Some weeds can meet the nutritional requirements of grazing livestock. It depends on the livestock species, its production cycle, the weed species present and the weed’s growth stage.

That’s according to Oregon State University researchers, who compared the feed value of 14 weed species to that of desirable forages in spring, summer and fall for a three-year period. The weeds included bog rush, bull thistle, Canada thistle, diffuse knapweed, French broom, gorse, Italian thistle, Scotch broom, spotted knapweed, yellow starthistle, Himalaya blackberry, sedge, Portuguese broom and meadow knapweed. Each species was tested for crude protein, acid detergent fiber, neutral detergent fiber, total digestible nutrients (TDN), net energy and mineral content at each growth stage.

Results indicate that some weeds have nutritional profiles similar to those of more desirable forage species, such as orchardgrass and ryegrass. Feed values are low at some developmental stages, however, suggesting that supplemental feeding would be required. Mineral profiles varied, indicating possible livestock health problems might occur, such as nutrient imbalances, if certain weeds were the only available feed. Several weed species, including the thistles and knapweeds, had very high levels of potassium, calcium and magnesium at all stages of plant development.

While forage quality of the weeds was generally high, it fluctuated over the sampling period. For many, crude protein content was highest in spring, then declined in summer and increased in fall. TDN appeared to fluctuate less, although it decreased in some species in summer.

Eleven of the 14 weeds met protein and TDN requirements of a 1,000-lb beef cow for the first five months of gestation, but most didn’t meet those requirements during the last four months. Requirements for a lactating cow were met by the knapweeds, French broom, Italian thistle and Himalaya blackberry.

The researchers conclude that intensive grazing of weeds can be an important biological control strategy as part of an integrated weed management plan. Grazing management will determine the success or failure of using livestock to help control weeds, they add. Read their full report.