The effects of herbage utilization during grazing on subsequent pasture quality : a dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Agricultural Science (Honours) in the University of Canterbury

Abstract

An existing grazing trial on an irrigated Wakanui silt loam at Lincoln College, Canterbury, was sampled to determine the effects of herbage utilisation at grazing on botanical composition and in vitro digestibility. Two stocking rates, high (27.5 ewes/ha} and low (17.5 ewes/ha) resulted in different levels of utilisation. Harvests began in January and continued for five consecutive grazings until July. Low utilisation (27%) produced a sward with a high dead material content (40 -50%) and low in vitro digestibility of the whole sward (< 70%). The high stocked sward, with a utilisation of 60%, had a larger percentage of leaf (31 - 63%) than the low stocked sward (28 - 39%), but total green yields were lower. The in vitro digestibility of the sward under the high stocking rate was over 70%, mainly as a result of a greater proportion of leaf and pseudostem, both of high digestibility (80 and 84% respectively). The proportion of dead material was less (under 26%), and it was more digestible than at the low stocking rate. During grazing, the percentage dead material increased in both swards, suggesting active rejection of this by animals. A sampling exercise indicated that greater numbers of samples were needed in the low stocked swards, to get an accurate estimate of herbage mass and. in vitro digestibility. This higher variability is probably a result of greater opportunity for animals to be selective in their grazing.