Effects of anthelmintics and gastrointestinal nematode infection intensity on growth and production performance and carcass quality in western Canadian feedlot cattle.

A randomized controlled trial determined the effects of anthelmintic treatments on growth and production performance, and carcass quality in western Canadian feedlot cattle. Fall-weaned, auction-derived steers (n = 234) were randomly assigned to three groups: untreated control (CTRL), injectable ivermectin (IVM), and injectable ivermectin combined with oral fenbendazole (IVM + FZ). Steers were dosed according to individual body weights and manufacturers' recommendations. Individual fecal and blood samples were collected at feedlot arrival to determine fecal egg counts (FEC) and serum O. ostertagi antibody titers. The effects of anthelmintic treatments on live body weights (BW), dry matter intake, average daily gain (ADG), gain-to-feed ratio, and carcass quality characteristics were analyzed at the pen level. Individual FEC and serum O. ostertagi titers were used to estimate effects of gastrointestinal nematode infection intensity on growth, production, and carcass traits at the steer level. Live BW (P ≥ 0.46), dry matter intake (P ≥ 0.48), ADG (P ≥ 0.60), and gain-to-feed ratio (P ≥ 0.36) were not associated with anthelmintic treatments in either the backgrounding (day 0-28) or finishing (days 29-177) phase. Hot carcass weight (P = 0.97), dressing percentage (P = 0.41), backfat thickness (P = 0.33), and rib-eye area (P = 0.97) also showed no differences between treatments. The IVM + FZ treatment increased the proportion of carcasses with modest marbling compared with CTRL and IVM (P = 0.002) and tended to produce more AAA carcasses than CTRL (P = 0.06), whereas IVM alone increased the proportion of yield grade 2 carcasses compared with the other treatments (P = 0.006). Higher FEC at arrival (mean 9.1 ± 10.2 eggs per g of feces) were associated with lower ADG during the finishing phase (P = 0.01), reduced hot carcass weight (P = 0.03), and a lower percentage of AAA carcasses (P = 0.02). Increasing serum O. ostertagi titers were linked to thinner backfat (P = 0.004) and a greater proportion of yield grade 1 carcasses compared to fatter yield grades (P = 0.04). The results indicate that anthelmintics used in western Canada improved certain beef cattle carcass qualities and that relatively low gastrointestinal nematode burdens may still be associated with measurable adverse impacts on feedlot performance under these conditions.