THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL
Understanding the Impact of Cow Size on Feed Efficiency
As we look at the scale ticket from shipping calves this year, are we pleased with what we see? Maybe so if we have a big check to cash, but does this mean we have big profits? Not necessarily; in large part profit depends on revenue from the sale of calves relative to cost of feed required to produce those calves. This relationship is highly influenced by cow size or maintenance energy requirements and milk production, essentially the ability of the cow to convert feed into pounds of weaned calf or what we refer to as biological efficiency.
To illustrate how cow size and milk production impact biological efficiency of the cow herd, we will compare a herd of 1000 pound cows with a herd of 1300 pound cows. First, let us compare the maintenance energy requirements between these cows. The maintenance energy requirement of the 1300-lb cow is 21% greater than the 1000-lb cow indicating that for the 1300-lb cow to have similar biological efficiency she must wean a heavier calf. The 1300-lb cow will require more feed for maintenance and so she has to wean a heavier calf to offset that increased amount of feed. If the 1300-lb cow has similar biological efficiency as the 1000-lb cow and assuming the 1000-lb cow will wean 50% of her body weight, the weaning weights of the 1000-lb and 1300-lb cows are 500 and 609 pounds, respectively. This translates into a weaning weight to cow weight ratio of 50% for the 1000-lb cow and 47% for the 1300-lb cow indicating that to have similar biological efficiency the 1300-lb cow does not have to wean as great a percentage of her weight.
If you can stock 100 1000-lb cows on your ranch, how many 1300-lb cows can you stock? The answer is fewer, right, because larger cows will require more feed and the ranch can only produce so much forage. Based on the maintenance energy requirements of the two cows, we estimated that 82 1300-lb cows could be stocked on the same ranch. So now the ranch will have fewer calves to sell but they will be heavier at weaning. These heavier calves may bring less per pound than lighter calves. The question is how heavy do the calves from 1300-lb cows need to be in order to bring the same total herd revenue at the sale barn? Using a price slide of $0.0565/lb, we estimated that the calves from the 1300-lb cows would need to weigh 660 pounds at weaning to bring the same total herd revenue as the 500 pound calves from the 1000-lb cows. The herd of 82 1300-lb cows and the herd of 100 1000-lb cows must both wean calves weighing 50% of cow weight to bring similar total herd revenue.
Let’s turn our attention to milk production for a minute. Here in south Florida we typically wean our calves at an older age, about 9 months. So for a calf to weigh 500 pounds at weaning, they need to gain 1.6 pounds per day over the 9 months. For the calf from the 1300-lb cow to weigh 660 pounds at weaning, they need to gain 2.2 pounds per day over the same time period. Based on some research data, we estimate that nursing calves would consume about 1.5% of their average body weight in forage. From this we calculated that the peak milk for the 1000-lb cow would need to be 13 pounds per day so that her calf would weigh 500 pounds at weaning. The peak milk for the 1300-lb cow would need to be 17 pounds per day so that her calf would weigh 660 pounds at weaning. Accordingly, the 1300-lb cow will require more and better quality feed to produce this much milk. The typical nutritive value of our forages in south Florida will not allow the 1300-lb cow to produce 17 pounds of milk, which means you will have to purchase more supplemental feed for this cow.
Another aspect associated with milk production is a 15% increase in maintenance energy requirements of cows with moderate to high milk potential. Seventeen pounds of milk per day would be considered to be moderate to high milk potential, indicating that the maintenance energy requirement of the 1300-lb cow would be 15% greater than what we first calculated. Increasing the maintenance energy requirement of the 1300-lb cow means that she needs to wean a 700 pound calf (54% of her weight) to have the same biological efficiency as a 1000-lb cow weaning a 500 pound calf (50% of her weight). Additionally, the increased maintenance requirements of the 1300-lb cows reduces the number of cows you can stock on the same ranch to 71 cows. Thus, the herd of 71 1300-lb cows would have to wean 795 pound calves (61% of cow weight) to bring the same total herd revenue as the herd of 100 1000-lb cows, and the herd of 1300-lb cows would have to produce 22 pounds of milk per day at peak lactation for calves to weigh 795 pounds at 9 months of age. It is highly unlikely that the forage resources in south Florida will support this level of milk production, again meaning that a significant amount of supplemental feed would need to be purchased.
