Proceedings of the First International Weed Control Congress, Melbourne 1992 273 Grazing animals as biological control agents Ian Popay2 and Roger Field2 grazing, have a natural advantage over many weed species. MAFTechnology, Flock House Agricultural Identifying a weed in grassland is difficult Centre, Private Bag, Bulls, New Zealand for farmers and scientists alike, even if it is 2 Plant Science Department, Lincoln University, defined simply as a plant species detrimentally Canterbury, New Zealand interfering with human activities. Barley grass may reduce sheep growth rates when seeding Summary but provides invaluable fodder when vegeta- Grazing animals are widely used as biologi- tive. Obvious weeds are either poisonous or cal control agents, but only in systems unpalatable, reducing the grassland available supporting crop /pasture and animal species to grazing stock. Species which may reduce rotations. Their main use is for food or grassland productivity are harder to identify as fibre, and weed control is incidental. To be weeds, and include some grasses and clovers. successful, weed control must become, Weediness is harder to define when plants temporarily, the main objective. The have redeeming features like high trace species, intensity and duration of grazing element content. animals can be controlled by fencing, allowing selectivity between weeds and Australian and New Zealand grazing desirable species. Many examples of success- ful biological control by grazing animals are systems given. Sustainability of control is offered by In Europe and North America, extensive use of increasing the control methods, and reduc- pesticides and fertilizers has allowed develop- ing their predictability. ment of simple agricultural systems. Some modem farms have no animals, whilst 62% of Introduction the 1982 U.S.A. beef market was controlled by large feed -lot production (22). This limits the Biological control cheaply and permanently scope for using grazing animals as biological reduces the impact of weed species. Tradition- control agents. ally, biological control redresses the natural Many New Zealand and Australian produc- balance by importing pests from the countries tion systems include pasture /crop rotations and in which the weeds originated. Increasing some rotation of cattle and sheep. Australian herbicide resistance, consumer preferences for farmers in the 70 million ha wheat and sheep pesticide -free food, and unprofitable pastoral zone can vary the area of wheat, and cattle or production make alternatives to herbicides sheep numbers (24). Australian and New more attractive. For weeds which are hard to Zealand grazing systems are relatively exten- eliminate, biological control may be the only sive and have to be sustainable at low input practical method. levels. Grazing animals can act as biological These agricultural systems often rely on control agents in both grassland and crops, plants or animals imported from Europe within either within the crop itself, or by reducing the last 200 years. The major weeds were also weed numbers or seed set in the preceding imported, often without the pests and diseases pasture or crop. Weeds may be more palatable which limited their impact in Europe. There, than desirable species and therefore grazed competitive pasture species, and grazing harder, or they may be less capable of recover- animals which ate the weeds and increased the ing from grazing. Grasses and the clovers tiller density and growth rates of the pasture (Trifolium spp.) or medics (Medicago spp.) species also helped to control the weeds. often associated with them, being adapted to 274 Proceedings of the First International Weed Control Congress, Melbourne 1992 Carter (4) examined how grazing animals used for `public -good' weed control. Feral control weeds in the Australian integrated goats have been encouraged in the Wairoa crop pasture - livestock system. He emphasized district of New Zealand to help reduce the the value of fencing and noted how grazing spread and impact of blackberry (Rubus animals in the pasture phase of a rotation can fruticosus). reduce seed set by potential crop weeds. Australian and New Zealand examples Extending the definition of biological Amor (1) pointed out that grazing by sheep is control the main method of biological control on Biological control treatises rarely discuss the dryland farms in Victoria, and that sheep are use of grazing animals for control of weeds. used extensively to suppress weeds on fallows Yet grazing animals, like classical biological and, to some extent, to reduce the seed produc- control agents, are often introduced from the tion of weeds in pastures before cropping. same native environment as the weeds. Sheep, control some weeds which become Selectivity