The effect of sheep and goat grazing on variegated thistle (Silybum marianum) populations in annual pastures
D.F. StanleyA, P.J. HolstA and C.J. AllanB
A NSW Agriculture, Cowra, New South Wales 2794, Australia.
B PO Box 1090, Orange, New South Wales 2800, Australia.
The effect of sheep and goat grazing on variegated thistle was studied over two years when sheep alone was compared with two ratios of sheep and goats. Measurements were taken on pasture production, thistle plants and thistle seeds in soil. Thistle measurements along a fixed transect included height and width, eaten score, capitula number and number of flowering stems eaten.
In each of the two years, sheep ate little variegated thistle whereas goats significantly (P<0.001) contained plant size and consumed all capitula (year 2; mature capitula per plant 5.41 v 0.0 for sheep and goats respectively). After two years, viable seed reserves in soil were 497 ± 157 in the sheep treatments and 126 ± 66 in the goat treatments (P<0.05) with no difference between a high or low ratio of goats.
It was concluded that sheep had relatively little impact on variegated thistle whereas goats preferentially grazed the thistle. The goats were particularly effective in reducing the number of capitula and the number of capitula consumed was a function of goat grazing pressure. Removal of capitula would reduce seed production and subsequent population of the thistle.
Plant Protection Quarterly (2000) 15 (3) 116-118.