Cattle breeds of African, European and Indian origins are being evaluated at Rockhampton for their suitability for beef production in northern Australia. In the current study, Belmont Adaptaur (HS), Belmont Red (AX) and Belmont BX (BX) dams were mated to produce straightbreds, and crossbred progeny by Brahman (B), Boron (Bo) and Tuli (Tu) sires. B dams were mated to produce straightbreds, and crossbred progeny by AX, BX, Bo, Charolais (Ch), HS and Tu sires. All animals were reared together in the presence of cattle ticks and gastrointestinal nematodes (‘worms’). Over a 10-month period, half of each genotype was treated every 3 weeks to control ticks and worms. The resistance of each genotype to these parasites was estimated from direct counts of maturing female ticks (‘tick counts’) and from a combination of worm eggs in fresh faeces (‘worm egg counts’) and response to treatment to control the parasites.

The ranking of the sire breeds for tick resistance was B, Bo > BX > AX, HS, Tu > Ch and for worm resistance was B > Bo, BX > AX, Ch, HS, Tu. Only the B × BX and B × Bo could match the resistance of the B to ticks and worms combined.

All genotypes responded to treatment to control parasites but at the low to moderate levels of infestation recorded throughout the study the response of the more resistant genotypes was too low to warrant the cost of treatment. However, the ranking of the genotypes for live-weight gains changed with parasite challenge. Progeny by B and by Bo bulls from each of the dam breeds had higher live-weight gains than the corresponding straightbreds irrespective of the level of parasite challenge but gains of Tu-sired progeny exceeded those of the corresponding straightbreds only at lower levels of parasite challenge. The difference in resistance between the B and the more resistant F2s was small and even at twice the parasite challenge experienced throughout the study, the Fts would still be expected to outgain the B. For those genotypes for which it was measured, heterosis was consistently significant for live-weight gains and for tick counts but not for worm egg counts. Heterosis for live-weight gain was consistently higher for control than for treated groups. Potential roles of the different breeds for crossbreeding is discussed in relation to the effects of different levels of parasite challenge on growth rates of their progeny.