The effect of weight and age on pregnancy rates in Brahman heifers in northern Australia

Abstract

The performance of Brahman heifers at their first mating as yearlings (4-year groups of ~100 per year) grazing on improved pasture in the Douglas Daly region and as 2 year olds (3-year groups of ~100 per year) grazing on native pasture in the Victoria River District was studied. The relationships between pre-mating weight and pregnancy rate were established for Brahman heifers mated as yearlings on improved pasture, and as 2 year olds on native pasture with pre-mating weights recorded in late October/early November (before the wet season starts) and in late December (just before the start of mating). These relationships were found to be different indicating that there is an interaction between age and weight that modifies the effect of weight. The relationships were used to model pregnancy rates that are likely to result from different pre-mating weights for these three scenarios. These estimates can be used to identify target mating weights for different situations, and to predict the pregnancy rates for groups of heifers, which will be useful in budgeting and assessing the profitability of different management strategies.

Growth and reproductive performance responses to post-weaning supplementation of early and normally-weaned Brahman crossbred heifers raised in tropical rangelands

“…Overall, the parameter that explained the greatest variation for occurrence of pregnancy was PM.LW, which was, in general, greater for the NW compared with EW heifers. This is consistent with previous studies which have found that the most important factor affecting pregnancy rates of maiden heifers is PM.LW [ 35 , 36 ]. This outlines the long-term consequence of first dry season supplementation and WW on pregnancy percentage, a practical objective of this experiment.…”

“…It suggests that two-year-old heifers need to have PM.LW of approximately 300 kg in October/November to achieve approximately 80% probability of pregnancy (67.1–90.3% for a 95% confidence interval), regardless of WW. Schatz and Hearnden [ 35 ] developed a similar model using data collected at the same location as this experiment. The authors proposed a target LW (curfew) of 253 kg measured in October/November would result in an 80% of pregnancy for two-year-old heifers.…”

Modelling the reproductive performance of tropical beef herds using long-term experimental grazing data on Urochloa humidicola pastures in the Llanos of Colombia

“…Similarly, age, if available, interacts with LW as shown in Figure 2 and Tables 1 and 4 early in reproductive life, whereas later reproduction performance during the 3 rd to 5 th parities is associated with fairly constant LWs (Table 5 and Vera et al, 1993) if the nutritional strategy persists. The current results are supported by data from very different, but still extensive, cattle systems of northern Australia (Schatz et al, 2011;Schatz and Hearnden, 2017) that were derived from several thousands of records pertaining to breeding heifers and cows in different physiological conditions that were also included for comparison in Figure 3. It should be noted that the "maiden" heifers of Schatz et al (2017) were at least one year older than their yearling heifers and showed much larger pregnancy rates at similar LW, thus adding support to the effect of the LW by Age interaction reported in the current results.…”

“…The current results are supported by data from very different, but still extensive, cattle systems of northern Australia (Schatz et al, 2011;Schatz and Hearnden, 2017) that were derived from several thousands of records pertaining to breeding heifers and cows in different physiological conditions that were also included for comparison in Figure 3. It should be noted that the "maiden" heifers of Schatz et al (2017) were at least one year older than their yearling heifers and showed much larger pregnancy rates at similar LW, thus adding support to the effect of the LW by Age interaction reported in the current results. The coincidence between our estimates and that of Schatz et al (2017) with their yearling heifers is remarkable (Figure 3).…”

Compensatory and catch-up growth in cattle

“…It suggests that two-year-old heifers need to weigh approximately 300 kg at PM.LW in October/November to achieve 80% of pregnancy percentage, regardless of weaning weight. Schatz and Hearnden (2016) developed a similar model using data collected at the same location of this experiment. The authors proposed a target LW (curfew) of 253 kg measured in October/November would result in 80% conception rates for twoyear-old heifers.…”