Productivity of Hereford, highgrade Simmental and Belmont Red beef herds in central Queensland. 3. Explanatory models for pregnancy rate

Abstract

Statistical models for pregnancy rate of Hereford, highgrade Simmental and Belmont Red cows were developed for brigalow pastures in central Queensland. Cows grazing improved pasture at 1 breeder/2 ha over 6 years, from 1979 to 1985, were mated to single sires for 78 days from early December each year and their calves weaned in May. Overall herd pregnancy rates were 74% Hereford, 69% highgrade Simmental and 86% Belmont Red. The 2-year-old maidens and non-lactating cows had higher rates than lactating cows for Hereford and highgrade Simmental. Mature, lactating and non-lactating cows had higher liveweights at the start of mating than first lactation cows and all were heavier than maidens. Pregnancy rate for maidens was not affected by liveweight, but 71% Hereford, 90% highgrade Simmental and 95% Belmont Red were above 300 kg at the start of mating. Response in pregnancy rate to liveweight at start of mating plateaued above 375 kg for first lactation Belmont Red and mature lactating Hereford; above 400 kg for mature lactating Belmont Red; and above 425 kg for mature lactating highgrade Simmental; but this liveweight was not important for first lactation Hereford and highgrade Simmental, nor for non-lactating cows. Mature cows calving in the first 6 weeks of the season had 13-30% higher pregnancy rates than those calving later. Previous lactation status did not affect pregnancy rate for highgrade Simmental and Belmont Red, but Hereford cows rearing a calf in the previous year had 14% higher pregnancy rates than those failing to rear a calf. Breed differences were apparent in the responses to the factors modelled. Pregnancy rates of lactating Belmont Red cows were sensitive to time of calving and liveweight at start of mating, even though these rates exceeded 80%. Mature, lactating highgrade Simmental were highly sensitive and Hereford intermediate, but first lactation and non-lactating Hereford and highgrade Simmental did not respond to these factors, even though pregnancy rates were low. Segregation of the breeding herd into high, intermediate and low expected fertility groups can be based on calving time and liveweight at start of mating. The high fertility group, consisting of cows calving in the first 6 weeks of the season and weighing greater than the plateau level, had advantages compared with the herd averages of 16 and 8% in fertility for first and mature lactating Belmont Red, respectively. Conversely, the low fertility group, consisting of late calving cows of low liveweight, had disadvantages of 30 and 18% for first and mature lactating Belmont Red. respectively.