Angus Australia


Maternal Heavy Grain Steer Indexes (MGN, MGN+)

The Maternal Heavy Grain Steer (MGN) and Maternal Heavy Grain Steer Plus (MGN+) indexes estimate the genetic differences between animals in net profitability per cow joined in a commercial self replacing herd targeting pasture grown steers with a 250 day feedlot finishing period for the grain fed high quality, highly marbled markets.

Daughters are retained for breeding and therefore female traits are of importance.

Steers are assumed to be slaughtered at 800 kg live weight (455 kg carcase weight with 30 mm P8 fat depth) at 24 months of age, with a significant premium for steers that exhibit superior marbling.

The MGN+ index is similar to the MGN index, but includes an additional objective of maintaining mature cow weight, whereas the MGN index does not aim to limit the increase in mature cow weight.

Figure 1 shows the traits that are considered in the MGN & MGN+ 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 MGN & MGN+ 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 MGN+ index, whereas this emphasis is given to calving ease and fertility in the MGN index.

Figures 2 & 3 show the selection advantage if animals are selected using the MGN & MGN+ indexes.

The selection advantage is calculated by ranking well used sires within the Angus breed on the 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 MGN index had 16 kg higher 400 Day Weight EBVs and 0.7 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 MGN & MGN+ indexes is in the growth  traits and mature cow weight. Animals ranking highly on the MGN index have higher growth EBVs relative to the MGN+ index, meaning selection on the MGN index will result in a higher increase in growth to 200, 400 and 600 days of age than selection on the MGN+ 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 MGN+ 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.