Why is forage digestibility important?

Outline 1. How fiber digestibility is assessed in the lab 2. Factors affecting forage quality 3. How lactating dairy cows respond to changes in fiber digestibility

Why is forage digestibility important? Forages typically comprise 40 60% of lactation diets Forages account for 30-50% of the energy in a typical lactation diet Forage digestibility has even more important functional roles: Impacts feed intake Impacts fiber effectiveness

Forage digestibility is determined by many factors

Forage digestibility is determined by many factors Rumen-degradable protein (RDP) must be adequate to allow for microbial growth

Forage digestibility is determined by many factors Rumen-degradable protein (RDP) must be adequate to allow for microbial growth Low ph inhibits cellulosic bacteria in the rumen, and therefore limits fiber digestibility Diets high in rapidly-fermentable carbohydrates can dramatically decrease fiber digestibility

How forage digestibility is estimated

How forage digestibility is estimated 24, 30, or 48 hours

How forage digestibility is estimated 24, 30, or 48 hours Measure residual NDF to assess relative rate of digestion

How forage digestibility is estimated 24, 30, or 48 hours Measure residual NDF to assess relative rate of digestion 120 or 240 hours

How forage digestibility is estimated 24, 30, or 48 hours 120 or 240 hours Measure residual NDF to assess relative rate of digestion Measure undf to assess fraction that is not ever digestible

IVTDMD vs. NDFd In vitro true dry matter digestibility (IVTDMD) Calculated as NDF residue initial DM weight Anything other than NDF is 100% digestible Neutral detergent fiber digestibility (NDFd) Calculated as NDF residue initial NDF in sample Not driven by overall nutrient composition

Determining NDF fractions 50% NDF, % of original sample mass 40% 30% 20% 10% 0% 0 20 40 60 80 100 120 140

Determining NDF fractions 50% 40% 30% NDF, % of original sample mass NDFD24 20% 10% 0% 0 20 40 60 80 100 120 140

Determining NDF fractions 50% NDF, % of original sample mass 40% 30% NDFD30 20% 10% 0% 0 20 40 60 80 100 120 140

Determining NDF fractions 50% NDF, % of original sample mass 40% 30% NDFD48 20% 10% 0% 0 20 40 60 80 100 120 140

Determining NDF fractions 50% NDF, % of original sample mass 40% 30% 20% 10% 0% 0 20 40 60 80 100 120 140

LAG Determining NDF fractions 50% 40% 30% 20% 10% NDF, % of original sample mass 0% 0 20 40 60 80 100 120 140

LAG Determining NDF fractions 50% 40% 30% 20% 10% NDF, % of original sample mass Kd: % disappearance per hour 0% 0 20 40 60 80 100 120 140

LAG Determining NDF fractions 50% 40% 30% 20% 10% NDF, % of original sample mass Kd: % disappearance per hour 0% undf 0 20 40 60 80 100 120 140

What value should I used to estimate 24 h NDFD? 30 h NDFD? 48 h NDFD? Lag, Kd, and undf? in vivo digestion rate?

What value should I used to estimate 24 h NDFD? 30 h NDFD? 48 h NDFD? Lag, Kd, and undf? in vivo digestion rate? Cow

What value should I used to estimate 24 h NDFD? 30 h NDFD? 48 h NDFD? Lag, Kd, and undf? in vivo digestion rate? Cow Flask

What value should I used to estimate 24 h NDFD? 30 h NDFD? 48 h NDFD? Lag, Kd, and undf? in vivo digestion rate? Cow Chewing, rumination, ph, passage rate Flask

Outline 1. How fiber digestibility is assessed in the lab 2. Factors affecting forage quality 3. How lactating dairy cows respond to changes in fiber digestibility

Primary determinants of forage Forage species Genetics Grain proportion Lignin and other fiber components Growing environment Fermentation digestibility

Variation by forage type CVAS, 2008

Do not evaluate across forage types! Grasses typically have greater NDFd than legumes This does not reflect reality in the cow Grasses are more filling and tend to limit milk Differences in physical properties? Cow doesn t eat finely-ground forage! Comparisons should be made within forage type

Corn silage genetics: IVTDMD vs. NDFd IVTDMD is driven primarily by grain content Across 32 corn silages 1, NFC content explained 61% of the variation in IVTDMD did not significantly correlate with NDFd NDFd is more valuable as a predictor of response to the forage Grain yield can be evaluated as a separate factor 1 Ferreira & Mertens, 2005

