A clearer understanding of the relationship between Rib Fat EBV and fertility was an outcome from Beef CRC research.
Research from the Beef CRC’s Maternal Productivity Program, led by Associate Professor Wayne Pitchford, has shed more light on the association between BREEDPLAN Rib Fat EBV and cow reproductive rate.
Rib Fat EBVs are estimates of the genetic differences between animals in fat depth at 12/13th rib site in a 300kg carcase. They are significant to beef producers because of their impact on fatness and meat yield.
Stock with positive Rib Fat EBVs are likely to produce progeny that are fatter and yield less on average, than stock with lower or negative Rib Fat EBVs.
The research program aimed to address the concerns of some producers that breeding cattle to produce higher yielding, leaner carcases by selecting negative Rump and Rib Fat EBVs may be compromising the ability of cows to maintain sufficient body condition for timely re-joining, especially during tough seasons or when nutrition is limited.
“To understand the impact that Rib Fat EBV has on reproductive rate, an intensive research station project was held over four years involving 391 Angus heifers specifically selected for divergence in BREEDPLAN Rib Fat EBV,” Wayne said.
“The heifers were divided into two groups; a high rib fat line, which had an average Rib Fat EBV of +1.0mm (compared to the 2010 Angus breed average of -0.1mm) while the low rib fat line had an average Rib Fat EBV of -1.3mm. The two groups represented approximately the top and bottom 10% of the Angus breed.”
The heifers were monitored for reproductive performance at two sites, Struan Research Centre,’ South Australia, and Vasse Research Station, Western Australia. At each site heifers were managed together so any differences in reproductive rate could be shown to be genetic.
Interpreting the data
“One of the most important outcomes was the large difference in pregnancy rates between the two groups of heifers,” Wayne said.
“The high Rib Fat EBV heifers had 8% higher pregnancy rate than low Rib Fat EBV heifers after a nine-week joining (91% vs. 83%).
“Under the increased pressure of a six-week heifer joining, the difference was greater, with a 13% variation in pregnancy rate between the lines (78% vs. 65%). Although the difference in actual fat depth between the lines was only 0.9mm (4.5mm vs. 3.6mm), this equated to a 20% difference in rib fat depth, and was associated with the large difference in heifer pregnancy rate.”
Wayne said it followed that as pre-joining rib fat depth increased up to 10mm, so did pregnancy rates with the greatest increases observed for leaner heifers with low levels of pre-joining rib fat depth.
“An increase in pre-joining rib fat depth from 2mm to 3mm was associated with a 6% increase in pregnancy rate while an increase from 8mm to 9mm was associated with less than 1% increase in pregnancy rate,” he said.
“For the first time we’ve been able to show that heifers with a very negative Rib Fat EBV (bottom 10%) have a greater likelihood of reduced pregnancy rate at their first joining.”
Next time around
However, differences in reproductive rate between the lines on their second and subsequent calves were much smaller. If dry heifers were culled based on pregnancy test, observed reproductive rates over four calving opportunities were:
•High Rib Fat EBV: 3.6 calves over 4 joinings; 82% of cows were pregnancy tested in calf in 4 successive years
•Low Rib Fat EBV: 3.5 calves over 4 joinings; 79% of cows were pregnancy tested in calf in 4 successive years
This finding has important implications; if producers cull based on heifer a pregnancy test, subsequent differences in cow reproductive rate between high and low Rib Fat EBV cows are small.
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