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A3. Estimates of Energy AvailabilityScope: Another major variable affecting the measurement of digestibility is the amount of selection allowed by the animal. Given a choice, most animals will eat the high protein, low fiber part of the feed (leaves) and leave the high fiber part (stems). Methods used to measure digestibility vary. Some scientists restrict the amount offered to the animal thereby encouraging the animal to consume it all. In this case a core sample of the feed represents what the animal consumed. However, most scientists measure ad libitum intake and digestibility in the same trial by offering the animals 5 to 15% more than they consume. Because they selectively consume the feed, a core sample may not represent the feed actually consumed and regression equations from these trials will be biased. Unfortunately it is difficult to uncover the exact techniques used to develop many of the equations for predicting energy value. Basic Principle: Total digestible nutrients (TDN) is the sum of digestible protein, digestible carbohydrates and digestible fat (fat is multiplied by 2.25 to adjust for its higher energy content). In general TDN is highly correlated with digestible dry matter (DDM) and digestible energy (DE). Estimated net energy (ENE) is a term formerly used to estimate net energy for production (weight gain or milk). Net energy of lactation (NEL) is the current term used by NRC for assessing the energy requirements and feed values for lactating cows. All equations express ADF, NDF, TDN and DDM as percentages (1 to 100) and ENE and NEL are expressed as Mcal/lb. Table 1. Prediction Equations from Midwest Legume and Grass Forages %DDM = 88.9  (0.779 x ADF)a Corn Silage %TDN = 87.84  (.70 x ADF)b Shelled Corn %TDN = 92.22  (1.535 x ADF)c NEL (Mcal/lb) = 0.905  (0.0026 x ADF)c Ear Corn %TDN = 99.72  (1.927 x ADF)c NEL (Mcal/lb) = 1.036  (0.0203 x ADF)c TDN conversion to NEL NEL (Mcal/lb) = (TDN x .01114)  0.054d ______________________________________________________________ a Source: Rohweder, Barnes and Jorgensen, J. Anim. Sci. 68:403 b Source: Schmidt et al., Agron. J. 68:403 c Source: Pennsylvania State d Source: NRC, Dairy Update, 1989 Table 2. Prediction equations from Pennsylvania Statea Legumes %TDN = 4.898 + (89.796 x NEL) ENE (Mcal/100 lb) = NEL x 82.6 NEL (Mcal/lb) = 1.044  (0.0119 x ADF) Mixed Forages %TDN = 4.898 + (89.796 x NEL) ENE (Mcal/100 lb) = NEL x 82.6 NEL (Mcal/lb) = 1.0876  (0.0127 x ADF) Grasses %TDN = 4.898 + (89.796 x NEL) ENE (Mcal/100 lb) = NEL x 82.6 NEL (Mcal/lb) = 1.0876  (0.0127 x ADF) Corn Silage %TDN = 31.4 + (53.1 x NEL) ENE (Mcal/100 lb) = NEL x 82.6 NEL (Mcal/lb) = 1.044  (0.0124 x ADF) Sorghum, Small Grain Forages %TDN = 4.898 + (89.796 x NEL) ENE (Mcal/100 lb) = NEL x 82.6 NEL (Mcal/lb) = 0.7936  (0.00344 x ADF) Complete Rations %TDN = 93.53  (1.03 x ADF) ENE (Mcal/100 lb)= 82.04  (0.76 x ADF) NEL (Mcal/lb) = (TDN x 0.0234)  0.5448 Grain Mixtures (CF = crude fiber) %CF = ADF x .80 %TDN = 81.41  (0.60 x CF) ENE (Mcal/100 lb) = 90.02  (1.0532 x CF) NEL (Mcal/lb) = (TDN x 0.0234)  0.5448 Ear Corn %TDN = 99.72  (1.927 x ADF) ENE (Mcal/100 lb) = TDN x 1.025 NEL (Mcal/lb) = 1.036  (0.0203 x ADF) Shelled Corn %TDN = 92.22  (1.535 x ADF) ENE (Mcal/100 lb) = TDN x 1.053 NEL (Mcal/lb) = 0.950  (0.0026 x ADF) Small Grains %TDN = 4.898 + (89.796 x NEL) ENE (Mcal/100 lb) = 96.0548  (0.8929 x ADF) NEL (Mcal/lb) = 0.9265  (0.00793 x ADF) _________________________________________________ a Source: Proceedings 41st Semiannual Meeting, 1981. Am. Feed Manufacturers Association. Lexington, Ky. p1617. Table 3. Equations from Western Regiona Alfalfa %TDN = 82.38  (0.7515 x ADF) NEL (Mcal/lb) = 0.8611  (0.00835 x ADF) ____________________________________________ aBath, Donald L. and Vern L. Marble. 