Why is forage digestibility important?

Transcription

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2 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

3 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

4 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

5 Forage digestibility is determined by many factors

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

7 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

8 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 Key: forage quality

9 How forage digestibility is estimated

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

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

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

13 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

14 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

15 Determining NDF fractions 50% NDF, % of original sample mass 40% 30% 20% 10% 0%

16 Determining NDF fractions 50% 40% 30% NDF, % of original sample mass NDFD24 20% 10% 0%

17 Determining NDF fractions 50% NDF, % of original sample mass 40% 30% NDFD30 20% 10% 0%

18 Determining NDF fractions 50% NDF, % of original sample mass 40% 30% NDFD48 20% 10% 0%

19 Determining NDF fractions 50% NDF, % of original sample mass 40% 30% 20% 10% 0%

20 LAG Determining NDF fractions 50% 40% 30% 20% 10% NDF, % of original sample mass 0%

21 LAG Determining NDF fractions 50% 40% 30% 20% 10% NDF, % of original sample mass Kd: % disappearance per hour 0%

22 LAG Determining NDF fractions 50% 40% 30% 20% 10% NDF, % of original sample mass Kd: % disappearance per hour 0% undf

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

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

25 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

26 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

27 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

28 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

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

30 Variation by forage type CVAS, 2008

31 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

32 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

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

34 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

35 Example: BMR vs. conventional CS Conv. BMR DM% NDF% ADF% NDFd% 30h Holt et al., 2010

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

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

38 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

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

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

41 Ensiled legumes have higher NDFd 60 NDFd, 30 h n = 10, n = 4, Legume hays Legume silages DairyOne

42 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

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

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

45 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

46 Forage filling effects Fiber Forage content & intake

47 Forage filling effects Fiber Forage content & intake

48 Forage filling effects Fiber Forage content & intake Passage

49 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

50 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

51 Example: BMR corn silage study Oba & Allen, 1999

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

53 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

54 Increased forage digestibility: DMI Oba & Allen, 1999

55 Increased forage digestibility: Milk Oba & Allen, 1999

56 Little effect on DM digestibility! Oba & Allen, 1999

57 Greater passage negates digestibility response Oba & Allen, 1999

58 Target-feed high quality forages

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

60 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

61 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

62 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

63 Less retention = less effective fiber While higher passage can benefit DMI, it can limit the physical effectiveness of the fiber 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

64 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

65 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 Example: 29% 38% Oba & Allen, 2000

66 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 Example: 29% 38% Oba & Allen, 2000

67 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 Example: 29% 38% Oba & Allen, 2000

68 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

69 Thank you! Questions/comments: Barry

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