Frequency distribution of sward height of kikuyu ('Pennisetum clandestinum') grass pastures intensively grazed by dairy cows

Abstract

Swards that are grazed continuously by cattle can become heterogeneous in structure, such that frequencies of height measurements have a skewed distribution. This structure has been best described by the double-normal, gamma and Weibull distributions rather than by the more common single-normal distribution. Limited analysis of the frequency distribution of sward height under dairy cattle grazing management is available. The current paper reports the frequency distribution of sward height under intensive dairy cow grazing of kikuyu (Pennisetum clandestinum) in a subtropical environment. Frequencies of sward height were collected during an experiment designed to investigate the effects of compressed sward height and grazing duration on grazing behaviour of dairy cows. There were two compressed sward heights (10 and 13 cm; Earle and McGowan 1979) and five grazing durations (1, 2, 4, 8 and 15 h). The frequency of height measurements taken were then fitted to four distributions – single-normal, log-normal, gamma and Weibull – to determine if treatments influenced the structure of the sward as grazing duration progressed. The frequency of sward height measurement was best described by the log-normal distribution. However,no one distribution described the frequency of height measurements consistently as grazing duration progressed, as determined by Akaike information criteria. The present analysis did not provide unequivocal evidence for either bimodal or unimodal distributions describing the height measurements for 10- and 13-cm compressed sward height treatments as grazing duration progressed.