Beef cattle research overview

A Beef Co-operative Research Centre (Beef CRC) was tasked with looking at whether recent pursuit of “genetic improvement in feedlot and abattoir performance of cattle” could lead to “a decline in maternal productivity (of breeding herds), especially under variable nutritional conditions”¹. Therefore a Beef CRC Maternal Productivity Project (Beef CRC ‘project’) with Angus (Bos taurus) cattle was undertaken over five years in southern Australia.

In the Beef CRC ‘project’, maternal productivity was defined as output (calf plus cow) versus input (feed intake), as measured by:

• weight of calf-weaned,
• cow weight change,
• energy intake, and
• progeny post-weaning production².

Figure 1. Major driving forces for Maternal Productivity

The Beef CRC ‘project’ noted that the balancing of cattle requirements for maternal productivity and feedlot or abattoir performance “is not straight forward, (but) is of critical importance”³. The researchers believe that producers should target management of “heifer pregnancy rates and interactions between the cow herd and (a variable) nutritional environment”3.

Suggestions for how to do this focused on heifer and cow genetics, in particular, estimated breeding values (EBVs) for rib fat and residual feed intake. The latter is an indicator of efficiency in feed conversion, and is defined as the difference between actual post-weaning feed consumption and the expected feed requirement for growth and maintenance.

Heifers with a higher EBV for rib fat had “significant (higher) pregnancy rates”⁴. Cows with higher EBVs for rib fat and intramuscular fat were more likely to be “earlier calving cows”⁵. This highlighted the importance of a heritable (genotypic) capacity to store feed energy as body fat. Similar to observations for physically (phenotypically) small-framed cows with lower feed maintenance requirements that allow them to store more feed energy as body fat, increased genotypic capacity to store body fat is perceived to provide a resilience for maternal productivity under variable seasonal conditions.

Cows with a lower EBV for residual feed intake had a lower weaning rate and delayed calving, but produced “heavier calves at weaning”⁵ and were “more efficient at producing weaner calves”⁶ under the range of ‘normal’ seasonal conditions in the Beef CRC ‘project’. This may sound counter-intuitive, but if the weaning rate is only slightly lower while the weaning weight of each calf is much higher, the average calf-weight weaned per breeder cow can still be higher. This latter finding illustrates the challenge in design of a breeding program that provides a regional balance between EBVs for improved reproduction (to increase the number of progeny), versus high growth (to increase the weight of individual progeny).

With regards to heifer fertility, the Beef CRC ‘project’ showed that “weight and fat depth were the largest contributing factors to variation in (heifer) pregnancy rates”⁴. The following example from the department’s Droughtmaster cattle at the Arid Zone Research Institute (AZRI sentinel heifers) illustrates the importance of managing nutrition for heifer fertility, i.e. to reach a target weight for mating, with moderate body condition to promote re-conception after calving.

Between 2013 and 2016, supplementary feeding of pellets with coccidiostat at weaning, enabled the AZRI sentinel heifers to sustain steady post-weaning growth, averaging 0.5 kg per head per day in the first 12 months and meeting a target average critical mating weight of 300 kg at 12 to 14 months of age. For these moderate frame-sized breeders, this target is within the recommendations reviewed for the Beef CRC ‘project’ (56 to 66% of mature cow weight ⁴). Controlled mating commenced in early December and after three months, resulted in pregnancy percentages ranging from 87 to 100% (see Table 1 ).

Table 1 . Summary of annual outcomes for management of AZRI sentinel heifer fertility

In 2017, supplementary feeding of pellets with coccidiostat was withheld at weaning and the heifers only grew at an average of 0.29 kg per head per day for four months post-weaning. They failed to meet the target average critical mating weight in time for controlled mating over summer 2017-18. At assessment in May 2018, these heifers were within the expected hip height range-for-age (medium frame), but the sub-optimal median body condition (fat) score of 2.5 (on scale of 1 to 5) and the absence of prospective calf production in 2018, is a legacy of their previous slow post-weaning growth.

Department research in the Top End⁷ has previously modelled the relationship between pre-mating weight and pregnancy percentage in yearling and two year old Brahman heifers, where pre-mating weight was recorded in either October or December (see Figure 2). In each case, pregnancy rates were higher with higher pre-mating weights. The average weight at conception for the yearling heifer groups that were used for the modelling, ranged from 300 to 333 kg. This weight range represented reproductive success (conception) in up to 36% of the control-mated heifers. In comparison, the slightly wider average weight range (304 to 343 kg) that was reported pre-mating for the control-mated AZRI Sentinel heifers, resulted in reproductive success in over 80% of heifers. So although those two weight ranges provide slightly different retrospective views of the interaction between management, nutrition and heifer fertility in the NT, they both emphasize the importance of weight targets over 300 kg for reproductive efficiency in moderate framed Bos indicus or Bos indicus-infused heifers.