We evaluated the weaning ratio (calf weaning weight divided by cow mature weight) in the crossbred Brangus cow herd at the Range Cattle Research and Education Center this year. We found that 1050-lb cows weaned calves weighing 533 pounds (51% of cow weight), and 1330-lb cows weaned calves weighing 563 pounds, which is only 42% of cow weight. With the forage resources (bahiagrass pasture, limpograss and stargrass hay) and supplemental feed (5 lb/d of 16% molasses) we use, the nutritional environment was likely not great enough to support the milk production necessary for 1330-lb cows to wean 50% of their weight. These results indicate that to improve biological efficiency and most likely profitability, depending upon price of feed relative to calf prices, beef cow producers should look to moderate the mature size and milk production of their cow herd. The optimum cow size and milk production is different for each ranch and depends upon nutritive content of the forage resources available, but 1000 to 1100 pound cows producing 13 to 15 pounds of milk at peak lactation is a good target for most ranches in south Florida.
Up to now we have been focusing on ranches that sell calves at weaning which applies to most ranches in south Florida, but some ranches retain ownership of calves through the feedlot. If the ranch retains ownership of calves through the feedlot, the situation changes a little bit. Larger cows can be more efficient because they will produce calves that grow faster in the feedlot and weigh more at slaughter which improves feed efficiency in the feedlot and increases carcass weight relative to maintenance energy requirements of the cow. In this situation, it will be more efficient to have cows with low to moderate milk production even though these are larger cows. Since the ranch is retaining ownership maximum carcass weight is the goal rather than maximum weaning weight, and it is more efficient to feed the calf in the feedlot than feed the cow to produce milk for the calf. However, one has to be careful about getting cows that are too large. When corn prices are high, calves can be grown on grass to reduce total cost of finishing, but calves from really large cows will reach the appropriate fatness at heavy carcass weights resulting in discounts for overweight carcasses. Thus, if cows are too large we are stuck in a bad place because cost of gain in the feedlot is too high if placed on feed at weaning and calves will be discounted for overweight carcasses if grown on grass prior to finishing.
A good management practice to increase the biological efficiency of the cow herd for ranches that sell calves at weaning or retain ownership through the feedlot is the use of two types of bulls. A moderate size bull that will produce daughters with moderate milk production can be used on a small group of cows to produce replacement heifers. This will produce moderate size cows for replacements. Then use a larger size bull with high growth potential on the rest of the cow herd to produce feeder calves. These bulls can be different breeds in a crossbreeding program or they can be two bulls of the same breed with different mature size. If we are in a situation where it is not feasible to keep bulls in separate breeding pastures, we can consider using artificial insemination in place of one of the bulls. Semen from a moderate size bull can be used to artificially inseminate a small group of cows to produce replacement heifers, and in this way two types of bulls can be used and only a small number of cows have to be artificially inseminated.
Do you know how big your cows are? The best way to know cow size is to put a set of scales under the squeeze chute and weigh each cow when she comes through the chute for routine vaccinations and pregnancy check. It will cost a little bit of money, but it is one of the best tools to evaluate performance of the cow herd. Another way is to look at what cull cows weigh at the sale barn, but remember these cows may not represent the average of the herd. Either way, always adjust the weight of each cow to a body condition score of 5. To do this record body condition score of cows as they go through the chute or as they are hauled to the sale barn. Then add 75 pounds for each condition score below 5 or subtract 75 pounds for each condition above 5. For example, a cow weighing 1050 pounds at body condition score 4 will weigh 1125 pounds at body condition score 5.
How do you go about moderating your cow size? At the Range Cattle Research and Education Center, we have reduced the mature cow size by about 150 to 200 pounds over the last 10 years. Prior to this time, our replacement heifers were selected by body weight with a focus on selecting the largest heifers as replacements. This objective resulted in a gradual, but continual, increase in cow mature size. A reversal was achieved by marketing our largest heifers and selecting our replacement heifers based on more moderate size, above average weaning weight, and weight per day of age.
In conclusion, cow size and milk production can significantly affect feed efficiency of the cow herd. With the forage resources in south Florida, cows with moderate mature weight and low to moderate milk production are likely to be more efficient than cows with larger mature weight. Having a scale under the chute will pay in the long run so that an accurate measurement of cow weight can be obtained and determine whether weaning weights are measuring up. Each ranch should determine what cow size is best suited for their nutritional environment and management plan (sell at weaning or retain ownership) and put together a plan to achieve that goal.