Abstract:
The effect of natural infestations of internal parasites (Cooperia species, Haemonchuspbcei, Oesophagostomum radiatum, and Trichostrongylus axei) and the cattle tick Boophilus microplus on the growth rate from weaning to 21 months of age of 93 F3 Africander x Hereford and Shorthorn (AX), Brahman x Hereford and Shorthorn (BX), and Hereford x Shorthorn (British) was studied. All animals were weighed, ticks counted, faeces sampled, and the animals treated at fortnightly intervals. Faecal egg counts were used as an index of internal parasite burdens, and tick infestations assessed according to the number of females between 0.45 and 0.8 cm in length on one complete side of the animal. Breed differences for the internal parasitic burdens were small and were only significant (P<. 0.05) for O. radiatum. The BX cattle carried the highest burdens. The BX cattle were significantly more resistant to tick infestations than the AX group, which in turn were significantly more resistant than the British group. Males carried significantly more ticks than females. Parasites had little or no effect on gains while the animals were gaining weight rapidly, and it was only after a cessation of gains due to deteriorating pasture conditions that significant treatment effects became evident. Treatment had the largest effect on the British and very little effect on the BX group. Regressions of gains on parasitic burdens within treatments were all negative. The repeatabilities of a series of egg counts for the species were significant, Haemonchus egg counts having the highest repeatabilities. Correlations between egg counts of Cooperia spp and 0. radiatum, and H. placei and T. axei were significant (r = 0.82, P < 0.001; and r = 0.23, P < 0.05). Internal parasites significantly depressed packed red cell volume, haemoglobin concentration, and plasma protein levels. There were significant negative correlations of egg counts of O. radiatum with haemoglobin concentration and packed cell volume of the Zebu crossbreds (r = 0.46 and v = 0.47, P < 0.01).