What is Optimum and What is Mediocre?
by
Gary Witherspoon, Ph.D.

Part I

Balanced EPDs are the buzz words of the 1990s. However, most bulls advertised as having balanced EPDs have EPDs at or even below the average of the breed. Is average optimal or is it simply mediocre?

Optimum is generally defined as the ideal, the best or the most nearly perfect point, position, condition or number. To maximize profits, the cow/calf operator needs a high percentage calf crop and the highest possible weaning weights. This means increasing weaning weights without negatively impacting fertility or calving ease. I believe that this can be done, but not as easily or as rapidly as we would probably like. Of course how much we can produce with a given set of cows in a particular environment depends on at what level of performance we begin.

Higher weaning weights can be generated in two principal ways. One is to increase the milk production of our cows. The other is to increase the growth potential of their calves. Both increases have built in hazards that must be monitored. Higher post-natal growth rates normally create higher birth weights because the growth genes that generate the increased post-natal growth also tend to increase prenatal growth. Added milk production in the cow if she is already producing a large amount of milk may impact time of re-breeding as well as udder and teat quality and durability. Feed availability for the cow and the calf may also be a significant factor in this equation.

Higher birth weights may affect calving ease, but calving ease is also affected by pelvic area and shape, calf shape, gestation length and nutrition. To maximize the total amount of calf weight at weaning, a cow/calf operator must optimize several interrelated traits. First, he or she must use a bull that does not follow the normal strong positive correlation between higher weaning weights and higher birth weights. Extensive data indicates that higher birth weights are not necessarily or absolutely connected to higher post-natal growth.

You can take any breed's sire summary and look at all the bulls that have about the same birth weight EPD, and you will find a significantly large variation in their EPDs for weaning weight. Although all the bulls in the group will tend to cluster at a mean, the range of the distribution might be as much as 30-40 lbs. of weaning weight.

Selection at the Northern Agricultural Research Center of Montana State University illustrates that it is possible to increase growth rates without proportionally increasing birth weights. By using a formula developed by Dr. Gordon Dickerson, bulls selected for two years of service in the Center's 100 head herd of purebred Havre Line 4 Herefords received deductions from their adjusted yearling weights of 3.2 times for each extra pound of birth weight they had. The result was that only bulls that were increasing growth faster than they were proportionally increasing birth weight were selected as herd sires.

During the 15 year period from 1975-89, the Havre L4s went from an average birth weight EPD of -0.5 to -2.3 while the Hereford breed in general increased its average birth weight EPD from -0.07 to +1.65 in the same 15 year period. When these variations in EPDs are translated into actual birth weights, their significance is more apparent. The average Hereford birth weight increased from 78.4 lbs. in 1978 to 87.1 lbs. in 1988. While proportionally decreasing birth weights in relation to breed average, the Havre L4s still kept pace with the average of the breed in weaning and yearling growth rates. The Havre L4Õs average weaning EPDs went from +9 to +24 and their average yearling EPDs went from +11 to +36. The averages for the breed in general went from a +.7 to a +21 for weaning weight and a +1.5 to a +32 for yearling weight during the same period. This data was reported in the American Hereford Journal by Don Anderson (July, 1992, pp. 212-214).

How is it possible for a bull to go against the grain of the positive correlation between birth weight and post natal growth? The answer may be largely due to differences in the timing of the increments of growth and to a lesser extent due to differences in gestation length. New research is showing that humans as well as other animals tend to grow in spurts rather than in evenly sustained increments. The growth increments of the progeny from different bulls appear to vary at various stages from embryonic conception to maturity.

If the typical bull with a birth weight EPD of +2 has a weaning weight EPD of +20, the breeder selecting for increased weaning weights should select one of the exceptional bulls with a +2 for BW but a +30 or maybe even a +35 for WW. That seems simple enough, but the whole picture is much more complicated.

Out of a group of bulls with the acceptable +2 for BW, there will be only a few that have a +30 for weaning weight and even fewer that will have a +35 for WW. The breeder must also pay attention to the milk EPD of the bulls chosen, because the short term gain in weaning weights may be offset by a long term lowering of milk production, resulting in a combination of milk and growth that might be lower than the present level of weaning performance in the herd. Higher growth rates are usually associated with lower levels of milk production. Therefore, the breeder must find a bull that also goes against the grain of the negative correlation between growth and milk production.

A bull that is both an exception from the positive correlation between growth and birth weight and the negative correlation between growth and milk would truly be a rarity, but such animals do occur in the range of genetic variation available in most breeds, and cattlemen would do well to extensively utilize such exceptional bulls, assuming such bulls do not have any serious faults in other important traits. This kind of selection is the essence of purposeful selection or genetic engineering as opposed to natural selection. The maintenance of genetic diversity within the herd, the breed and the species would add caution to the excessive use of any particular individual, however.

Selection for higher levels of optimal performance in multiple traits is more difficult, more complicated and slower than it is for single trait selection, but it is the only sound and profitable way to go in the long term.

Part II

It may be easier to find a balanced level of multiple traits affecting weaning weights around the breed average than it is to find an optimally correlated level of traits in the top 25% or top 10% of the breed. There are bulls in most breeds that are in the top 25% or even in the top 10% of the breed for all traits except birth weight. It is not likely that we can find a proven bull in the top 10% of his breed for both birth weight and weaning weight, but such a correlation is not necessarily required. We can maintain and even improve calving ease as it relates to birth weight by increasing birth weight at a pace which is proportionally slower than the pace at which growth is increasing.

