Effect of cattle and sheep grazing on the structure of Highland Sourveld swards in South Africa

Sward height has a major influence on the ingestive behaviour of cattle and sheep grazing sour (humid) grassland. We investigated the effects of mixed stocking on vertical sward structure and tracked temporal changes in award structure over a 6-year period. The effectiveness of a year's rest preceded and followed by a burn for eliminating grazing-induced patch structure was also assessed. Sward height data were collected (1990-1996) from a grazing trial stocked at 5 cattle:sheep ratios (1:0, 3:1, 1:1, 1:3, 0:1), each at 3 stocking rates (0.5, 0.71 and 1.0 animal unit equivalents/ha). Double-normal distributions were used to obtain measures of the mean height of the "short" and the "tall" components of the award and estimates of their relative proportions. Most of the height distributions were not normally distributed and became more skewed and uneven with time. A rest-and-burn after 3 years of grazing resulted in more uniform sward use in the following year in many treatments, but short patches developed again in most swards during the second grazing cycle. The height of the tall areas of the sward increased over time as grazing became increasingly concentrated on the short patches. The interaction between stocking rate and species ratio determined the height and extent of the short and tall components. Sheep at low and high stocking rates grazed patches very severely (<1.2 cm), but all swards, except those grazed predominantly by cattle at the low stocking rate, had short patches (<3.5 cm) in 1996. The most uniform grazing occurred at the higher cattle:sheep ratios and stocking rates, where usually less than one-third of the award remained unutilised. It is therefore recommended that cattle be stocked at a higher proportion than sheep to ensure even use of the sward and to enhance sheep performance.