Effect of Season-Long Weed Interference on Beef Heifer Performance in Cool-Season Pastures in Alabama

Abstract Weed interference in pasture-based livestock systems can alter animal performance through reductions in desirable forage yield and potential declines in nutritive value and palatability. To evaluate these effects on beef heifer growth, a grazing study was conducted using 12 replicated 1-hectare tall fescue (Schedonorus arundinaceus) paddocks at the Sand Mountain Research and Extension Center in Crossville, AL. Paddocks were overseeded with one of 3 species of cool-season annual forbs at a desired inclusion rate of 30%: broadleaf mustard (Brassica juncea), purpletop turnip (Brassica septiceps), and hairy vetch (Vicia villosa). Three yearling beef heifers were randomly assigned to each paddock and grazed from April 1 to May 23, 2025, with body weights (BW) collected every 28 days. Botanical separations of forage samples were performed biweekly to quantify weed biomass, and paddocks were adjusted with temporary electric fencing to maintain equal forage allowance across treatments. Average season-long forage allowance across all paddocks was 1.7 kg dry matter per kilogram of BW. Season-average weed interference across all paddocks ranged from 1.3% to 63.4% of total biomass. Across the grazing season, no relationship was observed between weed biomass and average daily gain (ADG), which ranged from 0.78 kg to 1.30 kg per animal per day across all paddocks (R2 = 0.0072, p = 0.6343). However, temporal analysis began to reveal differing trends across the transition from cool-season to warm-season weeds. During the early season (April 1 – April 30), increasing weed biomass showed a non-significant but positive association with ADG (R2 = 0.0820, p = 0.0953), while in the late season (May 1 – May 23), no significant relationship was observed between weed biomass and ADG (R2 = 0.0138, p = 0.5017). These findings suggest that while season-long weed interference did not reduce heifer performance, species shifts from cool-season to warm-season weeds may have nuanced effects on cattle growth. Ongoing analysis and further research will aim to explore how changes in weed species composition influence forage nutritive value and broader measures of animal performance.