Effect of Winter Cover Crop Grazing on Animal Performance and Antibiotic Resistance during Pre-Weaning Period in Beef Cattle.

We investigated the effect of winter wheat grazing on body weight gain and the level of antibiotic resistant bacteria in beef cattle. Calves and cows (16 each) were equally randomized into tall fescue or wheat pastures. Body weights and fecal samples were taken on d 0, d 7, d 14 and d 21. Samples were cultured for tetracycline (TET^r) - and 3rd generation cephalosporin (3GC^r) -resistant Escherichia coli. After a 4-week grazing period the average daily gain of the calves was significantly (P=0.0014) higher in the wheat group (2.7 kg) compared to the tall fescue group (1.98 kg); no significant (P=0.1945) difference was observed in the cows. TET^r E. coli concentration did not significantly (P>0.05) differ between treatments both in the calves and the cows. In the calves TET^r E. coli concentration increased by more than 5 log₁₀ CFU/g of feces for days 14 and 21 compared to baseline level. In the cows it increased by 2 and 1 log₁₀ CFU/g on d 14 and d 21 respectively. There was significant (P<0.001) sampling day by age (calves versus cows) interaction effect on TET^r E. coli concentration: TET^r E. coli concentration was higher in the cows on d 0, but was higher in the calves on days 14 and 21. Treatment or sampling day had no effect on the prevalence of 3GC^r E. coli both in the calves and the cows (P>0.05). Higher (P=0.062) prevalence of 3GC^r E. coli was observed in the cows (34.4%) than in the calves (12.9%). In conclusion wheat pasture grazing increases animal performance with no significant impact on the level of antibiotic resistant bacteria. The observed age dependent increase in the level of TET^r bacteria in the calves, and the role of cows for the maintenance and transmission of 3GC^r bacteria need further investigation.