ONA REPORT

published in

THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL


March 2015

Optimum Cow Size and Milk Production in a Bahiagrass System in South Florida


For questions or comments regarding this publication contact: Dr. Pillip Lancaster, University of Florida, IFAS


In a previous Ona Report (December 2013), we looked at comparing biological efficiency of beef cows with different mature weight. We concluded that cows with moderate mature weight would generally work best for the beef industry.  Previous research at the USDA Meat Animal Research Center indicates that efficiency of beef cows with different mature weight and milk production varies depending upon the nutritional environment. Thus, the most efficient cow type is not the same for all environments. In this article we will look at cows of different mature weight and milk production in a specific forage system.

Data on forage nutritive value were compiled for bahiagrass pasture in each month of the year and limpograss hay for winter feeding. Using nutrition models, feed intake of cows having mature body weight of 900, 1100, or 1300 lb, and peak milk production of 8, 16, or 24 lb/d was computed, as well as forage intake and weaning weight of their respective calves. The production system used in the model was as follows: calving date was set as November 1 and weaning date as August 1, limpograss hay was fed from December to March to supplement bahiagrass pasture, and molasses and cottonseed meal were fed as needed during winter months to support lactation. Growth and feed intake of calves from their respective weaning weights to an estimated slaughter weight based on cow mature weight were also computed using nutrition models. The goal of this analysis was to identify the target cow mature weight and peak milk production of the cow herd for this particular forage system depending upon marketing strategy: sell at weaning or retain ownership to slaughter. All of the comparisons described below are greatest (1300 lb cow or 24 lb peak milk) versus least (900 lb cow or 8 lb peak milk) of the parameters evaluated.

Weaning weight was greatest for the 1300 lb cow with 24 lb peak milk (684 lb) and least for the 900 lb cow with 8 lb peak milk (361 lb). However, weaning weight did not increase similarly for each cow mature weight category as milk production increased. Comparing cows with 24 lb peak milk to those with 8 lb peak milk, weaning weight increased 70% for the 900 lb cow, 64% for the 1100 lb cow, and 58% for the 1300 lb cow indicating that increasing milk production had a greater impact on growth of calves from smaller cows. Calculated weaning weight from this analysis increased 64% with increasing milk production, but only 14% with increasing cow mature weight. This indicates that milk production is more beneficial for increasing weaning weight than cow mature weight.
 
The amount of feed TDN (total digestible nutrients), which is a measurement of energy, consumed during the cow-calf phase was greatest for the 1300 lb cow with 24 lb peak milk and her calf (6,720 lb TDN) and least for the 900 lb cow with 8 lb peak milk and her calf (4,591 lb TDN).  Comparing cows with 24 lb peak milk to those with 8 lb peak milk, TDN consumed increased 16% for the 900 lb cow, 14% for the 1100 lb cow, and 13% for the 1300 lb cow. The amount of feed TDN consumed increased 28% with increasing cow mature weight, and also increased 14% with increasing milk production indicating that cow weight and associated increase in maintenance requirement had the most impact on feed TDN consumed.

Because weaning weight increased most dramatically with increasing peak milk and TDN consumed increased most dramatically with increasing cow mature weight, the biological efficiency (ratio of weaning weight / TDN consumed) was greatest for 900 lb cow with 24 lb peak milk (11.6 lb weaning weight/100 lb TDN) and least for the 1300 lb cow with 8 lb peak milk (7.2 lb weaning weight/100 lb TDN). The 1300 lb cow must have greater milk production to have the same biological efficiency as the 900 lb cow. Increasing milk production in the 900 lb cow resulted in a 47% increase in biological efficiency, whereas it resulted in only a 41% increase in biological efficiency in the 1300 lb cow. Milk production had a more dramatic effect on increasing biological efficiency than cow mature weight, indicating that more attention should be paid to milk production than mature weight. Increasing supplemental feed of the cow during early lactation to support greater milk production when forage nutritive value is low can increase weaning weight and thus improve biological efficiency. However, milk production will only increase to the maximum genetic milk potential of the cow, indicating that increasing supplemental feed to cows with low peak milk potential will negatively impact biological efficiency and only result in fat cows.

For producers retaining ownership of calves through the feedlot, biological efficiency of cows should be calculated from conception to slaughter. In the feedlot, increasing milk production of the cow decreased feed efficiency of the calf in the feedlot 13% due to calves being heavier relative to mature weight at feedlot entry. Differences in feed efficiency of calves were not consistent among cows of different mature weight across the different levels of peak milk. At 8 lb peak milk there was only a 1% increase in feed efficiency of the calf from the 1300 lb cow compared with the calf from the 900 lb cow, but at 24 lb peak milk the calf from the 1300 lb cow as 8% more efficient than the calf from the 900 lb cow. From conception to slaughter, biological efficiency increased 1% with increasing cow mature weight, and 3% with increasing milk production.  Thus, cow mature weight and milk production had minimal impact on biological efficiency over the entire production chain.

In this analysis, we compared a large-framed cow that will produce a calf with greater genetic potential for growth with a small-framed cow that will produce a calf with lower genetic potential for growth. The results are aimed at assisting producers in better targeting the optimum cow mature weight and milk production for this specific production system; other forages and production systems may have different results. On a per cow basis, the 900 lb cow with 24 lb peak milk had the greatest biological efficiency when calves were sold at weaning; however, all cow mature weights and peak milk levels had very similar biological efficiency when retained through the feedlot. Based on this analysis, biological efficiency can be greatly improved by increasing milk production of the cow herd in a bahiagrass forage system even with the additional feed required to support milk production. Additionally, increasing milk production does not appear to be detrimental to overall efficiency if ownership of calves is retained through the feedlot. However, this analysis assumes feeding cows to maximize the genetic milk potential.  Feeding cows with high genetic milk potential that have a greater maintenance requirement for a milk level less than their genetic potential will decrease weaning weight relative to cow maintenance requirements and significantly decrease biological efficiency. Although, the cow type with the greatest biological efficiency does not necessarily translate into the greatest economic efficiency. Thus, further analysis of economic efficiency is needed.


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