Nutrition
Nutrition Articles
Evaluating Forages from Feed Test Results
The main purpose for feeding forages to cows is for dietary energy. As the animal transitions from gestation to lactation generally there is not enough energy generated from daily intake of forages alone to meet their body’s needs so grain can be added to make up the difference. Feed Tests give us a number of important results that let us know how and when to use the various forages that are available to the producer. By testing forages we can strategically use inventory to achieve both best cost feeding programs and maintain Body Condition Score (BCS). Protein is also an important substance but in general there is a tendency to feed excess protein during gestation so this note will concentrate on energy which can often be deficient.
Energy is supplied for the most part by carbohydrates. Some protein is broken down and metabolized for energy but not large amounts. Examination of a forage plant shows contents of sugars, starch, cellulose, hemi-cellulose and lignin. Lignin is not a carbohydrate but is part of NDF. Moving through this list sugars are the most digestible and lignin essentially not digestible. So the relative amounts of each carbohydrate render the ingredient either highly digestible or of low digestibility. When making hay the cattle producer can control this outcome somewhat by changing when the forage is cut for hay or silage. In the natural growth cycle the less digestible carbohydrates develop as the plant grows to give it structural strength but reduces digestibility of the resulting hay. Cell walls and stem components are lignified when the plant has grown sufficiently to produce a seed head and is mature. The cattle producer wants high volumes of hay from each acre but trades digestibility for it. By selectively altering date for harvest it is possible to increase feeding value of some portion of the hay crop and improve cattle production for milk thereby reducing dependence on grain slightly. Forages of higher Relative Feed Value (RFV) would be suitable for gestation months seven and eight in substitution for straw which can be fed in the first six months of gestation. Month eight and nine are transition months where the calf is growing rapidly and the cow’s body is adapting to increased energy demands from the calf. High quality hay should be used in this period of growth of the calf. Failing to recognize this change will cost the cow since the calf will draw nutrients from the cow’s body reserves lowering BCS and causing an energy negative metabolism in the cow. Sacrificing yield for quality has a cost but the resulting higher quality forages can replace even higher priced grain for rations in the last three months of gestation and through lactation making sense economically. With both lower and higher quality forages in hand blending selectively for each stage of the cow’s life cycle can be accomplished using the Highline® AMX850 twin screw pull type mixer or the Accumix™ 1000s self-propelled, self-loading mixer. Accuracy counts when engaged in precision feeding.
Terms from feed test results can have more meaning when understood in the context of this fibre type. A plant’s structural components are the source of NDF, specifically the cell walls. “NDF is measured by boiling the forage in a neutral detergent solution, then measuring the insoluble residue. ADF is a measure of the plant components in forages that are the least digestible by livestock, including cellulose and lignin. ADF increases digestibility decreases, so forages with high ADF concentrations are typically lower in energy. Acid detergent fiber (ADF) is what remains after NDF is boiled in acidic detergent solution for 1 h. In this process, hemicellulose is lost and cellulose and lignin remain.1” You generally want to choose forages with low NDF percentages because these levels increase as forages mature. Together, ADF and NDF factor into the crude fiber of forages, which is the measure of total fiber content. It’s helpful to know ADF and NDF percentages because they can accurately predict forage intake as it relates to bulk; in other words, how much an animal will eat before its stomach is full and it stops eating. These figures are also key for measuring energy levels and animal performance. The animal is restricted to 1.2% of body weight for daily intake of NDF to avoid impaction.
Having an understanding of these two measures of fibre and how they affect intake and digestion help to evaluate forages and assist in ration formulation. Relative Feed Quality has been reported also on Feed Tests and differs from RFV by taking into consideration digestibility of fibre. For reference a Relative Feed Quality value (RFQ) of 175 is standard for dairy cattle and an RFQ of 100 is standard for beef cattle.
Fig. 1 from University of Wisconsin 2
1. Nutrition and feeding of dairy cattle, Chapter 9, Peter S. Erickson, Kenneth F. Kalscheur, Department of Agriculture, Nutrition, and Food Systems, University of New Hampshire, Durham, NH, United States; U.S. Dairy Forage Research Center, USDA-ARS, Madison, W.I
2. Comparison of Relative Forage Quality (RFQ) to
Relative Feed Value (RFV) by Dr. Dan Undersander, University of Wisconsin