was always maximal at the time when the dusk grazing bout ceased indicating that rumen fill is more likely to play a major role in signalling the termination of the dusk grazing bout.