Milk2000: Combining Yield and Quality into a Single Term

R. Shaver^1,3, D. Undersander^2,3, E. Schwab¹, P. Hoffman^1,3, J. Lauer^2,3, D. Combs^1,2, and J. Coors²

Departments of Dairy Science¹ and Agronomy²

University of Wisconsin-Madison^1,2

University of Wisconsin-Extension³

Introduction

Undersander et al. (1993) developed a method for estimating milk per ton of forage dry matter (DM) as an index of forage quality. The milk per ton index of Undersander et al. (1993) is based on energy content of the forage predicted from acid detergent fiber (ADF) content and DM intake potential of the forage predicted from neutral detergent fiber (NDF) content. The milk per ton index has been used in corn silage hybrid performance trials at the University of Wisconsin - Madison (Lauer et al., 1997).

The milk per ton index of Undersander et al. (1993) has now been modified (Schwab and Shaver, 2001), and an easy to use Excel 5.0 spreadsheet called Milk2000 has been developed. MILK2000 uses forage analyses (crude protein, NDF, in vitro NDF digestibility, starch, and non-fiber carbohydrate) to estimate energy content using a modification of the NRC (2001) summative approach and DM intake from NDF (Mertens, 1987) and in vitro NDF digestibility (Oba and Allen, 1999) to predict milk production per ton of forage DM. In MILK2000, the intake of energy from forage for a 1350 lb. milking cow consuming a 30% NDF diet is calculated and the cow’s maintenance energy requirement (proportioned according to the percentage of forage in the diet) is then subtracted from energy intake to provide an estimate of the energy available from forage for conversion to milk (NRC, 1989). Forage DM yield multiplied times the milk produced per ton of forage DM provides an estimate of the milk produced per acre and combines yield and quality into a single term.

Development of MILK2000

A published summative energy equation (Weiss, 1996), with crude protein (CP), fat, non-fiber carbohydrate (NFC) and NDF components and corresponding digestibility coefficients, was adapted as follows: the CP and fat components were not altered, and the NDF digestibility coefficient based on lignin content was replaced by an in vitro NDF digestibility (NDFD) measurement. The NFC component with constant digestibility was left unchanged for alfalfa and grasses but, for corn silage, was replaced with starch and non-starch NFC components with the starch digestibility coefficient varied in relationship to whole-plant DM content and kernel processing (Schwab and Shaver, 2001).

For corn silage, regression equations were developed from literature data to predict apparent total tract starch digestibility from whole-plant DM content for unprocessed and processed corn silage. Slopes of the unprocessed and processed CS starch digestibility regression equations indicate that DM content has a greater impact on the starch digestibility of unprocessed than processed CS. At 35% DM, predicted apparent total tract starch digestibility for unprocessed and processed CS were 86 and 91%, respectively. At lower DM contents the difference between processed and unprocessed silage was smaller and increased as DM content increased. Minimum and maximum restrictions of 70% (unprocessed corn silage) to 80% (processed corn silage) and 95%, respectively, were placed on predicted apparent total tract starch digestibility coefficients (Firkins, et al., 2001). Predicted starch digestibility coefficients were adjusted to a maintenance level of intake using a 4-percent decline in digestibility per multiple of maintenance (NRC, 1989) with a maximum true starch digestibility of 98% (NRC, 2001; Weiss, 1996) for use in the Schwab-Shaver summative equation. The concentration of the non-starch NFC component of CS was approximated by subtracting percent starch from percent NFC, and a digestion coefficient of 98% was assigned to this component according to Weiss (1996). A 48-hour or maintenance intake NDFD measurement was used in the summative equation as described in NRC (2001).

For both alfalfa/grass and corn silage, the MILK2000 model (Schwab and Shaver, 2001) used Schwab-Shaver net energy for lactation estimates along with DM intake estimated from both NDF content (Mertens, 1987) and NDFD (Oba and Allen, 1999) to estimate milk per ton of DM and per acre. In the spreadsheet, the cows’ maintenance energy requirement (proportioned according to the percentage of forage in the diet DM) is subtracted from energy intake to provide an estimate of the energy available from forage for conversion to milk (NRC, 1989).

