| Just how big are our beef cows? |
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| The Cattle Show | ||||||||||||||||||||||||||||
 Answering this question requires that we take a journey back in time; back to what I like to refer to as the beginning of the “genetic revolution” in the U.S. cattle business.
The first is genetics and the associated animal breeding technology that contributed to the genetic revolution. Since 1975 the influence of European breeds has had a substantial impact on increasing the weaning weight of calves. Crossbreeding and the exploitation of complementarity and heterosis have contributed as well. The development of EPD technology as a selection tool for growth facilitated further advancements in weaning weights.
The improvement in our health programs in terms of vaccinations, parasite control and antibiotics has contributed to the increase in weaning weights. The management of breeding systems and more controlled calving seasons has helped producers wean more uniform and heavier calves. Our nutrition programs in general have improved. Even though our native range country has declined in species diversity, overall quality and quantity, we have had some success with improving forage varieties and incorporating legumes into our tame pastures. Supplementation strategies have improved based on a better understanding of the nutrient gaps that exist between requirements and what is supplied in the forage. This is especially true with regard to the seasonally variable mineral supplied in the forage and the ever-changing requirements of the cow as she progresses through the four phases of production. As a result of these improved technologies we have increased weaning weights in our calves about 200 pounds in the last 30 years. It should come as no surprise that during this same period we have increased the carcass weights of both fed steers and heifers substantially. During this same period we, not so coincidently, increased the carcass weights of both slaughter bulls and cows. This of course makes perfect sense considering that these same bulls and cows are the sires and dams of these larger fed steers and heifers. Just as weaning weights have increased significantly since 1975, so have the carcass weights of all classes of cattle. During this thirty-year period we selected cattle for growth, more specifically ADG and yearling weight, and it worked. It worked so well in fact that there has been considerable discussion around carcasses that yield cuts too large to “fit the box.” What also “worked” was exactly what animal breeders warned us about if we focused too intently on direct growth traits, the mature size of our cattle would increase as well because of the high genetic correlation between growth and mature size.
The carcass weights of our cows have increased by nearly the identical amount as the steers. This too is not surprising after all they are the mothers of the steers. The bulls increased more than the others simply because we focused our selection efforts on the sires and castrated those that did not reach our standard. The additional fifty-pound increase in the heifers at first observation seems questionable; until one considers the improvement in implant programs for heifers along with the introduction and increasing use of MGA. Our efforts to get feeder heifers to perform more like steers appear to have been effective.
The average carcass weight for slaughter cows in 1975 was 475 pounds; by 2005 they were 621 pounds, an increase of 146 pounds. The interesting and valuable question for cow/calf producers, if we have increased cow carcass weights by 146 pounds, just how much have we increased the mature weight of our cows.
In order to calculate the increase in live weight at slaughter we need to know the average dressing percent of the cows at slaughter. One of the major factors that impacts dressing percentage is the amount of fatness at the time of slaughter. Body condition scoring (BCS) can give us a range estimate of dressing percent. If we can determine the average BCS of our cows at harvest time then we can reasonably approximate their average dressing percent especially when the sample size is large. According to the National Market Cow and Bull Beef Quality Audit-2007, a survey of 23 packing plants, the average BCS for 2800 head of cows was 4.53. Based on a body condition score of 4.53 the expected range in dressing percent is from 45 to 49% and the average to approximate 47%. Using .47 as the average dressing percentage we can calculate with reasonable accuracy the live weight change at slaughter of those cows in the audit by dividing the carcass weight difference by the dressing percentage (146 lbs. / .47 = 311 lbs.). Based on these assumptions the increase in cow live weights at slaughter from 1975 to 2005 has a range from 298 to 324 pounds with an expected average of 311 pounds. So if we have increased our cow weights at slaughter by 311 pounds then what is the average live slaughter weight of those cows? By dividing the average carcass weights of the cows, 475 in 1975 and 621 in 2005 by 47% we approximate the live weights at slaughter. That calculation estimates the live slaughter weight of cows in 1975 and 2005 as 1010 and 1321 pounds, respectively. A 1321-pound cow at slaughter is a big cow, but that is not considered to be her mature weight. The mature weight for cows is established at a constant BCS of five. Because the body condition score was estimated at 4.5 the average slaughter weight must be adjusted to a BCS of 5.0 to determine her mature weight. The factor we use to make that calculation according to the NRC is .965. Therefore the cows slaughtered in 2005 in a body condition score of 5 on average would have a mature weight 1369 pounds (1321 lbs. / .965 = 1369). Assuming the BCS and dressing percent averages were the same for the 1975 cows they had a mature weight of 1047 pounds.
So what? One might ask. A thirteen hundred pound cow can be a productive cow in some environments. True enough in some environments, but not in all environments. The more important issue is not so much with the average itself but with the distribution around that average, more specifically the heavier cows. If the mature weight of our cows is a normal distribution around the average then by definition one half of our cows have a mature weight over 1350 pounds. And that is where the problem lies in terms of production efficiency. That leads us to an even more important question. How has this increase in mature weight impacted production efficiency for cow/calf producers? |
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