An evaluation of a modified preventive drenching programme on commercial sheep farms

The effectiveness of a commonly advocated five drench preventive drenching programme in lambs was assessed on nine farms in the Gisborne, Hawkes Bay and Wairarapa districts between December 1983 and March 1985. The productive performance of two groups of preventively drenched lambs was compared with lambs in which parasitism was suppressed by fortnightly drenching from weaning until mid-August 1984. One of the groups received the standard five drench programme commencing at weaning and finishing in March 1984. The other group received six additional 4-weekly drenches, with the last drench in mid-August 1984. All the other lambs on the farms were drenched according to the five drench programme. The availability of infective larvae on pasture and the build-up of parasitic infection in the lambs was monitored. The effects of the different drenching programmes on liveweight and faecal egg counts was followed until March 1985, when the animals were 18 months old. Between March and July 1984 on all nine farms there was a significant depression of growth rates of the standard five drench group when compared with the suppression-drenched group (P<0.01). The mean liveweight difference from December 1983 to July 1984 for all farms was 4.0 kg (P<0.05). Hogget wool weights were significantly depressed (mean difference 0.37 kg, P<0.05). Faecal egg counts rose steadily after the completion of the five drench programme in March, exceeding 1000 eggs per gram on seven farms and 4000 eggs per gram on three of these farms. Trichostrongylus spp. was the major constituent of the winter parasite increase except on one farm when Cooperia spp. succeeded a previous Haemonchus dominance. Two farms had major build-ups of Haemonchus spp., one in early May, the other in June. Six farms salvage-drenched the standard five drench groups. The lambs in the standard drench groups on these farms recovered about half the bodyweight difference between August and March. The mean weight gains to July and wool weights of the modified drench programme groups were reduced slightly compared with the suppression-drenched group (mean liveweight difference 1.4 kg; mean wool weight difference 0.15 kg). No major rises in faecal egg counts occurred. Herbage larval counts exceeded 1000 larvae/kg at some stage on seven farms and, of these, the counts exceeded 4000 larvae/kg on three farms.