Summary
Tropical beef cattle in northern Australia face numerous challenges such as parasite infestation, hot climatic conditions and seasonal nutrition. To make this industry economically viable and environmentally friendly, it is essential to breed cattle that are fast growing, highly fertile, parasite and heat resistant. But to maintain the long-term sustainability and profitability of northern Australian beef cattle industry in the face of environmental concerns against use of chemicals to control parasites and competition from other livestock products, productivity must be increased with minimum input.
Productivity is a complex trait in northern Australian beef herds and encompasses components such as growth, tropical adaptation, and fertility and survival traits. No one breed excels in all the bioeconomic traits influencing the profit function in any livestock production system, more so in beef cattle as proven by many studies across various environments. Crossbreeding is one of the most useful tools to improve the performance of various bioeconomic traits by exploiting high levels of heterosis or hybrid vigour. However optimising the suitable crossbreeding system for any particular environment "market" management specifications depends on an understanding of the differences in performance of various breeds as heterosis depends on the genetic distance between contributing breeds in the traits of interest.
This crossbreeding study outlines the differential performance of various breeds of African, European and Indian origins in harsh tropical environments and quantifies the heterotic advantage gained through crossing breeds of distinctly different genetic background. This experiment was conducted during 1992-1997. Various crosses were pooled together into 31 genotypes derived from tropically adapted British (B), Sanga derived (S), Zebu cross (Zx), Zebu (Z) and Continental (C) beef cattle breed groups to compare their performance based on growth, adaptation, temperament, fertility, survival and carcass and meat quality traits. British cattle in this study were the Belmont Adaptaur (Hereford-Shorthorn cross), which is a tropically adapted line selected for tick resistance and 550-day liveweight. Hence it does not represent temperate British breeds. Belmont Red and Tuli constituted Sanga derived breeds and Brahman and Boran were grouped under Zebu. Charolais and Simmental constituted the sire breeds of Continental origin. Female fertility and calf survival traits were analysed as traits of the dam to study the differences in the parental breeds.
Major results from the study are: Growth traits Breed differences were significant and in general, crossbred calves performed significantly better than purebred calves. Calves out of Zebu dam breeds had lower birth weights, and Zebu sire breeds and Sanga derived dam breeds resulted in heavier birth weights. ZC (Zebu dam crossed with Continental sire), and ZC cross with Sanga and Zebu sires had heavier weights and higher weight gains until 18 months of age. Direct breed additive effects of C, Z and S expressed as a deviation from the British mean were high and significant for all the growth traits indicating their better realised growth than the British in this environment.
The magnitude of dominance effects, causing heterosis, was higher in taurine x indicine crosses e.g., Brahman cows x Belmont Adaptaur bulls, indicating the advantage of Bos taurus x Bos indicus crosses. Taurine x indicine crossbred dams provided the best maternal environment resulting in crossbred dams rearing bigger calves. The crossbreeding parameters estimated for growth traits are useful in developing prediction models for predicting the performance of untested genotypes in similar environments. The percent heterosis estimates were highest in Zebu x British crosses for growth traits (8 to 19%). Adaptive traits and temperament Zebu and its crosses had better parasite and heat resistance than taurine crosses. No clear breed differences were noticed in flight time, a measure of temperament. The better adaptability of Zebu and its crosses was evidenced by the significantly negative (favourable) breed additive component in all adaptive traits relative to the British. Sanga derived and tropically adapted British breeds had similar tick resistance but Sanga derived breeds had lower worm resistance than the British as evidenced by the additive genetic components.
Continental breed group tick resistance levels were also lower than the British. Tick, worm and heat resistance levels increased in crossbreds especially in taurine x indicine crosses due to the significant and favourable dominance effects. High and significant heterosis percentages were observed in Zebu x British crosses for all the adaptive traits. Favourable and significant heterosis percentages were also observed in Sanga x British crosses for tick resistance. Treatment to control ticks and worms significantly increased postweaning live weight gains in many genotypes. As the Bos indicus proportion in the genotype increased, the response to treatment in live weight gain reduced. Taking into consideration the economic cost it is prudent to breed parasite resistant taurine-indicine crosses rather than to treat against ticks and worms. Fertility and survival traits High scrotal circumference in British bulls and taurine crossbreds (British x Sanga) as opposed to Zebu and its crosses was noticed indicating potentially good semen characteristics and seminal volume of British bulls. Most of the heterosis in scrotal circumference was due to the heterosis in body weight.
Heterosis estimates were not significant when scrotal circumference was adjusted for body weight. Among purebreds, calving success was high in Belmont Adaptaur wet (lactating) cows and high in Brahman and Brahman cross in dry (non-lactating) cows and heifers. There was no concrete evidence of low fertility of Brahmans from the present study. In general, crossbreds had better calving success than purebreds especially in dry cows and heifers. There was also an advantage of crossbred dams over purebred dams in days to calving. Even though there was a crossbred advantage in fertility traits, heterosis was generally low and insignificant except in crosses between Brahman and Belmont Red. Preweaning calf survival proportions of crossbred dams were in general higher than those of the purebred dams. High mortalities in calves born to British dams from Zebu sires were noticed indicating the incidence of dystocia. Low and insignificant heterosis was observed in survival traits. Carcass quality traits The major observation of the results from carcass quality traits was that the tenderness of grilling and roasting cuts of meat from the predominantly Brahman-based beef herd of northern Australia can be improved through crossbreeding with taurine breeds.
There was no evidence of heterosis in any eating quality attribute. For both feedlot finished and pasture finished steers, striploin steaks from British steers were most tender, and Sanga and Zebu x Continental cross steaks were more tender than Zebu steaks. Evidence existed for a Brahman sire whose progeny had both high marbling scores and intra muscular fat. This signifies the possibility of farming tropically adapted high marbling crossbreds or composites targeting markets preferring high marbling. Knowledge of performance of various breed crosses and the parameter estimates obtained from this study can be used to develop models for predicting the performance of untested genotypes in similar environments.
They are also useful in deriving the optimal breed proportions in tropical beef composites. With the diversified target markets available for beef cattle trade, the importance of development of composites that can perform well under tropical conditions is ever growing. So to address such specific requirements, precision in the design of breeding programmes is crucial. This can be realised through the development of decision support software which can utilise the parameters obtained from this study and from other published literature and combine it with the economic weightings needed for various traits under different environments.
Besides helping in the development of these precise prediction models, the present study also allows us to draw some simple conclusions such as the higher benefits through heterosis from crossing taurine and indicine breeds as opposed to maintaining a single breed stock. Crossbreeding also helps to improve the traits that are not influenced by heterosis, such as meat quality characteristics, by producing genotypes that perform at an average value of the parental breeds.
