The northeastern U.S. is blessed with abundant and potentially high-quality forage. For growing cattle, high-quality forages can support adequate levels of gain with little supplementation other than vitamins and minerals. In this article, we’ll look at the prediction of average daily gain (ADG) and the forage quality necessary to achieve a desired/target ADG. The information is adapted from Fact Sheet 1097, “Summary of Nutrient Requirements for Growing and Finishing Beef Cattle,” in the Cornell University Beef Production Reference Manual.
There are two ways to arrive at the expected weight at USDA low Choice. The most precise method is to determine the frame score by measuring hip height. The frame score can be converted to the expected weight at USDA low Choice. Details on these procedures can be found on the Cornell Beef Cattle Management website (http://beefcattle.ansci.cornell.edu/software-and-reports). The second method is to base it on dam weight. On average, a steer at low Choice will weigh the same as his dam; a heifer will weigh approximately 75 to 100 pounds less.
Determine nutrient requirements
The most commonly used energy value is total digestible nutrients (TDN). In general, if the TDN of the forage is greater than or equal to 60 percent, then an acceptable gain can be achieved.
While energy drives growth, protein levels need to reach a certain threshold so rumen microbes can utilize the available energy. This is where it gets complicated. Dr. Dan Fox, Cornell professor emeritus, noted in the revised fact sheet:
“Expressing protein requirements as crude protein (CP) is no longer adequate, because we have more knowledge of the requirements for rumen fermentation. The CP is made up of two types of protein: rumen-degradable protein (RDP) and rumen-undegradable protein (RUP). The total amount of RDP and RUP supplied by the feed may not match the animal’s requirement for each. The RDP is needed to meet the requirements for bacteria that ferment the forage and grain consumed. These bacteria are then digested to supply microbial protein to meet the animal’s requirements for maintenance and growth. The RUP supplements the microbial protein produced in meeting the animal’s requirement for maintenance and growth. Sometimes the feed CP will meet the RDP but not the RUP required. In that case, more CP will be required to provide the RUP needed. For example, corn silage protein has a high proportion of RDP and a low proportion of RUP. Balancing the ration to meet both the RDP and RUP requirement for backgrounding calves on a corn silage-based ration will result in the ration CP being higher than with other feeds that have a CP in which the RDP and RUP more closely match the animal’s protein requirement for maintenance and growth.”
Requirements for energy, protein and minerals are listed in Table 1, which was extracted from Fact Sheet 1097 (revised). This fact sheet provides nutrient requirements for heifers and steers ranging from 1,000 to 1,400 pounds at USDA low Choice grade (or maturity) over nine stages of growth. Within each growth stage, the protein and fat content of gain is similar across five weight categories.
Determine the chemical composition of feed
Book values work OK for grains; however, forages vary to such an extent that a forage analysis is necessary. Therefore, the next step in developing a feeding program is to have your forages analyzed so requirements for ADG can be determined and rations can be accurately balanced.
The value for TDN can be taken directly from the forage analysis; however, the RUP and RDP are not necessarily listed and may need to be recalculated. In a forage analysis from Dairy One, you will receive a value for degradable protein shown as a percent of the CP. The value you need is rumen-degradable protein (RDP). This is calculated as RDP = CP × degradable protein. RUP is calculated as RUP = CP ×(1 – degradable protein).
An analysis of the mixed (mostly grass) hay fed to the bulls in the 2012-2013 Cornell All Forage Fed Bull Test showed 16.4 percent CP and 65 percent degradable protein. Doing the calculations, RDP was 10.7 percent (0.164 × 0.65) and RUP was 5.7 percent (0.164 × [1 – 0.65]) on a dry matter (DM) basis.
Your county extension agent can help you get feeds analyzed, or you can contact Dairy One via http://www.dairyone.com.
Here’s an example using the feed mentioned above that was fed to the bulls entered in the 2012-2013 All Forage Fed Bull Test.
1. Cattle description: The bulls arrived in December 2012 and went on test at an average weight of 658 pounds. The average hip height at 309 days of age was 46 inches, which corresponds to an expected weight at low Choice of 1,136 pounds.
2. Determine nutrient requirements of cattle: Using Table 1, find the column with the value closest to 1,136 pounds (1,100 pounds). Go down that column until you find the value closest to 658 pounds (660 pounds). The TDN value from the Dairy One forage analysis is 64 percent. The closest value (65 percent) indicates that this bull should gain 1.71 pounds per day and eat 15.6 pounds DM. On that same row, you find that the CP, RDP, RUP, Ca and P requirements are 10.8 percent, 8.5 percent, 2.3 percent, 0.37 percent and 0.21 percent, respectively.
3. Determine feed composition: From the Dairy One feed analysis, TDN = 64 percent, CP = 16.4 percent, degradable protein (percent CP) = 65 percent, Ca = 1.36 percent and P = 0.29 percent. Using the calculations shown above, RDP = 10.7 percent and RUP = 5.7 percent.
4. Matching nutrient supply with requirements: If you’re satisfied with a projected ADG of 1.71 pounds, then your job is done, because protein and mineral requirements are met to support that gain with the TDN of the forage. To achieve higher levels of gain, you need to supplement with energy, most likely a grain. At this point, the ration formulation process becomes more complex, and it is recommended that you use the Cornell Net Carbohydrate and Protein System (CNCPS) as implemented in the Large Ruminant Nutrition System (LRNS), because it contains Level 1, which was used to develop Table 1.
The LRNS (http://nutritionmodels.tamu. edu/lrns.html) is a computer program that estimates beef and dairy cattle requirements and nutrient supply under specific conditions of animal type, environment (climatic factors), management, and physicochemical composition of available feeds. The TDN and net energy for maintenance and gain (NEm and NEg) in LRNS Level 1 are predicted with the same equations used by Dairy One, and the analytical values shown on the Dairy One feed analysis report can be put into the LRNS so it can compute accurate energy, protein and mineral values to balance rations on each farm.
5. Observed versus actual performance:
The observed ADG was higher than predicted by the modeled values in the table, which is a result of higher intake. If ADG were lower, this could be the result of:
a. Not adjusting feed fed on a DM basis. Most hay will be 88 to 90 percent DM. If you’re feeding 15 pounds of hay per day, the cattle are only getting 13.3 pounds DM (15 x 0.89 DM).
b. Higher maintenance requirements due to cattle having wet and/ or muddy hair coats with little access to wind protection.
c. Restriction of voluntary intake due to poor feed quality, not having feed available at all times, or crowded conditions.
d. Failure to meet RDP and RUP requirements. Since the proportion of the CP percentage that is RDP and RUP varies from feed to feed and within feeds and both are needed, the CP percentage required in the ration will often have to be higher than those in the table to meet both the RDP and RUP percentages.
High-quality forage is our competitive advantage in the northeastern U.S.; it will support adequate gain for growing cattle, replacement heifers and grass finishing. The key is to describe your cattle, determine their requirements and analyze your forage. This is not rocket science (though it has taken years of research to understand and accurately produce the values in this table), but if you want cattle to perform on forage, you need to apply these basic principles.