Sheep and legumes as partners to control crop weeds

By Dean Thomas, The University of Western Australia

Understanding the grazing behavior of livestock can help producers control weeds in crop fields. In Australia, annual ryegrass invasion is a persistent problem in many wheat fields, and herbicides are becoming less effective due to an increase in herbicide-resistant weeds. Producers can reduce the abundance of weeds by planting cropland to pasture with a legume that is not preferred by livestock. Livestock grazing the legume-ryegrass pasture eat more annual ryegrass and less legumes, thus reducing the number of ryegrass weed seeds produced. When the legume, Casbah biserrula, was planted in a ryegrass infested pasture and then grazed by sheep, seed set of annual ryegrass was reduced by 90% in one year compared to pastures where other legumes were planted. The additional nitrogen and organic matter produced by the legume also benefits the subsequent crop.

Casbah biserrula is avoided because it may contain secondary compounds that are aversive to sheep. Sheep grazing pastures containing a high proportion of biserrula avoided biserrula, while those grazing pastures with a low proportion or no biserrula had a higher preference for biserrula when offered a choice. In addition, biserrula has been reported to cause photosensitization in sheep, although this is rare. Growth is not adversely affect in sheep grazing biserrula pastures.

Figure 2. Sheep have consumed most of the ryegrass in the pasture sown in casbah biserrula (left) but not in the pasture sown in the yellow serradella.

Choosing the right species for the pasture is essential for effective weed control. These researchers recommend using a legume that is not highly preferred by livestock, tolerates grazing, and is suited to the climate and soil of the area. It must grow vigorously and compete effectively against weeds.

Where animals have difficulty selecting preferred plants from those that are less preferred, pastures might need to be sown to allow animals to easily select the target plant. Planting the legume in rows spaced far apart may increase control because animals can easily graze grass sprouting between the rows. Researchers recommend that sheep are most effective at controlling ryegrass when forage availability is between 1000 to 1500 lbs/acre. If the legume is too abundant, sheep have difficulty finding and grazing ryegrass. On the other hand if the pasture becomes overgrazed, animals will consume all forage making the legume less competitive