Sorting Gate
Consider long-term effects of selection for $M and $W.
October 1, 2024
The American Angus Association’s maternal weaned calf value ($M) and weaned calf value ($W) indexes both focus on the same stage of the production cycle — gauging value differences from birth to weaning within a commercial cow-calf setting.
Both breeding objectives are defined as targeting a commercial cow herd that is replacing 20% of its females annually with heifers retained from its own supply. The remaining cull females and all male progeny are sold as feeder calves after weaning.
However, the long-term outcomes for your cow herd of selecting on one index vs. the other are quite different.
An evolution
The introduction of $M represented a significant evolution in how the Association approached maternal indexes. In 2018, the Association began revising and incorporating more traits into its maternal index to provide the most robust profitability predictions possible. The result was $M, which offered a more comprehensive and economically relevant tool compared to the traditional $W.
While the breeding objectives remain the same, $M includes a broader range of economically relevant traits, offering a more well-rounded assessment of profitability in a cow-calf operation. While $W strictly focuses on four traits (birth weight, weaning weight, maternal milk and mature cow size), $M replaces birth weight with calving ease. Additionally, $M includes traits for heifer pregnancy, foot conformation and docility, while still incorporating weaning weight, maternal milk and mature cow size. This broader range of traits leads to re-ranking individuals when comparing the two indexes.
Profitability for both indexes is focused on the total pounds of weaned calf. Like any profit function, the revenue from these pounds is linked to the costs involved. These costs cover feed resources for maintaining and breeding females, replacing breeding-age females, and any other direct costs of the calf up until weaning age.
Looking long term
The biggest difference producers notice between the two $Values is the focus on cost reduction for the cow-calf operation. Fig. 1 shows response to selection graphs for both $M and $W.
For $W, there is a strong emphasis on increasing weaning weight and maternal milk and as a result, mature cow size. While focusing solely on these traits will increase overall pounds of weaned calves, it doesn’t account for diminishing returns on maternal milk and only slightly addresses mature size related to feed costs.
Fig. 1: Response to selection on maternal weaned calf value ($M) vs. weaned calf value ($W), trait responses to 1 SD* of selection (~ 10 years)
Fig. 2: Top 200 registration sires: top 25% maternal weaned calf value ($M) performers vs. mature weight (MW) expected progeny difference (EPD)
In contrast, $M places greater emphasis on reducing the cost of the cow-calf operation, including feed resources. Over a 10-year selection period, $M aims to reduce costs by limiting cow size, introducing diminishing returns on maternal milk, and emphasizing other maternal traits while maintaining current weaning weight levels.
Mature weight is crucial for cost savings in cow-calf operations because larger cows have higher maintenance energy needs. Cows with higher requirements need more calories just to maintain themselves.
Top $M performers and MW EPDs
Take a look at Fig. 2. It shows the top 200 sires for progeny registrations who rank in the top 25% for $M. The top 25% $M performers are compared against their mature weight (MW) EPDs.
While there is a relationship between $M and MW EPDs, the top 25% of $M performers have MW EPDs that vary widely — some are above average, and some are below. For example, the second-highest $M bull in the top 200 has an above-average MW EPD. This is because, despite his larger size, which increases costs due to maintenance energy requirements, he compensates by creating daughters who can also wean off larger calves as seen by his top 2% ranking for weaning weight (WW) EPD.
Over the years, as new data has become available and genetic understanding has deepened, the conversation about supporting two tools with the same breeding objective has come to light. Is it time to retire $W? This conversation reflects growing recognition of the importance of a broader range of traits in driving profitability from conception to weaning.
While $M and $W both have served important roles in genetic selection, $M provides a more holistic approach by considering a wider array of traits and focusing on long-term maternal efficiency. As the cattle industry continues to evolve, tools like $M will be crucial in helping producers navigate the complexities of modern breeding strategies.
Editor’s note: “Sorting Gate” is a regular Angus Beef Bulletin column featuring herd improvement topics for commercial producers using Angus genetics. Kelli Retallick-Riley is president of AGI. For additional information on performance programs available through the American Angus Association and AGI, visit www.angus.org.
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