Evaluation of Feeds II Digestibility 1 16 23 Evaluation of Feeds II Digestibility of Nutrients Learning Objectives for Evaluation of Feeds Digestibility of Nutrients I How do we evaluate digestibility of feeds II How does digestibility relate to energy value of feeds III Understand the primary factors that affect nutrient digestibility of feeds IV and the differences and similarities between species in how these factors affect digestibility V How do we measure TDN value of feeds VI Why is TDN not always the best energy value to use Chemical analysis vs nutrient digestibility Nutritional value The potential nutritional value of a feed can be assessed by chemical analysis e g crude protein The actual nutritional value of a feed can only be determined after making allowances for losses associated with o Digestion o Absorption o Metabolism Steer fitted with a fecal collection harness for conducting a digestion trial 1 2 3 1 Evaluation of Feeds II Digestibility 1 16 23 Chemical analysis vs nutrient digestibility Digestibility Defined as that proportion of feed that is not excreted in the feces and is thus assumed to be absorbed by the animal Fecal losses of nutrients represents the single largest loss of the potential energy value of a feed Methods to determine nutrient digestibility Conventional digestion trial The most accurate method to measure nutrient digestibility Objective To accurately quantify nutrients consumed and nutrients excreted in feces for a particular feed or diet over a given period of time Digestion trials are typically conducted for 5 to 7 d Procedures Adapt animals to metabolism stalls and to consuming the test diet Precisely measure the amount of the diet fed and feed refusals each day Methods to determine nutrient digestibility Procedures Collect and weigh fecal output daily Avoid urine contamination of feces Prepare feed and fecal samples for chemical analyses Conduct chemical analysis e g moisture protein fiber of both feed and fecal samples 4 5 6 2 Evaluation of Feeds II Digestibility 1 16 23 Methods to determine nutrient digestibility Procedures Dry matter DM or nutrient digestibilities can be calculated as shown below Apparent DM or Nutrient Digestibility DM or Nutrient In DM or Nutrient Out x 100 DM or Nutrient In Example calculations for DM and acid detergent fiber ADF digestibilities from a sheep trial shown in table Item Amount consumed or excreted wet basis g d Dry matter Amount dry basis g d DM digestibility Feed Fecal 1 150 91 972 40 1 046 389 1 046 389 1 046 x 100 62 9 ADF DM basis 40 ADF amount g d 156 ADF digestibility 377 156 377 x 36 377 100 58 6 Factors affecting apparent digestibility Nutrient composition 1 Nutrient composition Digestibility of a feed is closely related to its chemical composition Both the amount NDF and type of chemical fiber lignin as fraction of NDF greatly affect the digestibility of a feed An inverse relationship exist between fiber CHO concentration and feed DM digestibility Carbohydrate digestibility NDF ADF ADF or NDF concentration NDF Neutral detergent fiber Cellulose Hemicellulose Lignin ADF Acid detergent fiber Cellulose Lignin Factors affecting apparent digestibility Nutrient composition Low starch diet High starch diet 70 0 6 4 57 9 66 1 3 84 10 65 43 8 26 7 86 5 4 27 Item Hay Corn Starch NDF Dry matter digestibility Digestible energy kcal kg Expressed on DM basis Swanson et al 2000 7 8 9 3 Evaluation of Feeds II Digestibility 1 16 23 Factors affecting apparent digestibility Plant species 2 Plant species Plant forage species differ in digestibility In general the digestibility of forage species rank as follows 1 Legumes highest 2 Cool season grasses intermediate 3 Warm season grasses lowest In general annual grass legume species are more digestible then perennial grass legume species Red Clover Legume Wheatgrass Cool season Bahiagrass Warm season Common legume and cool and warm season forages Legumes Alfalfa Red clover White clover Alsike clover Birdsfoot trefoil Arrowleaf clover Crimson clover Vetch Winter pea Peanut Kudzu Lespedeza Cowpea Alyceclover Cool season grasses Warm season grasses Timothy Kentucky bluegrass Smooth bromegrass Orchardgrass Tall fescue Ryegrass Wheatgrass Oats Barley Triticale Indiangrass Switchgrass Side oats gramma Buffalograss Lovegrass Bermudagrass Dallisgrass Bahiagrass Kleingrass Johnsongrass Forage sorghum Sudangrass Crabgrass Pearl millet Sorghum sudan hybrids Digestibility Legumes Cool season Warm season grasses Factors affecting apparent digestibility Plant species 3A Within species differences Tifton 85 is a hybrid of coastal bermudagrass that was selected for improved yield and digestibility Rouquette et al 1985 found that stocker calves gained faster while grazing Tifton 85 compared to common coastal bermudagrass ADG lb day 1 01 Treatment Coastal Pasture only Coastal 2 lb supplement Tifton 85 Pasture only Tifton 85 2 lb supplement Gain per acre during 84 day trial 1 69 1 30 2 02 Gain per acre lb 279 360 465 550 10 11 12 4 Evaluation of Feeds II Digestibility 1 16 23 Factors affecting apparent digestibility Plant species 3B Within species differences A non transgenic low lignin alfalfa variety AFX 1060 contains 7 to 10 less lignin than conventional alfalfa AFX 1060 alfalfa plants also have a higher leaf to stem ratio AFX 1060 s reduction in lignin increases fiber digestion by 5 to 10 and improves animal intake Increased crude protein by 3 to 5 units Net impact for dairy cattle Additional 2 to 3 lb of milk per cow per day Factors affecting apparent digestibility Plant species Percent of Maximum Yield or Quality 3C Within species differences Alfalfa has been genetically modified to contain less lignin Low lignin GMO alfalfa is higher in digestibility and intake than conventional alfalfa Low lignin GMO alfalfa can be harvested later higher yield while maintaining the same nutritional quality compared to conventional alfalfa 140 120 100 80 60 40 20 0 GMO Alfalfa Quality Non GMO Alfalfa Quality Forage Yield 5 10 15 25 20 35 Days of Regrowth 30 40 44 Factors affecting apparent digestibility Forage maturity 4 Forage maturity As forage plants approach physiological maturity the leaf to stem ratio decreases and DM yield increases As the plant matures o Cell wall content increases NDF content increases o DM digestibility decreases o Protein content decreases Thus an inverse relationship exists between forage quality vs quantity as the plant matures Peak DM yield Peak in Dry Matter