Search

In a study on nematodes recovered from dairy barn mangers, stalls, calf pens, and exercise yards at The Ohio State University, over 99% of the nematodes were nonparasitic species of the family Rhabditidae. The only parasitic species recovered was Strongyloides sp, a rhabditoid nematode of minor clinical importance to adult cattle. These findings were in disagreement with widely publicized claims that adult dairy cattle in confinement are continuously exposed to trichostrongylids and that all cows should be routinely treated with anthelmintics.

The effect of 2 prophylactic anthelmintic treatments with albendazole (7.5 mg/kg) was evaluated in 2 groups of Holstein dairy replacement heifers treated 3 and 6 weeks after turnout to spring pasture on May 2, 1983. The lightweight group consisted of twelve 7- to 9-month-old heifers with a mean initial weight of 201.1 kg. The heavyweight group consisted of ten 11- to 15-month-old heifers with a mean weight of 319.4 kg. Within each group, the heifers were paired by initial weight, and one member of each pair was randomly assigned to the treatment group. Each of the 4 resulting groups grazed separate, contaminated pastures until winter housing on Oct 31, 1983. The albendazole-treated, lightweight heifers had bodyweight gains 28.9% greater than that of the control heifers at the end of 23 weeks (P less than 0.001). There was no significant difference in gain for the treated heavyweight heifers, which had a lower feeding rate/kg of bodyweight than did the lightweight heifers. At the end of the study, there was a sevenfold difference in mean pasture L3 counts between treatment (1,303 L3/kg) and control (9,153 L3/kg) replicates. It was concluded that it is desirable to treat all heifers in northern regions during their first spring at pasture. This strategy would likely provide substantial economic returns to the farmer in years with a wet summer, which would favor early migration of L3 to herbage, but would provide smaller returns in dry seasons, which would delay this pattern of migration.

Eleven dairy replacement heifers (mean body weight, 262.0 kg) were given two treatments with levamisole in the spring to reduce worm egg output and consequent pasture contamination with infective nematode larvae during the summer/autumn grazing season. The treatments were given 3 weeks and 76 weeks after the heifers were moved from confinement to permanent pastures in may 1979, at which time they were 5 to 12 months old. Treated heifers gained significantly (P < 0.01) more weight than a group of 11 control heifers (mean body weight, 262.4 kg) and outweighed them by mean values of 9.2, 19.7, 31.0, and 39.5 kg at the end of July, August, September, and October, respectively. Control heifers had highest egg counts (47-51 eggs/g) from July to September, but there was a 2-month delay before treated heifers attained peak concentrations (42 eggs/g). This delay occurred at a critical time of the year (July, August), when environmental conditions were favorable for the rapid development of eggs to infective larvae. When the egg count for treated heifers peaked at the end of September, environmental conditions were no longer favorable for optimal development of free-living stages.

Dairy replacement heifers given two strategic treatments with ivermectin (0.2 mg/kg) three and eight weeks after turnout to spring pastures in northern USA had mean adjusted weight gains (62.3 kg) that were 62 per cent (P less than 0.02) greater than those of control heifers (38.5 kg) at the time of winter housing. Pasture infectivity was consistently higher in control pastures and there was a sixfold difference between treatment (56 L3/kg) and control (358 L3/kg) pastures by the time of winter housing on October 18. This study confirmed earlier studies in northern USA demonstrating the value of two strategic treatments in achieving significant increases in weight gains and hastening the time of first breeding by one to three months at a saving of $40 to $140 per heifer. It was concluded that this dosing strategy is a satisfactory method of parasite control for dairy replacement heifers in northern USA without undue selection for drug resistance.