For example, bull A is at breed average (50% level) for both birth weight and weaning weight. Bull B is at the top 40% level for weaning weight and at the 60% level for birth weight. Bull B would not be an improvement over bull A because while bull B's progeny's weaning weights increased 10% in relation to breed average, his birth weights also increased 10% proportional to breed average.

Likewise, a bull that is at the top 10% level for weaning weight and at the lower 90% level for birth weight has increased weaning weight at the expense of proportionally higher birth weights. I do not know of any data or research that directly deals with whether increased calving difficulties offset the increased weaning weights produced from the higher growth rates, but we do know that a dead calf has a very poor growth rate.

What we can say with some certainty, however, is that a bull (C) that is in the top 30% for weaning weight and the top 50% for birth weight is better - other things being equal - than a bull that is at the 50% level for both BW and WW. What about a bull D that is at the top 20% level of his breed for WW and at the 55% level for BW. In relation to bull C, bull D is adding weaning growth in relation to BW in a proportionally desirable manner, and might, therefore, be considered to be a better bull than bull C. Actually there is nothing absolutely definitive about these proportions, but relating them to breed averages gives one a reference point with which to estimate whether one is improving post-natal growth proportionally more rapidly than one is increasing prenatal growth or birth weight.

The high growth bull that increases weaning and yearling weights proportionally more than he increases birth weights is a bull that probably will also eventually improve calving ease despite the fact that the actual birth weights of his progeny will be greater than breed average. This assumes that the higher growth rate bull will also proportionally increase pelvic area, so that his daughters will be able to have heavier calves more easily. We do not know at this point if this is indeed true, but we assume larger, high growth cattle will have larger pelvic areas. But that correlation cannot be taken for granted in individual cases.

In making genetic progress, as distinct from environmental or managerial progress, in the total amount of calf weight weaned from a given set of cows, we must seek optimal correlations between and among the traits of birth weight, post-natal growth, pelvic area, maternal milk and fertility. We want to increase post natal growth more rapidly than we increase prenatal growth, and we also want to increase pelvic area proportionally greater than we increase birth weights.

Maternal milk is another important factor in this equation. Weaning weights can be increased by either increasing milk or growth. However, there is a negative correlation between milk and growth. Chances are that increasing milk will decrease growth, and vice versa. Here again we must look for the bull that goes against the grain of the negative correlation between growth and milk, and he must also go against the positive correlation between growth and birth weight. These extraordinary bulls can be found, but they are frequently overlooked and not used for reasons such as having scurs, having an unpopular animal in the pedigree or having been bred by a breeder one does not like, having too much white on the legs, being too short or too tall, or having too much leather under the chin.

Sometimes the bull that is the best at going against the grain of these genetic antagonisms has a significant fault or two. Then we have to decide if we can compensate for this or correct it through the females to which he is bred, or we have to use another bull that is not quite as good at overcoming the antagonisms but has no liabilities to be corrected or weaknesses to be compensated for. This slows our progress but it need not deflect it. Because it is difficult to produce cattle that excel in all traits, we just cannot afford to deter our progress with insignificant factors or priorities such as those mentioned at the end of the paragraph above.

While increasing milk production, we cannot do so at the expense of udder and teat quality and longevity. However, function and performance should be our guide rather than some absolute and ideal model of the perfect udder and set of teats. If a cow can raise 15 calves without needing human assistance in getting her calves to nurse and without developing problems for herself or her calf, then I say she has a good enough udder and set of teats, regardless of whether they would score a 10 or a 7 or an 8 on someone's model of perfection.. If she requires human assistance and treatment to get her calves nursing, then I say she needs to be corrected or discarded.

The point I am trying to make here is that if we become too absolute and too rigid about even important traits, we are going to inevitably sacrifice progress toward universal excellence. Therefore, it is important to eliminate insignificant breeding priorities and to not take significant ones to any greater extreme than is functionally necessary.

Fertility is another very important factor in this optimal equation. Fertility might not affect EPDs or adjusted weaning weights for the purebred producer, but percentage bred and calving intervals of those bred have a huge impact on the total pounds of calf weight produced at weaning in a commercial herd. Therefore, the best indices of fertility - scrotal circumference, semen quality, libido and cow calving intervals - also need to be monitored and improved to realize the optimal outcome of increased weaning weights from a given set of cows in a particular environment.

Genetic improvement in multiple traits that affect the total amount of calf weight weaned from a given set of cows in a particular environment is a very complicated matter. It takes careful attention to relative levels of many traits. But improvement can be made and total pounds weaned can be increased if we make good selection decisions and if we limit our breeding priorities to significant traits and functional levels of acceptability of those significant traits.

Progress toward total excellence will be less dramatic than happens in the single trait selection process, but the tools available to us today with which to accomplish total excellence are vast and profound. To those of us who were breeding Herefords in the 1970s, the North American Sire Summary is a marvelous tool we never dreamed of having. The ability to compare cattle across all herds, both horned and polled, and across international boundaries is astonishing. And to have an accuracy level attached to each of those traits is miraculous from a 1970s perspective.

We should be in love with the Sire Summary, and make the greatest possible use of it without being blind to the important traits not yet measured by the EPD analysis. I find it amazing and bewildering that when we breeders have been blessed with the means of flying toward total excellence, many of us insist on walking in circles trying to get rid of a few white spots here and a little horny tissue there, defending that position with what amounts to the analogical equivalent of those who said that if God had intended us to fly, he would have created us with wings. In the sire summary, we have invented the wings with which to fly.

Why not excellence?

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