The author is an associate professor of dairy cattle genetics at Penn State University.

Many breeders believe that today's cows lack the strength needed to withstand the rigors of high production. In recognition of the need to emphasize a balance between high yield and cow fitness, a new category called Dairy Strength was added to the PDCA (Purebred Dairy Cattle Association) scorecard in 2009. The intent was to shift emphasis away from ultra-thin dairy cows and toward a balance of dairy character with strength.

While many assume that selecting for strength will result in more robust cows, genetic research has failed to document evidence that selecting for higher strength will be beneficial. Studies have compared strength scores to herd life, fertility and disease incidence. Strength scores are rarely correlated in a favorable manner with these measures of health and fitness, and are often correlated in an unfavorable direction.

Despite research to the contrary, I believe dairy producers are not mistaken about a general lack of strength in some of our dairy cattle breeds. However, the manner in which we describe and evaluate strength leads to some disconnect between the results we obtain when selecting for strength and what we hope to achieve.

A deeper look
I've summarized evaluations for about 8,300 Holstein bulls born between 1995 and 2005 that have at least 100 daughters in their productive life PTA (predicted transmitting ability) in order to demonstrate the relationship of strength to herd life. The correlation between genetic evaluations for strength and productive life is -0.21, which is unfavorable. Correlations of strength with stature (0.71) and body depth (0.89) are very strong. In other words, selecting for more "strength" will result in larger cows but will not result in healthier daughters that are destined to stay in the herd longer.

Based on that information alone, we might conclude that we do not need stronger cows; however, a different picture emerges if we begin to evaluate strength after accounting for differences in body depth. I've stratified our bulls into three groups.

The first group consists of about 1,600 bulls with strength evaluations that are at least 0.5 points lower than their body depth evaluations. We might consider their daughters to be frail relative to their body depth, and I will refer to the group as "relatively frail." The second group consists of about 1,000 bulls with strength scores at least 0.5 points higher than body depth, or whose daughters I will refer to as "relatively strong." The third group is the remaining bulls who have similar strength and body depth score (within 0.5 points of each other).

The average productive life PTAs for bulls with relatively frail or relatively strong daughters are shown in Figure 1. The values shown in the chart are compared to the third group of bulls that had similar strength and body depth scores.

Productive life differences among Holstein bulls with contrasting strength and body depth evaluations

As you can see, bulls with relatively high strength scores had daughters that lived longer. The relatively strong group also had more favorable values for daughter pregnancy rate, cow conception rate, heifer conception rate and somatic cell score. The opposite effects were true for bulls in the relatively frail category. So, yes, strong cows are more robust. The challenge is one of determining which sires have daughters that are truly "strong" rather than just big.

Productive life PTA differences for bulls with tall, short and high yielding daughters

The second figure breaks down our strength versus body depth comparison more fully by demonstrating productive life PTA for bulls with very tall daughters (stature greater than 2), very short daughters (stature less than -2) and daughters with high milk yield (PTA milk greater than 1,000 pounds).

You can see that bulls with tall daughters have below-average productive life PTA, in general. Nevertheless, our relationship holds and the combination of tall daughters and relatively frail is highly unfavorable in regards to productive life.

The comparison of the tall, relatively frail group to the short and relatively strong group is informative. The average strength score for the tall, relatively frail group is +1.06, and their body depth score is +1.88; this compares with averages of -1.03 for strength and -1.94 for body depth in the short, relatively strong group.

This helps to emphasize the point that we should not evaluate a bull's strength score without considering how it relates to body depth. Small cows are consistently given a low strength score during classification, and this shows up in their sire's genetic evaluation. The short and relatively strong group had strength scores that exceeded their body depth score by 0.91 points. In other words, they were strong for their size.

These results also suggest to me that we need to be very careful how we interpret the meaning of "Dairy Strength." My primary concern is that cows we evaluate as having dairy strength are often just ultra-thin cows with deep and wide skeletons. This will push us further toward the tall and relatively frail group that is so unfavorable for fertility and longevity.

Same for other breeds
While I've presented results for Holsteins in the figures, the same general relationship is true for other breeds. Jersey and Brown Swiss genetic evaluations do not report body depth scores, but bulls with extremely high stature and low strength tend to have lower productive life evaluations in both breeds.

I want to emphasize that I'm not suggesting that everyone should start pouring through bull catalogs comparing strength to body depth scores in order to breed for more robust cows. While the figures show that there is a general relationship among strength, body depth and longevity, there are also many important exceptions. You will make more progress selecting directly for longevity and fertility than trying to improve such traits indirectly.

My goal is to provide a better understanding of what selection for "strength" will achieve. Those breeders who are trying to improve strength need to avoid bulls whose strength scores are significantly less than their scores for body depth. Sires with a strength score of +3.0 may be tempting, but if they have a body depth of +4.0, you might well end up with a generation of cows frailer and less healthy than the last - the exact opposite of what you intended.

This article appears on page 614 of the September 25, 2013 issue of Hoard's Dairyman.