Decreasing Forage Allowance Can Force Cattle to Graze Broom Snakeweed (Gutierrezia sarothrae) as a Potential Biological Control

Abstract

Broom snakeweed (Gutierrezia sarothrae [Pursh] Britt. Rusby) increases and dominates rangelands following disturbances such as overgrazing, fire, and drought. However, if cattle can be forced to graze snakeweed, they can be used as a biological tool to control it. Grazing trials were conducted in May and August 2004, 2005, and 2006 on a crested wheatgrass (Agropyron cristatum L.) seeding that had been invaded by broom snakeweed. Narrow grazing lanes were fenced with temporary electric fence and the cows were moved to a new lane each day. Forage allowance was limited to 24%-75% of the intake requirement. There were significant negative correlations (P < 0.05) between forage allowance and snakeweed utilization, suggesting it was the main factor driving snakeweed consumption. In the 2004 experiment, 7 cows in low body condition (4.6 body condition score, BCS) and 7 cows in high body condition (6.8 BCS) were grazed in separate lanes. The low body condition group grazed more snakeweed in the evening grazing period (26% of bites) than the high body condition group (20% of bites, P = 0.03). In the 2005 experiment, one group (6 cows) received a protein/energy supplement high in bypass amino acids required for detoxification of terpenes; the second group received no supplement. There was no difference in snakeweed consumption between the supplement groups (P = 0.63). The major difference in diets in both years occurred in grazing periods during the day. Cows grazed perennial bunchgrasses first, then turned to cheatgrass (Bromus tectorum L.), and grazed snakeweed only when all other forage was depleted (20% of bites in the evening grazing periods). Cattle grazed 62%-95% of snakeweed plants and utilized 50%-85% of snakeweed biomass. Cattle can be forced to graze snakeweed by confining them to small areas and limiting alternative forage. Grazing reduced the snakeweed population.