is important in both cases. Conven- dominant under cattle grazing. Under sheep tional biological control agents are released if grazing, the size, importance and flowering they reproduce only on the target species. performance of ragwort (Senecio jacobaea) Grazing animals feed both on target species, and giant buttercup (Ranunculus acris) are and on desirable crop species. The target reduced (8,28). Judicious timing and intensity species must be more damaged by grazing, or of grazing with sheep can help control Califor- regrow less well after grazing. nian thistle (Cirsium arvense) (13), Scotch Biological control, whether conventional or thistle (C. vulgare in New Zealand) (9), and by grazing animals, is more sustainable than Paterson's curse (Echium plantagineum) (4). continuous pesticide use. Increased diversity Seed output from barley grass and other of control agents makes the system more grasses is reduced by timely, hard grazing resilient to change. Herbicide resistance is pressure with sheep (7). Sheep can be impor- unlikely to develop. Farming a mixture of tant in the control of gorse (Ulex europaeus) crops and animal species allows the farmer seedlings in pasture developed after burning of more flexibility in management and marketing mature gorse (10). They are commonly used strategies. for general weed control in orchards, vineyards Grazing animals are introduced primarily and waste places. for food or fibre, and their use for weed Weed control can be enhanced by applica- control is of secondary concern. However, if tion of sub lethal herbicide rates which make weed control becomes the main objective, weeds more palatable, so that they are killed some sacrifice of animal production may be by grazing rather than by the herbicide. This necessary. Classical biological control agents technique is known in Australia as "spray - are introduced primarily for weed control, and grazing" (6). rarely have any other value. Fish species can Bracken (Pteridium aquilinum var. be introduced either for grazing control of esculentum) has been controlled by cattle aquatic weeds or for food, or both (15). treading, or more often, by crushing and The difference in size between conven- burning, oversowing and fertilizing, followed tional biocontrol agents, usually smaller than by mob - stocking with cattle (21). Cattle, at a the target species, and larger grazing animals, reasonable stocking rate, can give good control has several consequences. Grazing animals can of Californian thistle (12). be contained, so that grazing pressure and Goats have been used to control many intensity achieve maximum selectivity be- weeds which are resistant to grazing by sheep tween weeds and desired species. The benefits and cattle. They seem to cope well with spiny of biological control can be captured privately, plants and have given effective control of which means that weeds outside the fence blackberry ( Rubus fruticosus) (30), sweet briar remain uncontrolled. Spraying insecticide, or (Rosa rubiginosa) (19), serrated tussock augmentation by releasing extra insects or (Nasella trichotoma) and galvanized burr spores are the only ways in which established (Sclerolaena birchii) (2), gorse (33), hore- conventional biological control agents can be hound (Marrubium vulgare) (14), matagouri manipulated. Grazing animals are, however, (Discaria toumatou) (5) and rushes (25). Goats Proceedings of the First International Weed Control Congress, Melbourne 1992 275 eat flowering thistles, but are not attracted to In. Britain, bracken (Pteridium dquilinum the vegetative rosette stage. Thistles controlled ssp. aquilinum) increased dramaticallywith include Illyrian thistle (Onopordum illyricum) the replacement of cattle by sheep on rougher (3), saffron thistle (Carthamus lanatus) (23), grazings in the 18th and 19th centuries, and variegated thistle (Silybum marianum) (2), increasing cattle numbers has been suggested nodding thistle (Carduus nutans), Scotch as a means of controlling, bracken (27). thistle (Onopordum acanthium in Australia) (18) and artichoke thistle (Cynara Other grazing animals used for biological cardunculus) (18). control Variation of grazing intensity and timing, Geese are used for weed control in peppermint using the normal grazing species, can give (Mentha x piperita) in Oregon, and in Oregon control of some weed species. Although and California for weed control in garlic Californian thistle is a major problem of (Allium sativum), tomatoes (Lycopersicon cattle grazed pasture, intensive cattle grazing, esculentum), cucumber (Cucumis sativus), especially through spring, when the shoots are cotton (Gossypium hirsutum), orchards and soft and newly emerged, reduces its incidence vineyards (20). Weeder geese have also been (12). Similarly,