Pasture and stock management of Californian thistle

Abstract

A combination of mowing and grazing by sheep has been shown to successfully control Californian thistle (Cirsium arvense) in small plot and paddock scale trials. Between 1994 and 2000 the technique was applied to 15 paddocks on nine farms in Otago, Southland and Mid- Canterbury. Thistle-infested pastures were mown as low as possible, usually in December after weaning. Thistles at this time were up to 1m tall at the mid to late bud stage. Mobs of at least 350 ewes/ha were introduced at mowing or a day or two before, with all mown herbage eaten within 3 days. Pastures were re-grazed with this or similar sized mobs at approximately threeweek intervals to remove re-growth thistles which had emerged since the previous grazing and before their spines hardened. Each grazing forced the plant's root system to produce a new crop of aerial shoots and this ultimately used up the root reserves causing the thistles to die. The farmers involved were responsible for implementing recommendations on mob size, time and length of grazing. Three or four repeat grazings markedly reduced thistle numbers the following summer, resulting in almost complete elimination by autumn in year two after the programme was repeated. With fewer grazings in year one, control took longer. The grazing regime had no apparent effect on ewe health and resulted in improved pasture quality. Keywords: Californian thistles, mob size, repeat grazing, thistle numbers