Low density of prickly acacia under sheep grazing in Queensland

Populations of an introduced woody weed, prickly acacia (Acacia nilotica (L .) Delile ssp. indica (Benth,) Brenan syn. Acacia arabica (Lam,) Willd. ssp . indica Benth.), were surveyed at 4 sites in central Queensland. There is a significantly lower frequency of plants of < 3 m in height within populat ions which have been grazed by sheep, indicating that browsing by sheep red uces regeneration. There were higher losses of seedlings at a sheep-grazed site than at cattle-grazed sites. These results support previous assertions that prickly acacia is regenerating more successfully on cattle properties , because cattle both disperse seeds and are less effective herbivores. In regions of low annual rainfall, prickly acacia is capable of forming dense stands (up to 2,700 shrubs ha(-1)) in lowland landscape types, Stands are l ess dense in upland landscapes (maximum of 718 shrubs ha(-1)). Of most conc ern is that in regions of high annual rainfall prickly acacia can form extr emely dense thickets across most landscape types (up to 3,400 shrubs ha(-1) ). We suggest that prickly acacia is most likely to become a management pro blem on cattle properties, and an extreme problem in high annual rainfall a reas. The inclusion of sheep in livestock rotations may be an effective con trol measure in the Mitchell Grasslands, but this may not always be possibl e. A high priority is to prevent prickly acacia from expanding its range in to equivalent high rainfall areas within Queensland, and also in the Northe rn Territory, northern New South Wales, and Western Australia. This could b e achieved by quarantining livestock which have come front infested propert ies until seeds have passed through the digestive tract, after about 6 days . Management strategies at the property level should aim to prevent further spread of prickly acacia by controlling cattle movements between paddocks during periods when cattle are ingesting pods and seeds.