Managing herbaceous broadleaf weeds

The invasion of broadleaf weeds represents one of the greatest conservation and land management challenges in the modern era. Most broadleaf weeds originated from Eurasia, and once introduced to North America spread rapidly across public and private land. These weeds can be highly nutritious and livestock will readily graze them during the growing season. The age and breed of the livestock best used to control herbaceous weeds depends upon the grazing situation.

Animals grazing cause the most damage to broadleaf weeds when the plants are initiating flower production and rapidly elevating flower stalks. Broadleaf weeds provide the most nutrition during their rapid growth phase when water and high nutrient uptake facilitates cell expansion. The period of highest nutritional need for ewes and nannies generally coincides with the time of highest forage value in the weeds.

Broadleaf weeds include:

• Diffuse Knapweed
• Russian Knapweed
• Spotted Knapweed
• Kudzu
• Leafy Spurge
• Sericea
• Tansy Ragwort
• Bull Thistle
• Canada Thistle
• Musk Thistle
• Scotch Thistle
• Yellow Starthistle
• Hoary Cress
• Perennial Pepperweed

By nature, the large mouth and tongue of cattle prevent them from prolific weed consumption. Plus, cattle have less effective digestive and metabolic systems to detoxify the deleterious plant compounds often found in weedy forbs. Sheep naturally prefer forbs over grasses, making them good candidates for weed control. However, sheep also graze grasses, making them less desirable for weed control in cattle pastures. Goats generally prefer shrubs and small trees so they compete less with cattle, but goats may cause unwanted damage to wooded riparian areas. Animals like variety in their diet, just like humans. Therefore, managers need to consider this when grazing.

Few studies have compared whether certain breeds of sheep or goats are better weed eaters than others. White-face breeds may be more appropriate for a herded situation as they form a tighter flock. Black-face sheep work well under permanent fence, which may limit their ability for large scale weed control. Yearlings experienced in eating specific weeds consume more of those weeds early in the summer than naïve yearlings. However, as the summer progresses, both groups consume equal amounts. It appears that sheep must learn to eat certain weeds by following social models. A herd of sheep that has never eaten a weed, may not consume the weeds until they learn from other sheep that the weed is nutritious and acceptable.

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