Current Status of MILK2000

Near infrared (NIR) calibrations and (or) wet chemistry techniques are available in commercial forage testing laboratories for all nutrients needed to calculate the Schwab-Shaver NE_l on forage samples. However, few laboratories have the capability of analyzing forage samples for NDFD. The University of Wisconsin Marshfield Soil and Forage Analysis Laboratory now performs wet chemistry NDFD, and NIR calibrations for NDFD on alfalfa/grass and corn silage samples are available from the UW Marshfield Soil and Forage Analysis Laboratory and a number of commercial forage testing laboratories. The summative equations have been made available to commercial forage testing labs and some have programmed these equations into their reporting system.

Use of Milk2000

Milk per ton and milk per acre calculations provide relative rankings of forage samples, but should not be considered as predictive of actual milk responses in specific situations for the following reasons:

1) equations and calculations are simplified to reduce inputs for ease of use.

2) farm to farm differences exist.

3) genetic, dietary, and environmental differences affecting feed utilization are not considered.

Do not use different values for yield or quality measurements that are not statistically different. Animal response calculations are more sensitive than our measurement techniques of yield and quality. The spreadsheet will show a milk/ton difference when yield and quality may not be statistically different.

Standard inputs that are needed for MILK2000 include DM percentage and yield, CP percentage, 48-hour in vitro NDF digestibility (not dry matter digestibility), NDF percentage, and starch percentage (corn silage only). Ash and ether extract should be entered if available, but book values can be entered instead (for normal corn silage, 4.3% for ash and 3.2% ether extract and for alfalfa/grasses, 10.0% ash and 2.7% ether extract, are recommended). Non-fiber carbohydrate and non-starch NFC are calculated values within the spreadsheet.

The MILK2000 alfalfa/grass worksheet contains NRC (2001) RFV100 and high quality alfalfa in rows 12 and 13 as a quality reference. You can begin entering your samples in row 14: sample identification in column A, quality data in columns B through G, and DM yield in column H. Calculated results are found in columns I through T. Depending on your spreadsheet settings, it may be necessary to push F9 after entering data for calculation of results

The MILK2000 corn silage worksheet contains NRC (2001) “normal” corn silage in row 12 as a quality reference. An example sample entry is included in row 13. You can begin entering your samples in row 14: sample identification in column A, processing in column B, quality data in columns C through I, and DM yield in column J. Calculated results are found in columns O through AB. Depending on your spreadsheet settings, it may be necessary to push F9 after entering data for calculation of results.

Normal Ranges of Values

Average and normal ranges for alfalfa/grass forage analysis of farmer samples are as follows:

CP NDF NDFD NFC EE ash milk/ton

% % % of DNF % % % lb/ton DM

Average 19.0 43.0 53.0 26 3.0 10.0 3,000

Minimum 10.0 30.0 30.0 10 1.0 6.0 1,600

Maximum 30.0 60.0 70.0 40 4.0 16.0 3,800

Average and normal ranges for corn silage forage analysis of farmer samples from the UW Marshfield Soil and Forage Analysis Laboratory are as follows:

CP NDF NDFD starch NFC ash EE milk/ton

% % % of NDF % % % % lb./ton DM

Average 9.0 47.0 59.0 26.0 35.0 5.0 3.0 3,400

Minimum 5.0 29.0 43.0 6.0 6.5 1.0 1.0 1,600

Maximum 13.0 78.0 82.0 43.0 54.0 10.0 5.0 4,450

Average and normal ranges for corn silage variety trial (Lauer et al., 1997) quality parameters are as follows:

CP NDF NDFD starch NFC milk/ton

% % % of NDF % % lb/ton DM

Average 7.0 46.0 52.0 30.0 41.0 3,100

Minimum 6.0 40.0 41.0 20.0 25.0 2,640

Maximum 9.0 53.0 63.0 40.0 50.0 3,500

Ash and ether extract are assumed constant at 5% and 2.5%, respectively.

