Application of the Double-Normal Distribution to Measurements of Sward Height for Kikuyu Pastures Grazed by Dairy Cows at Different Grazing Durations

Sward height (SH) distributions are frequently summarised by the sample mean and standard error. This is most appropriate when the population distribution is approximately symmetrical. However, in continuously grazed swards a combination of relatively 'short' and 'tall' patches in pasture are created and maintained by cattle through selective grazing (Edwards et al. 1997). The relative proportions of 'short' and 'tall' areas are largely affected by management factors such as grazing intensity and the length of the grazing session. Sward heterogeneity is important for a number of reasons. It will affect subsequent utilisation by grazing animals which may in turn affect patterns of nutrient return and treading damage. It may also affect inter-plant competition and potentially the species composition of the sward in the long term. Frequencies of SH usually show a skewed distribution and the representation of a single mean SH is potentially misleading (Gibb and Ridout 1986). Sward height data from an experiment designed to measure the behaviour of dairy cows grazing kikuyu pastures at two different sward heights (SH: 10 and 13 cm) and grazing durations (1, 2, 4, 8 and 15h) was used to determine the ability of a double-normal (DN) distribution to describe changes over the grazing treatments in the mean heights of the 'short' and 'tall' components of the sward.