A Guide to Parasite Control at Turn-out
3. Appropriate Use of Anthelmintics Anthelmintic resistance is a key consideration in sustainable control of parasites. Therefore, these drugs must be used carefully in order to benefit both animals and farmers. Because of differences in susceptibility resulting from acquired immunity to worms, it is easiest to consider cattle in three different age categories: adults, second grazing season (SGS) and first grazing season (FGS). 1. Adult cattle It is rare for adult cattle to show any clinical signs of gut worm infestation, yet many studies, particularly in dairy cows, have shown evidence of subclinical infections with production losses. Reported benefits from treatment generally relate to milk yield; however, in a few studies, beneficial effects on body condition and fertility have also been observed. 2. Second season grazers These animals are not fully immune to gut worms and can experience production losses and occasionally disease. Lack of exposure to infection during the FGS, as seen in late-born calves, beef suckler calves or under intensive anthelmintic treatment regimes may result in lower levels of immunity at the start of the SGS. 3. First season grazers These calves initially have no immunity to parasites and are at risk of clinical, as well as subclinical gut worm infestations. Suckler calves and dairy calves must be considered separately in terms of risk factors. Beef Suckler Calves Initially Beef Suckler calves are at a low risk as they are grazing with their mothers and have low herbage intake. There is also an apparent effect of milk on parasitic roundworms such that the adverse effects of these worms are reduced. Their greatest risk periods are after weaning in late summer and autumn. Growth rates should be monitored and faecal samples from 10 - 15 calves (which can be pooled) can be checked to determine how many worm eggs are present. If nutrition appears adequate, significant numbers of worm eggs are present and growth rates are below target, then the use of anthelmintics is justified. Results of recent research indicate that satisfactory group performance can be maintained by treating only those animals with growth rates that fall below target.
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