BENCHMARKING FORAGE NUTRIENT COMPOSITION AND DIGESTIBILITY

Forage quality impacts diet DM intake and energy density, lactation performance, supplemental grain and protein needs, cow health and feed costs. Forage quality is highly variable both among and within forage types. Benchmarks for forage quality and variation in composition for selected nutrients and digestibility parameters are summarized herein from Dairy NRC (2001) tabular data and results from commercial feed testing labs.


Introduction
Forages can comprise over half of the dry matter (DM) in diets for high producing dairy cows depending upon forage quality, inventory and price.Forage quality impacts diet DM intake and energy density, lactation performance, supplemental grain and protein needs, cow health and feed costs.Recent increases in corn and protein supplement prices have been unprecedented, making forage quality of paramount importance for reducing purchased feed costs and improving income over feed cost.The projected impact of forage neutral detergent fiber (NDF) content and in vitro NDF digestibility (IVNDFD; % of NDF) on diet concentrate proportion, amount and cost when meeting minimum diet NDF from forage constraints are provided in Table 1.Forage quality is highly variable both among and within forage types.Forage species, variety or hybrid, stage of maturity at harvest, cutting, environmental factors, production and harvest practices, storage method (i.e.hay vs. silage, bunker vs. bag, etc.) and ensiling practices all are factors that contribute to this variation.This paper will benchmark forages for quality and variation in composition for selected nutrients and digestibility parameters.

Nutrient Composition
Means and standard deviations for crude protein (CP) and neutral detergent fiber (NDF) and calculated means for total digestible nutrients at a maintenance level of intake (TDN 1x ) and nonfiber carbohydrate (NFC) of selected forages from NRC (2001) table 15-1 are presented in Table 2. CP is highest and NDF is lowest for legume forages.The TDN 1x estimate is reasonably similar between legumes and grasses, mainly because the less lignified NDF for grasses compared to legumes results in a higher calculated digestible NDF for grasses, which offsets their lower NFC and CP contents when using the NRC (2001) summative energy equation.However, forage DMI is negatively related to its NDF content in high producing dairy cows (Mertens, 1987), which may reduce energy intake from grass compared to legume forages.The NDF content of corn silages can be comparable to legume forages, primarily due to dilution with grain that comprises a high proportion of whole-plant corn silage harvested at normal to advanced stages of maturity.Essentially the high NFC content of corn silage results in high TDN 1x estimates relative to other forages when using the NRC (2001) summative energy equation.Coefficients of variation (standard deviation divided by the mean times 100) across forages ranged from 12% to 46% and 7% to 16% for CP and NDF contents, respectively.
A survey was conducted of forage analytical data that has been posted on the internet by Cumberland Valley Analytical Services Inc. (CVAS; www.foragelab.com/),Dairyland Laboratories Inc. (Dairyland; www.dairylandlabs.com/),Dairy One Forage Laboratory (Dairy One; www.dairyone.com/Forage/),University of Wisconsin -Madison Soil & Forage Analysis Lab (Marshfield; uwlab.dyndns.org/marshfield/),and Rock River Labs Inc. (Rock River; www.rockriverlab.com).Data (n, average and standard deviation) for selected nutrients and forages are summarized in the tables.DM, CP and NDF contents for legume, grass and mixed hay-crop silages and corn silages are provided in Tables 3, 4, 5 and 6, respectively.The starch content of the corn silages is summarized in Table 7. Averaged across labs and years, the starch content of corn silage was 30.6 ± 6.7.Averaged across labs and years, the starch content of corn silage was 30.6 ± 6.7%.

Fiber
DMI and milk yield are positively related to IVNDFD (Oba and Allen, 1999).IVNDFD data (n, average and standard deviation) for legume, grass and mixed hay-crop silages are provided in Tables 8 (48-h) and 9 (30-h).Both 48-h and 30-h IVNDFD data for corn silage appear in Table 10.Averaged across labs and years, the IVNDFD of corn silage was 58 ± 6% and 51 ± 7% for 48-h and 30-h incubations, respectively.

Starch
Total tract digestibility of starch by dairy cows is variable ranging from 70% to 100% (Firkins et al., 2001).Schwab et al. (2003) developed regression equations to predict corn silage starch digestibility (StarchD) through the total digestive tract in dairy cows.Slopes of the regression equations indicate that DM content had a greater impact on total-tract StarchD of unprocessed than processed corn silage.At 35% whole-plant DM, predicted total-tract StarchD was 5%-units greater for processed than unprocessed corn silage.At lower DM concentrations the difference between processed and unprocessed silage was smaller and increased as DM concentration increased.This may be due to the starch in dryer kernels being less available for digestion.The effect of whole-plant DM content and its interaction with processing will likely vary depending on hybrid type, soil type, growing conditions, and dry-down rate.The processing effect will vary depending on chop length and roll clearance.Therefore, several laboratory methods have been developed to assess StarchD of diverse corn silage samples (i.e.highly variable DM content, chop length, roll clearance, kernel hardness, etc.).Results from various assays to assess StarchD performed at commercial feed testing labs are summarized in Table 11.------------------------DM basis------------------------

Table 1 .
Impact forage NDF content and digestibility on diet concentrate proportion, amount and cost when meeting minimum diet NDF from forage constraints.Calculated assuming DMI of 50 lb.per cow per day. 2 Calculated assuming concentrate price of $0.15 per lb. of DM.

Table 3 .
Analysis of legume silages obtained from testing lab summaries.

Table 4 .
Analysis of grass silages obtained from testing lab summaries.

Table 5 .
Analysis of mixed hay-crop silages obtained from testing lab summaries.

Table 6 .
Analysis of corn silages obtained from testing lab summaries.

Table 11 .
Results from lab assays to assess starch digestibility (StarchD) for corn silage samples.