Use of antibiotics in beef production

Abstract

In the first section of this thesis, literature is reviewed on low-level feeding of antibiotics to ruminants, on incidence of liver abscesses and rumen lesions in cattle fed high-cereal diets, on possible modes of action of antibiotics in enhancing animal growth and on public health aspects of the use of antibiotics in animal feeds.

In a second section two experiments are described which had the object of investigating possible modes of action of oxytetracycline and bacitracin. These experiments involved 18 calves fed a barley-beef diet and weighing on average 130 kg in expt. 1 and 162 kg in expt. 2. Neither oxytetracycline nor bacitracin influenced dry-matter digestibility, apparent nitrogen digestibility, nitrogen retention, concentration of volatile fatty acids in the rumen or rumen pH.

Oxytetracycline significantly increased rumen ammonia and blood urea content, but bacitracin had no such effect. In expt. 1 blood glucose levels were not influenced by the feeding of either antibiotic, in expt. 2 calves on both experimental diets showed a tendency for a greater increase in blood glucose content during the interval from 2 to 10 hrs. after feeding time. A third section reports an experiment, which was conducted on 14 commercial barley-beef units in Scotland, using a total of 339 early~weaned Friesian male calves. On each unit the trial batch of calves was divided into two groups, which were then allocated to a control diet (85 % barley+ 15 % protein supplement) or the same diet supplemented with 20 mg chlortetracycline per kg of feed. The experiment was conducted over the live-weight range 115 to 360 kg. The chlortetracycline significantly (P<O.Ol) improved live-weight gain(+ 4.9 %) and feed utilization (+ 4.8 %). This overall effect was largely due to a high response during the first 12 weeks.

From 13 to 28 weeks the chlortetracycline had only a marginal effect, which was not significant.

There was no significant difference in the feed intakes of the two groups. There was a significant (P<O.Ol) negative correlation between the effect of the chlortetracycline on live-weight gain at each unit and the growth rate of the control animals at that unit; the response was greatest on farms with poor hygiene.

The animals given chlortetracycline showed a significantly (P<0.05) lower incidence of liver abscesses (ll.8 %) than the controls (28.2 %).

Macro£copic examination of the rumen showed that rumenitis is common among intensively fed cattle, but no correlation could be found between incidence of liver abscesses and rumen lesions.