THE EFFECTS OF THE BONT TICK, AMBLYOMMA-HEBRAEUM, ON MILK-PRODUCTION OF SANGA AND SANGA X BRAHMAN CATTLE

The effects of adults of the bent tick, Amblyomma hebraeum on the milk production of Sanga and Sanga x zebu (Brahman) cattle were measured o ver a period of 11 weeks in the lowveld of Zimbabwe in the summer of 1 986. Four groups of lactating cows, consisting of two breeds, each div ided into a high and low tick treatment, were exposed to very low or h igh challenges of ticks and their milk production measured by weighing their calves before and after suckling. The liveweight gains (LWG) of the calves were also measured. Tick burdens on the infested groups av eraged around fourteen engorging females of A.hebraeum per day, which amounted to infestations of about 150 adult ticks. That is greater tha n most observed field infestations. This caused no significant reducti on in milk yield or calf growth over the whole period, provided the te ats of the dams had not been damaged by ticks. Mismothering occurred w hen teats were damaged. No breed differences were observed so all data was pooled for further analysis. Average calf LWGs of the high tick g roups were reduced by 2.2 kg (P < 0.01) during one 4-week period but o verall the 3.9 kg difference in LWG of the tick treatment groups was n ot quite significant (P < 0.10). Although there was a poor relationshi p between tick numbers and reduced milk yield or calf LWG, the effects were always in the direction expected. The effects averaged 6 +/- 10 g reduction of milk and 2.6 +/- 1.8 g loss of LWG of calves for every female tick that engorged. It was concluded that milk production is no t an important consideration when estimating the losses in production caused by A. hebraeum on Brahman x Sanga or Sanga breeds of cattle. Lo sses due to tear or udder damage could be much more important and need to be quantified.