Replacement index a key tool, saysGrange study

The Teagasc maternal herd was established in 2012 to determine if the ICBF replacement index is a useful tool in identifying cows with superior maternal performance. A herd of 120 maiden heifers were sourced from both the suckler herd and beef cross heifers from the dairy herd. Both these groups contained a sub-group of high- and low-replacement-index heifers. The high group consisted of heifers with four or five stars for the replacement index and the low group consisted of heifers with one or two stars for the replacement index. Although each heifer was sourced on the basis of their overall replacement index, particular emphasis was placed on the maternal key profit traits, such as daughter calving difficulty, daughter milk and daughter calving interval. Only heifers sired by bulls with a reliability of greater than 70% for the replacement index were selected. Heifers were sired by either Angus (early maturing) or Limousin (late-maturing) bulls.

The objective of the maternal herd was to maximise the difference in replacement index within the high and low groups. However, this was not achieved in the dairy cross cows (Table 1) due to the dairy cow influence on the replacement index of this group. Because of this, differences in cow and calf performance cannot be detected between the two dairy cross groups. However, due to the large difference in the replacement index between the two suckler groups (Table 1), one should expect to see large differences in the maternal performance if the replacement index is true to purpose.

The first calves were born to the maternal herd in spring 2014 and to date maternal performance has been collected on 218 cows across four calving seasons. This has provided a large amount of data to evaluate the role of the replacement index on cow and calf performance.

Calving performance

Table 2 shows the performance of the four groups of cows. Both high- and low-replacement-index cows sourced from suckler herds had a similar calving date (mid-March), calf birth weight and level of assistance at calving. However, the high-replacement-index suckler cows had a 4% lower calf mortality at birth and a 9% lower overall mortality at weaning. In a 50-cow herd, this would equate to four or five fewer calves for sale at weaning with low-index suckler cows.

Fertility performance

The breeding season began in late April each year, with AI used for the first nine weeks and stock bulls introduced for a four-week period thereafter. As all progeny were destined for slaughter, the entire herd were mated to easy-calving, high-terminal-index bulls.

Across all groups, the age at first calving was 24 months. Across the four years there was a significant difference in the pregnancy rate between the high- and low-replacement-index suckler groups. On average, 86% of the high-index sucklers were scanned in calf, whereas only 77% of the low-index suckler cows were scanned in calf. This had a large knock-on effect on the overall cow survival rate between the suckler high and low groups. An additional 15% of low-index sucklers were culled when compared with the high-index suckler group. This would result in a higher replacement rate and therefore replacement costs associated with the low-index suckler group.

Calf Performance

Calf performance to slaughter is monitored on all calves born in the maternal herd. Milk yield was measured across all groups. Results show that from birth to weaning the milk yield in the high-index suckler cows was 600g per day greater than the low-index suckler cows. This resulted in an average weaning weight of 281kg for calves born to high-index suckler cows compared with 271kg from calves born to the low-index suckler cows.

All calves generated from the maternal herd are destined for slaughter. To date, 179 calves born in 2014 and 2015 have been slaughtered and these preliminary results show the progeny (heifers and steers) from suckler high-index cows were 607kg liveweight at slaughter compared with 588kg liveweight for the low-index progeny. This resulted in a 6kg difference in carcase weight recorded between the progeny from the high-index and low-index suckler cows. However, slaughter data from progeny born in 2016 and 2017 will also be included before final results on carcase performance are generated.

What’s next for the maternal herd?

By 2019, four years of data on both cow and calf traits will be available from the current maternal herd. This will provide a robust validation of the replacement index. However, a number of changes have taken place within the replacement index since the establishment of the maternal herd in 2012, most notably the advent of genomic selection in the beef herd. Therefore, a second phase of the maternal herd will commence in spring 2018 whereby replacement heifers will be sourced based on their genomic proofs for the replacement index.

Three separate groups will be formed: an elite suckler group (containing cows in the top 10% for replacement index), a national average suckler group (replacement index €55 to €73) and a dairy cross group (sourced irrespective of replacement index). The second project on the maternal herd will be phased into the existing herd over a number of years and detailed measurements of key performance traits will be collected.

Results from the last four years of data generated from the maternal herd has shown that large differences existed between high- and low-replacement-index suckler cows on some key performance traits such as calf mortality, milk yield, reproductive performance and cow survival.

Preliminary economic analysis indicates this would result in an advantage of €110 per year in favour of the high-replacement-index suckler cow compared to a low-replacement-index cow sourced from a suckler herd.

An interesting point to note is that the weaning weight from the low dairy cross cows was 9kg ahead of the high suckler cows and this is why the next phase of the project is to be welcomed.Genomic proofs should minimise fluctuations in indices of animals selected in the next phase.

Farmers should follow this trial closely and take note of results when making breeding decisions on their farms. Farmers have invested a lot of time and money in altering breeding programmes any they need to be sure that this is the right path on which to proceed.