Abstract
The effect of previous suppressive anthelmintic treatments after weaning on parasitological parameters and weight gain of cattle was studied in the Pampeana region of Argentina. The study was carried out at two grazing fattening periods: April 1995/July 1996 and April 1997/July 1998. During both periods, 60 weaned calves that grazed contaminated pastures, were divided into three groups during the first part of the periods: GY1 group was treated every 2 weeks with doramectin while GY2 and GY3 groups remained untreated. During the second part of the periods, from October onwards GY1 and GY2 remained untreated and GY3 was treated every 2 weeks. In this second period two new groups of 20 weaning young calves were added: TG (treated every 2 weeks) and UG (untreated). Egg counts (EPG), larval cultures, pasture larval counts, serum pepsinogen (Pep) and live weight gain (LWG) were recorded monthly. Ostertagia, Cooperia, Trichostrongylus and Haemonchus were the predominant genera. Despite low levels of previous infection during the first part of the period, slight differences of EPG between GY1 ( P<0.09) or UG ( P<0.05) and GY2 were detected in the second part of the fattening period in 1995/1996. In 1997/1998 moderate infection levels during the first part of the period were observed. During the second part of this period, GY1 and UG showed higher ( P<0.001) EPG than GY2, and only GY3 and TG had ( P<0.05) lower Pep levels. Also, during the second part of 1997/1998, LWG responses of GY3 were higher ( P<0.001) than those of GY1 and GY2. Live weight gain of GY2 exceeded GY1 by 10.7 kg ( P<0.006). Higher EPG and lower LWG of GY1 suggest that suppressive treatments negatively affected the level of resistance to infection of yearlings, but these effects were influenced by previous levels of nematode infection. The lack of differences between yearling (GY1) and calves (UG) groups suggest that, under the conditions of this study, there was no evidence that resistance to infection and the related parameters are influenced by the age.
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