The chemical composition of fiber is extremely complex Secondary Wall Hemicellulose Lignin Cellulose Primary Wall + many other minor components

Genetics influence fiber composition Example: brown midrib (bmr) mutation Naturally-occurring mutant that is low in lignin Even ignoring bmr varieties, there is wide variation in fiber composition among forages

Example: BMR vs. conventional CS 45 40 35 30 25 20 15 Conv. BMR 10 5 0 DM% NDF% ADF% NDFd% 30h Holt et al., 2010

Mature alfalfa has lower NDF Dig. Hoffman et al., 2003

Mature corn silage has lower NDF Dig. Hoffman et al., 2003

Mature forages have lower NDF Dig. As forages mature, cell walls become increasingly lignified Protein, sugars also decrease during this time This leads to the well-recognized inverse relationship between total NDF content and NDF digestibility of forages

NDFd can be assessed at harvest for corn silage Cherney et al., 2007

NDFd, 30 hour But heating of legumes has a large impact on NDFd CVAS

Ensiled legumes have higher NDFd 60 NDFd, 30 h n = 10,661 50 n = 4,469 40 30 20 10 0 Legume hays Legume silages DairyOne

Outline 1. How fiber digestibility is assessed in the lab 2. Factors affecting forage quality 3. How lactating dairy cows respond to changes in fiber digestibility

How do cows respond to NDF Increased in vivo NDF digestion? Increased dry matter intake? digestibility? Increased milk production? Milk fat depression? Yes, sometimes!

Digestibility responses Digestibility can be improved, but can be derailed by: Increased passage rate 70 60 50 40 30 20 Disrupted rumen 10 function 0 Conv. BMR In vitro NDFd% 30h Diet total-tract %NDF dig. Holt et al., 2010

NDF digestibility likely alters passage as well as digestion rate Particle breakdown occurs as structural carbohydrates are degraded by ruminal microbes Decreased particle size results in passage from the rumen

Forage filling effects Fiber Forage content & intake

Forage filling effects Fiber Forage content & intake Passage

Shorter retention time can promote greater DMI In cases where gut fill limits feed intake, that fill is largely fiber particles Fill is a function of both forage intake and mean retention time Therefore, if retention time is decreased, forage intake can be increased with the same gut capacity

Expected response to increased NDFd 1 unit increase in NDFd (30-h values): 0.3 lb/d increase in dry matter intake 0.5 lb/d increase in 4% FCM yield Oba & Allen, 2005

Example: BMR corn silage study Oba & Allen, 1999

Digestibility and production responses 45 40 CON BMR +2.6 kg/d 35 Only 3 units 30 +2 kg/d 25 20 Total-tract NDFd, % DMI, kg/d 3.5% FCM, kg/d Oba & Allen, 1999

Experimental Design Pre-trial Period 1 Period 2 Common Diet Group 1 Group 2 A B B A Response = Change between diets (B-A) Response is plotted against pre-trial milk Oba & Allen, 1999

Increased forage digestibility: DMI Oba & Allen, 1999

Increased forage digestibility: Milk Oba & Allen, 1999

Little effect on DM digestibility! Oba & Allen, 1999

Greater passage negates digestibility response Oba & Allen, 1999

Target-feed high quality forages

Target-feed high quality forages Improved forage digestibility has the greatest benefit for high-producing cows.

Target-feed high quality forages Improved forage digestibility has the greatest benefit for high-producing cows. In these cows, the higher passage rate relieves constraints on DMI

Target-feed high quality forages Improved forage digestibility has the greatest benefit for high-producing cows. In these cows, the higher passage rate relieves constraints on DMI In lower-producing cows, a greater increase in digestibility can occur

Less retention = less effective fiber While higher passage can benefit DMI, it can limit the physical effectiveness of the fiber 70 60 50 40 30 20 10 0 Conv. BMR In vitro NDFd% 30h Diet total-tract %NDF dig. 3,4 3,2 3 2,8 2,6 2,4 2,2 2 Milk fat % Holt et al., 2010

Milk fat depression can result from very high NDFd General advice is to feed higher forage diets Increase pendf to offset shorter retention time Energy differential vs. concentrates is less Oba & Allen, 2000

Summary Fiber digestibility plays huge role in determining feed intake and productivity Particle breakdown allows for passage from rumen, opening up space for more feed Metabolism of fiber provides energy as VFAs BUT more rapid digestion and passage also decreases its physically effective value

Thank you! Questions/comments: Barry Bradford bbradfor@ksu.edu @AnimNutr

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