1989. Testing Alfalfa for Its Feeding Value. Univ of CA. Cooperative Extension. Leaflet 21457. (WREP 109). Table 5. Prediction equations from D.R. Mertens (personal communication) Legumes %TDNm = 86.2  (0.513 x NDF) NEL (Mcal/lb) = 1.054  (0.0098 x NDF) %TDNm = 84.2  (0.598 x ADF) NEL (Mcal/lb) = 1.011  (0.0113 x ADF) Grasses %TDNm = 105.2  (0.667 x NDF) NEL (Mcal/lb) = 1.297  (0.119 x NDF) %TDNm = 97.6  (0.974 x ADF) NEL (Mcal/lb) = 1.120  (0.0159 x ADF) ______________________________________________ Table 4. Prediction Equations from New York State Grasses %TDN = 34.9 + (53.1 x NEL) ENE (Mcal/lb) = NEL x 0.826 NEL (Mcal/lb) = 1.085  (0.0150 x ADF) Legumes %TDN = 29.8 + (53.1 x NEL) ENE (Mcal/lb) = NEL x 0.826 NEL (Mcal/lb) = 1.044  (0.0123 x ADF) Mixed Forages %TDN = 32.4 + (53.1 x NEL) ENE (Mcal/lb) = NEL x 0.826 NEL (Mcal/lb) = 1.044  (0.0131 x ADF) Complete Feed %TDN = 95.88  0.911 x ADF ENE (Mcal/lb) = 1.0123  (0.01432 x ADF) NEL (Mcal/lb) = 0.866  (0.007 x ADF) Grain mix %TDN = 81.41  (0.48 x ADF) ENE (Mcal/lb) = 0.9002  (0.0084 x ADF) NEL (Mcal/lb) = [(TDN x 0.0245)  0.12] x 0.454 Ear Corn %TDN = 99.72  (1.927 x ADF) ENE (Mcal/lb) = TDN x 1.025 NEL (Mcal/lb) = 0.94  (0.008 x ADF) Shell Corn %TDN = 92.22  (1.535 x ADF) ENE (Mcal/lb) = TDN x 0.01053 NEL (Mcal/lb) = 0.94  (0.008 x ADF) Corn Silage %TDN = 31.4 + (53.1 x NEL) ENE (Mcal/lb) = NEL x 0.826 NEL (Mcal/lb) = 0.94  (0.008 x ADF) __________________________________________________________________________ Table 6. Comparison of TDN prediction equation for alfalfa and legumes. Table 3 Table 1 Table 2 Table 4 Table 5 NRC Western Midwest Penn St NY Mertens Mertens Alfalfa Alfalfa Gr.&Leg Legume Legume Legume Legume ADF %TDN %TDN %TDN %TDN %TDN %TDNm %TDN3X __________________________________________________________________________ 27 68 62.1 67.9 69.8 67.6 68.1 62.6 29 63 60.6 66.3 67.7 66.3 66.9 61.5 31 60 59.1 64.8 65.5 65.0 65.7 60.4 33 57.6 63.2 63.4 63.7 64.5 59.3 35 58 56.1 61.6 61.2 62.4 63.3 58.2 37 55 54.6 60.1 59.1 61.1 62.1 57.1 39 53.1 58.5 57.0 59.8 60.9 56.0 41 51.6 57.0 54.8 58.5 59.7 54.9 43 50.1 55.4 52.7 57.2 58.5 53.8 ___________________________________________________________________________ Table 7. Comparison of TDN prediction equations for grasses. Table 1 Table 2 Table 4 Table 5 NRC Midwest Penn St NY Mertens Mertens Grass Gr.&Leg Grass Grass Grass Grass ADF %TDNa %TDN %TDN %TDN %TDNm %TDN3X ________________________________________________________________ 27 67.9 71.8 71.0 71.3 65.6 29 74 66.3 69.5 69.4 69.4 63.8 31 71 64.8 67.2 67.8 67.4 62.0 33 69 63.2 64.9 66.2 65.5 60.2 35 67 61.6 62.6 64.6 63.5 58.4 37 64 60.1 60.4 63.0 61.6 56.6 39 62 58.5 58.1 61.5 59.6 54.8 41 60 57.0 55.8 59.9 57.7 53.1 43 57 55.4 53.5 58.3 55.7 51.3 ________________________________________________________________ a NRC Grass is an average of bromegrass, orchardgrass and ryegrass. NRC timothy was 7 to 8 %units lower in TDN at each level of ADF. 

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