Anticoccidial activity of narasin in broiler chickens reared in floor pens.

The anticoccidial activity of the ionophore narasin was tested in 3 floor-pen experiments. Narasin was tested at levels of 40, 60, 80, 100, or 120 ppm and compared against feeding ration containing 80, 100, or 121 ppm monensin or no medication. Feeding at all levels of narasin significantly prevented coccidiosis-induced mortality and improved weight gains and feed conversion ratios compared with the same parameters in groups given unmedicated feed. Protection with narasin was equal to or slightly greater than the protection obtained with monensin. The reduction in intestinal lesion scores was greater with narasin medication than with monensin. Analysis of pooled data from these trials indicated that a level between 48 and 96 ppm narasin would provide the optimum, depending on whether maximum weight gain or optimum feed conversion ratio was desired. Testing for coccidial immunity using the immunity challenge technique indicated that, based on the parameters of weight gain and lesion scores, even levels as low as 40 ppm narasin had enough efficacy to significantly reduce the amount of immunity which developed in medicated birds compared with the level of immunity developed in birds receiving unmedicated feed.

Efficacy of different feeding levels of monensin in the control of coccidiosis in broilers.

The feed level of monensin which gave maximal protection from coccidial infection in broilers reared in floor pens depended on the severity of the coccidial exposure and the criteria of infection studied. With indirect seeding of pens with coccidia (light coccidial exposure), 40 ppm was as efficacious as 100 ppm in improving weight gain, lesion score, and feed conversion. Statistical analysis with direct seeding (seeder birds; severe coccidial exposure) showed that feed levels of 84 and 102 ppm gave maximal improvement of weight gain and 4-week feed conversion, respectively. Further increasing the monensin level from 100 to 121 ppm did not improve weight gain and feed conversion at 8 weeks. Conversely, the relationship of plasma pigmentation and total lesion scores to monensin feed levels indicated that increasing the dosage of monensin from 100 to 121 ppm improved the performance of the broilers on the basis of these 2 infection indicators.

Effect of dietary methionine status on response of chicks to coccidial infection.

Three experiments determined if the methionine requirement of broiler chicks was affected by coccidial infection. Chicks were fed a corn-soy or a corn-soy-pea basal diet containing 0.73% and 0.62% total sulfur amino acids (TSAA), respectively. Levels of 0 to 0.45% DL-methionine were added, with and without 0.01% monensin sodium. In two experiments, the chicks were inoculated at two weeks of age with a mixture of oocysts of E. acervulina, E. maxima, E. tenella, E. necatrix and E. brunetti. Lesion scores on the intestines and ceca, and blood carotenoid levels were determined at three weeks. The experiments were terminated at four weeks. A level of methionine greater than 0.47% and of TSAA greater than 0.83% was necessary to obtain maximum growth rate in uninoculated chicks. No evidence of dermatitis was observed. Growth rate and feed efficiency of chicks infected with coccidiosis were more severly depressed when the diet was not supplemented with methionine. Infections of coccidia and low levels of methionine, which in themselves did not produce any significant change in weight gain, did give a significant weight depression in combination. Adding monensin to the diet prevented a reduction in growth rate and feed efficiency of inoculated chicks fed adequate methionine. Monensin did not completely prevent the adverse effects of a coccidial infection, based on feed efficiency, when chicks were fed diets inadequate in methionine. Blood carotenoid levels were not affected by methionine level, but were significantly lowered by coccidial infection in the absence of monensin. Intestinal and cecal lesions in inoculated chicks were significantly reduced by including monesin in the diet. Although the coccidial infection more severly affected the performance of chicks fed diets deficient in methionine, satistical analysis of pooled data indicated no difference in the quantitative requirement of chicks for methionine. Therefore, a level of methionine and cystine adequate for optimum growth under the coccidial-free conditions should be adequate for chicks when infected with coccidia.