References

Firkins, J. L., M. L. Eastridge, N. R. St-Pierre, and S. M. Noftsger. 2001. (In Press). Effects of grain variability and processing on starch utilization by lactating dairy cattle. J. Anim. Sci.

Lauer, J., K. Kohn, P. Flannery, and K. Hudelson. 1997. Wisconsin Corn Hybrid Performance Trial Results : Grain and Silage. A3653. Univ. WI Extension. Madison, WI.

Mertens, D. R. 1987. Predicting intake and digestibility using mathematical models of ruminal function. J. Anim. Sci. 64:1548-1558.

National Research Council. 2001. Nutrient requirements of dairy cattle. 7^th rev. ed. Natl. Acad. Sci., Washington D.C.

National Research Council. 1989. Nutrient Requirements of Dairy Cattle. 6^th rev. ed. Natl. Acad, Sci., Washington, DC.

Oba, M. and M. S. Allen. 1999. Evaluation of the importance of the digestibility of neutral detergent fiber from forage: effects on dry matter intake and milk yield of dairy cows. J. Dairy Sci. 82:589-596.

Schwab, E. C., and R. D. Shaver. 2001. Evaluation of corn silage nutritive value using MILK2000. Pages 21-24 in Proc. of 25^th Forage Production and Use Symposium. WI Forage Council Annual Mtg. Eau Claire, WI.

Undersander, D.J., W.T. Howard, and R.D. Shaver. 1993. Milk per acre spreadsheet for combining yield and quality into a single term. J. Prod. Ag. 6:231‑235.

Weiss, W. P. 1996. Estimating available energy content of ruminant feeds. Pages 1-11 in Proc. California Nutrition Conference, Fresno, CA.

Appendix of Equations in Milk2000

Equations are the same for the alfalfa/grass and corn silage spreadsheets except that tdStarch and tdNSTNFC terms are replaced by a single tdNFC term using a true digestibility coefficient of 98% in the alfalfa/grass spreadsheet.

TDN (maintenance) = tdCP + (tdFA*2.25) + tdNDF + tdStarch + tdNSTNFC - 7

tdCP = CP*0.93

tdFA = 0.97*(EE –1)

tdNDF = NDF/NDFD * 100

tdStarch = Starch*(adStarch*1.12); maximum of 98% for adStarch*1.12

adStarch_UNPROCESSED% = 144.8 – (1.67 * DM%); min/max of 70% and 95%

adStarch_PROCESSED% = 121.6 – (0.88 * DM%); min/max of 80% and 95%

tdNSTNFC = 0.98*(NFC - Starch)

NFC = 100 – (NDF + CP + EE + ash – NDFCP)

where,

td = true digestibility

CP = crude protein

FA = fatty acids

NDF = neutral detergent fiber

NFC = non-fiber carbohydrate

NSTNFC = non-starch NFC

EE = ether extract

NDFD = in vitro 48-hour digestible NDF expressed as percent of NDF

adStarch = apparent total-tract starch digestibility

DM = dry matter

NDFCP is a constant of 1.3 (for corn silage) or 3.8 (for grasses and legumes).

Net energy of lactation at 3x maintenance (NRC, 1989):

NE_L (Mcal/lb.) = ((TDN * 0.0245) –0.12)/2.2.

Dry matter intake:

Calculated with base NDF intake set at 1.15% of body weight (Mertens, 1987) divided by 0.3 (assumes 30% NDF in ration) and then adjusted + or - 0.374 lbs for each 1% change in NDFD above or below an average NDFD (Oba and Allen, J Dairy Sci 82: 589-596).

Milk from forage:

Calculated as total energy from forage minus the cows’ maintenance energy requirement (proportioned according to the percentage of forage in the diet) divided by an NRC (1989) energy to milk conversion factor:

((NE_L * forage intake) – (0.08*613.64^.75 * percent forage in ration))/0.31