The Angus Maternal Breeder (AMB) and Angus Maternal Breeder Plus (AMB+) indexes estimate the genetic differences between animals in net profitability per cow joined in a typical commercial self replacing herd using Angus bulls.
These selection indexes are not specific to a particular production system or market end-point, but identify animals that will improve overall profitability in the majority of commercial, self replacing, grass and grain finishing beef production systems. Daughters are retained for breeding and therefore female traits are of importance.
The AMB+ index is similar to the AMB index, but includes an additional objective of maintaining mature cow weight, whereas the AMB index does not aim to limit the increase in mature cow weight.
Figure 1 shows the traits that are considered in the AMB & AMB+ indexes, and how much they contribute to the overall balance of each selection index. The larger the segment, the greater the impact on the selection index.
In both the AMB & AMB+ indexes, there is a focus on improving growth, carcase, calving ease and fertility traits. The main difference is the increased emphasis on mature cow weight in the AMB+ index, whereas this emphasis is given to calving ease and fertility in the AMB index.
Figures 2 & 3 show the selection advantage if animals are selected using the AMB & AMB+ indexes.
The selection advantage is calculated by ranking well used sires within the Angus breed on each respective selection index, and comparing the average EBVs of the sires in the highest 10% with the average EBVs of all sires from which they were selected. For example, the sires ranked in the highest 10% based on the AMB index had 18 kg higher 400 Day Weight EBVs and 0.6 kg lower Birth Weight EBVs than the average EBVs of the sires from which they were selected.
The selection advantage is indicative of the long term direction and relativity of response that will occur in individual traits if selection is based on each respective selection index. The actual response that is observed will vary depending on the features of the individual breeding program.
The main difference in the selection advantage between the AMB & AMB+ indexes is in the growth traits and mature cow weight. Animals ranking highly on the AMB index have higher growth EBVs relative to the AMB+ index, meaning selection on the AMB index will result in a higher increase in growth to 200, 400 and 600 days of age than selection on the AMB+ index, however this is also associated with higher mature cow weight EBVs, meaning an increase in mature weight will also be observed. By comparison, the selection advantage of the AMB+ index for mature cow weight is close to 0, meaning that selection on this index will maintain mature cow weight, while still increasing growth to 200, 400 & 600 days of age.