Effects of previous suppressive anthelmintic treatments on subsequent nematode infection in fattening cattle in Argentina

The effect of previous suppressive anthelmintic treatments after weaning on parasitological parameters and weight gain of cattle was studied in the Pa mpeana region of Argentina. The study was carried out at two grazing fatten ing periods: April 1995/July 1996 and April 1997/July 1998. During both per iods, 60 weaned calves that grazed contaminated pastures, were divided into three groups during the first part of the periods: GY1 group was treated e very 2 weeks with doramectin while GY2 and GY3 groups remained untreated. D uring the second part of the periods, from October onwards GY1 and GY2 rema ined 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 week s) and UG (untreated). Egg counts (EPG), larval cultures, pasture larval co unts, serum pepsinogen (Pep) and live weight gain (LWG) were recorded month ly. Ostertagia, Copperia, Trichostrongylus and Haemonchus were the predomin ant 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 secund part of the fattening period in 1 995/1996. In 1997/1998 moderate infection levels during the first part of t he 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 acid GY2. Live weight gain of GY2 exceeded GY1 by 10.7 kg (P < 0.006). Higher EPG and lower LWG of GY 1 suggest that suppressive treatments negatively affected the level of resi stance to infection of yearlings, but these effects were influenced by prev ious levels of nematode infection. The lack of differences between yearling (GYI) and calves (UG) groups suggest that, under the conditions of this st udy, there was no evidence that resistance to infection and the related par ameters are influenced by the age. (C) 2001 Elsevier Science B.V. All right s reserved.