Reproduction records from 2137 cows first mated at 2 years of age and recorded through to 8.5 years of age were used to study the genetics of early and lifetime reproductive performance from two genotypes (1020 Brahman and 1117 Tropical Composite) in tropical Australian production systems. Regular ultrasound scanning of the reproductive tract, coupled with full recording of mating, calving and weaning histories, allowed a comprehensive evaluation of a range of reproductive traits. Results showed components traits of early reproductive performance had moderate to high heritabilities, especially in Brahmans. The heritability of lactation anoestrous interval in 3-year-old cows was 0.51 ± 0.18 and 0.26 ± 0.11 for Brahman and Tropical Composite, respectively. Heritabilities of binary reproductive output traits (conception rate, pregnancy rate, calving rate and weaning rate) from first and second matings were generally moderate to high on the underlying scale. Estimates ranged from 0.15 to 0.69 in Brahman and 0.15 to 0.34 in Tropical Composite, but were considerably lower when expressed on the observed scale, particularly for those traits with high mean levels. Heritabilities of lifetime reproduction traits were low, with estimates of 0.11 ± 0.06 and 0.07 ± 0.06 for lifetime annual weaning rate in Brahman and Tropical Composite, respectively. Significant differences in mean reproductive performance were observed between the two genotypes, especially for traits associated with anoestrus in first-lactation cows. Genetic correlations between early-in-life reproductive measures and lifetime reproduction traits were moderate to high. Genetic correlations between lactation anoestrous interval and lifetime annual weaning rate were –0.62 ± 0.24 in Brahman and –0.87 ± 0.32 in Tropical Composite. The results emphasise the substantial opportunity that exists to genetically improve weaning rates in tropical beef cattle breeds by focusing recording and selection on early-in-life female reproduction traits, particularly in Brahman for traits associated with lactation anoestrus.

Eighty-four calves were weaned at five weeks of age on to barley-based pellets containing 0, 2, or 4 per cent of mixed buffers. An additional treatment was to provide access or no access to straw chaff. Growth rates from six to twelve weeks of age were 0.94 and 0.71 kg day- 1 for calves with and without access to straw chaff. With 0, 2, and 4 per cent buffers in the pellets the growth rates after weaning were 0.79,1.00, and 1.02 kg day-1 respectively for calves eating chaff and 0.66,0.71 and 0.77 kg day-1 respectively for calves without access to chaff. There were two deaths due to bloat at each of the 0 and 2 per cent levels of buffers when chaff was not available. The responses to buffers and roughage are discussed in relation to saliva production and rumen function.

Eighty-four calves were weaned at five weeks of age on to barley-based pellets containing 0, 15 or 30 per cent oat pollards. An additional treatment was to provide access or no access to long straw. Growth rates from six to ten weeks were 0.75 and 0.50 kg day-1 for calves with and without access to long straw, respectively (P < 0.001 ). As the content of oat pollards in the pellets increased, pellet intake increased so that growth rates were maintained when calves had access to long straw, and increased when long straw was not available. However, the highest growth rate of calves without long straw was still 0.14 kg day-1 lower than the lowest average growth rates of calves with access to long straw. These results are discussed in relation to the buffering capacity of the rumen in young calves.

No abstract