Dr. Bill Weiss, Professor and Extension Dairy Specialist, Department of Animal Sciences, The Ohio State University, Wooster
The equation used by many feed testing labs to estimate net energy for lactation (NEL) values is based on equations developed at OARDC in 1984 and 1992. Minor adjustments have been made since then, but our lab is in the process of making a major update to the equation. Because the equation is summative, an equation component for one nutrient can be changed without affecting the other components. The original simplified equation estimated the digestible energy (DE) provided by crude protein (CP), non-fiber carbohydrate (NFC), fat, and neutral detergent fiber (NDF) as:
Protein DE (Mcal/lb) = 0.023*CP
NFC DE (Mcal/lb) = 0.018*(100-NDF – Ash – CP – Fat)
Fat DE (Mcal/lb) = 0.042*Fat
NDF DE (Mcal/lb) = 0.015*((NDF-Lignin)*(1-[(Lignin/NDF)0.667]))
where all nutrients are entered as % of DM.
Those values are summed and metabolic fecal energy (0.14 Mcal/lb) is subtracted to yield the DE concentration (Mcal/lb) of the feed. Then standard equations are used to convert DE to metabolizable energy (ME) and finally to NEL:
ME (Mcal/lb) = (1.01*DE) – 0.20
NEL (Mcal/lb) = 0.66* ME
When our equation was developed, commercial feed testing labs did not measure starch, but it is now a routine assay. One improvement we are making to our equation is to replace NFC with starch and a fraction we call residual organic matter (ROM) which is comprised of sugars, soluble fiber, fermentation acids, and several minor components. The concentration of ROM is calculated as: 100 – Ash – CP – NDF – Fat – Starch (all nutrients as % of DM). The NFC fraction of grains and corn silage is mostly starch, but for many other feeds, ROM predominates.
From experiments conducted at Ohio State, we found that the true digestibility of ROM is very high (96%) and is uniform across a diversity of diets. On the other hand, starch digestibility by dairy cows can vary from < 80% to essentially 100%, depending on the feed, and we know many of the factors responsible for that variation. Our revised equation will be as the one above except the NFC term is deleted and replaced with these 2 terms:
ROM (Mcal/lb): 0.017*(100-NDF-Ash-CP-Fat-Starch)
Starch (Mcal/lb): StDig*0.019*Starch
where StDig is the digestibility of starch, which varies depending on the feed. Current best estimates of starch digestibility for major starch sources are in Table 1. For feeds not shown, assume a starch digestibility of 92% (entered in the equation as 0.92). The data in Table 1 are from a variety of published sources.
Table 1. Average starch digestibility for major starch sources.1
|Finely ground dry corn (particle size <1000 um)||0.92|
|Medium ground dry corn (1500-3000 um)||0.89|
|Coarse ground dry corn (>3500 um)||0.80|
|Ground high moisture corn (>27% moisture)||0.96|
|Rolled high moisture corn (>27% moisture)||0.90|
|Steam-flaked corn (<28 lb/bushel density)||0.94|
|Rolled dry barley||0.91|
|Rolled dry wheat||0.93|
|Immature corn silage (<30% DM)||0.91|
|Normal corn silage (32-37% DM)||0.88|
|Mature corn silage (>40% DM)2||0.84|
1Ranges in particle size and DM are not continuous, indicating the uncertainty with the estimates. If particle sizes or DM are not in the table, you can interpolate between rows.
2If kernel processed through rollers with < 3 mm gap, increase starch digestibility to 0.87 (Ferraretto and Shaver, 2012; Prof. Animal Scientist 28:141).
Replacing the NFC term in our energy equation with ROM and starch improves the accuracy of predicting NEL, especially for feeds that are not ‘average’, such as mature corn silage, very finely or very coarsely ground corn, or extensively processed steam- flaked corn.
Future articles will outline additional changes we are making to the equation.