Using genetics to control cattle parasites—the Rockhampton experience
Introduction
The use of agricultural chemicals and their impact on food safety and the general environment have never before been subjected to such intense public scrutiny. The need to increase productivity of agricultural enterprises, no longer to feed a hungry world but to survive the effects of competition, has never been greater. The ability to reduce costs, increase market flexibility, and provide increasingly critical consumers with the product they demand are all essential components of the drive towards increased productivity. The northern Australian beef business is not exempt from these requirements. In parasite infested areas of the region, parasites must be controlled if productivity is to be increased. Many of the public concerns could be allayed, costs of control could be contained and the certainty of breeding chemical-resistant parasites could be removed if parasite control was achieved through the use of cattle that were inherently highly resistant to parasites. Over the last few decades, Brahmans have played an important and essential part in this role. Of the breeds evaluated at the National Cattle Breeding Station, ‘Belmont’, for resistance to cattle ticks (Boophilus microplus) and the tropical species of worms present at ‘Belmont’ (mainly Cooperia, Haemonchus, Oesophagostomum and Trichostrongylus spp.), the Brahman has the highest overall resistance [1]. The Brahman is now the dominant breed in northern Australia and the principal control agent for cattle ticks and worms. Nevertheless, Brahmans do not meet all market requirements, are not totally resistant to the parasites present in northern Australia, and are inherently less productive than F1(Brahman x taurus) crossbreeds [2].
Potentially, these problems could be overcome by selecting for increased overall resistance, improved meat quality, higher reproductive potential and improvements in other characteristics in which the Brahman is deficient. Such an approach is however far more complex and the rate of improvement far slower (and therefore commercially less attractive) than the immediate improvement achieved by crossing the Brahman to a taurine breed in which the desired characteristics are already well developed. The principal drawback to this approach is that if a European (British or Continental) breed is used as the taurine component of the cross, overall parasite resistance of the resulting F1 is lower than that of the Brahman. Under conditions of high parasite challenge, the F1 is then more affected than is the Brahman [1].
All of these issues could be resolved by crossing the Brahman with a highly parasite-resistant European breed. Unfortunately, compared with the Brahman, all of the European breeds are lowly resistant. This affects not only the resistance of F1(Brahman x European) crossbreeds, but also the ability of straight-bred European bulls to survive and sire acceptable numbers of F1 progeny in parasite infested regions. Adult cattle are generally highly resistant to the gastrointestinal nematodes present in northern Australia. As far as parasites are concerned, the most important consideration for European breed bulls destined for use in northern Australia is then their resistance to cattle ticks, which can cause morbidity or death, and to a lesser extent, buffalo flies (Haematobia irritans exigua), which can cause extreme irritation.
It was these considerations that led in 1980 to the intensification of selection for high tick resistance in the HS line at the National Cattle Breeding Station, ‘Belmont’, near Rockhampton. The HS (Hereford x Shorthorn) line occupied a unique position among cattle breeds. It was one of the few populations of solely British origins remaining in the tropics anywhere in the world and since it was on a research station, there was no immediate need to generate a commercially viable level of revenue from the line. Its evolutionary history was such that it had the potential to yield major genes for tick resistance that could then be introgressed into other breeds [3]. The existence of a major anti-tick gene in the line was later verified [4], and the effect of introgressing the gene into both taurine and indicine breeds was reported [3]. The study itself provided a clear demonstration of the outcome of sustained, long-term selection for high tick resistance. In addition, success in achieving the goal of very high, or preferably total, tick-resistance would lay the base for a potentially valuable commercial product that could then be utilised to fund further development of the line.
Unfortunately, outside forces intervened before the goal was achieved. Nevertheless, considerable progress has been made, the line has been partially commercialised as the Belmont Adaptaur and a breed Association formed [5]. Progress in selection towards total resistance to ticks is reported here.
A complementary approach to selecting within the HS line for increased tick resistance was the search for other taurine breeds that already possessed inherently high resistance but that also had other advantageous attributes that were not well developed in either the Brahman or in the European breeds. This led to the introduction of the Tuli and the evaluation of the Tuli, Belmont Red and other breeds for their potential role in increasing the productivity of the northern Australian herd through crossbreeding with Brahmans. Both the Tuli and Belmont Red are taurine breeds of sanga derivation. Until recently, commercial beef producers have generally ignored sanga breeds in favour of European breeds for crossbreeding in northern Australia. However, the need to not only improve meat quality characteristics and increase productivity but also maintain the high resistance of the Brahman has sparked renewed interest in crossbreeding between Brahmans and well-adapted taurine breeds. Results from an extensive crossbreeding study conducted at ‘Belmont’ show that this interest is justified [3]. Further results are reported here.
Section snippets
Materials and methods
The foundations of the HS line were laid in the 1950s when reciprocal crosses were made at the National Cattle Breeding Station, ‘Belmont’, between Herefords and Shorthorns, the two breeds that at the time dominated beef production in northern Australia. The reciprocal crosses were interbred to form a line that was nominally 50%Hereford50%Shorthorn. Until the late 1970s, the line was used solely for experimental purposes. It was maintained at about 100 breeding females replaced at about
Results and discussion
Fig. 1 shows the genetic trends in the HS line for tick counts and worm egg counts from 1983 to 1998.
Over the entire period, tick count declined almost linearly (R2 = 0.93) at a mean rate of 7 ticks/year from a mean of 275 ticks/animal/day at the start of selection in 1983, to about 40 ticks/animal/day in 1998. Worm egg count declined very slowly at a mean rate of 2 eggs/year. However, the decline was not statistically significant (R2 = 0.09).
Comparison of the differences in the responses for
Conclusions
The economic advantage of controlling parasites of cattle in tropical regions is very dependent on both the production system and the genotypes used. As the push continues to increase productivity by reducing the age at first joining and at slaughter, the advantages of minimizing the detrimental effects of parasites will increase. In the extensive pastoral regions of northern Australia, the use of breeds that have inherently high parasite resistance makes sound economic sense and avoids many of
Acknowledgements
We gratefully acknowledge the assistance of staff at the Tropical Beef Centre, particularly A. Day, J. Quilty, G. Halford and G. Weldon for care of the animals and for assistance with data collection. M. D’Occhio and T. Whyte are thanked for progesterone assays. A major part of funding for the various projects involved was provided by the Meat Research Corporation through ownership of ‘Belmont’ and through provision of a grant that enabled the studies to proceed.
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