The objective of this study was to assess the effects of horn fly and gastrointestinal (GI) nematode control on cow-calf performance with a spring-calving system. A total of 508 cow-calf records from two locations in Louisiana were available for study over two trials (2005 and 2006). Cow and calf pairs were stratified by cow age, calf sex, calf age, calf sire breed and calf sire and randomly assigned to three replicate treatment groups across the two locations. The treatments were: (1) no horn fly or GI nematode control; (2) horn fly control administrated to cows with insecticidal ear tags applied in late-May (start of trial); (3) GI nematode control administrated to calves in late-May, mid-July and late-August (each calf received fenbendazole at the rate of 5 mg kg(-1) body weight); and (4) horn fly and GI nematode control as stated above. Length of the trials ranged from 115 to 124 days (late-May to weaning in late-September or early-October). Cows and calves were weighed at the start of the trial, mid-July and weaning. Weekly horn fly counts were conducted. Individual calf fecal samples were collected in late-May, mid-July, late-August, weaning and 10-14 days after late-May and late-August collections. Cows treated for horn flies had lower (P<0.05) fly counts compared with untreated cows for each trial; however, overall fly counts for the treated cows at one location exceeded over 70 flies per side. Calves treated with fenbendazole had lower (P<0.01) GI nematode fecal egg counts (FEC) for the 10-14 day post-treatment collections. Calves treated GI nematodes and whose dams were treated for horn flies had numerically higher or greater (P<0.05) total calf gains (8.6 kg and 11.8 kg) in both trials than untreated calves. Weight gain differences for fenbendazole-treated calves compared with untreated calves were of greater magnitude from mid-July to weaning than weight gain differences from late-May to mid-July in both trials. There were no treatment differences for any of the cow response traits. In conclusion, improvement in calf growth from both horn fly and GI nematode treatments were realized even with marginal levels of fly control and a greater response in calf growth resulted when deworming of spring-born calves occurred when they were 4 months of age or older.