Strategies for coping with poor forage digestibility in dairy rations: response to enzyme supplements
Cell wall is measured by determining neutral detergent fiber (NDF). The detergent fiber system was developed during a period when nutritionists were establishing uniform feed fractions. A uniform feed fraction is a chemically measured feed component that is always the same from a nutritional standpoint. This concept is the basis of the OSU energy equation (Weiss, 1996). If cell wall was a uniform feed fraction or were composed of two or more uniform feed fractions, dairy cow nutrition would be a much less demanding career!
While the detergent fiber system has had an enormous impact on feeding dairy cows, detergent fibers cannot be considered uniform feed fractions.
Neutral detergent fiber successfully represents fiber that will stimulate rumination and rumen function (providing particle size is adequate). However, digestibility of NDF varies for both grasses (including corn silage) and legumes. Therefore, from an energy and digestibility standpoint, NDF is NOT a uniform feed fraction. Acid detergent fiber (ADF) was intended to isolate the cell wall components that were more resistant to digestion than other fiber components. Unfortunately, ADF does not recover ALL truly indigestible cell wall components, although it tends to be more highly correlated with digestibility than any other feed fraction.
Because of the correlation between ADF and feed energy, the detergent fibers are widely used to estimate the energy content of feeds. Based on practical experience, many nutritionists have reservations about this practice.
One of the developers of the detergent fiber system even warns against using it this way: “The use of either NDF or ADF to predict value of forages are mis-uses, because of the failure to consider rational models of the availability of forage fractions…” –P.J. Van Soest (1993).
The failure of detergent fibers to predict energy content can be traced to the mechanisms that control cell wall digestibility. These include lignin encrustation of the plant cell wall (physical effect), how tightly cellulose strands are woven together, and the amount of open space within the plant cell wall that is accessible to fibrolytic bacteria and/or enzymes.
Unpredictable (generally lower than expected) digestibility of forage fiber is a major problem in balancing dairy rations that perform as expected.
Every year at FARME Institute, we evaluate forage digestibility on thousands of samples where cow performance does not correspond to the predicted energy content of the ration. A common problem is illustrated in Table 1.
For these two corn silage samples, similar chemical composition leads to identical predictions of energy content. Digestibility of both cell wall and cell contents differed between the two silages. As a result, their actual ability to support milk production was very different.
Table 1. Comparison of energy content of two corn silages predicted by regression on chemical composition or predicted using measured ruminal digestibility.
Troubleshooting digestibility problems on the farm
In the northeastern US, corn silage is a frequent culprit when we investigate digestibility-related milk production problems. Evaluating digestibility of the grain is easy; simply look at the manure to see if there is a problem. A macro digestibility (digestibility of an unground, undried sample) evaluation can quantify the seriousness of the problem.
Over the past five years, progress in corn silage hybrid selection, harvest and ensiling management in combination with the growing use of kernel processors has dramatically changed digestibility patterns of the grain in corn silage. As a result, we can no longer consider the absence of grain in the manure as an unequivocal positive. Instead, we must also consider the balance between rapidly and more slowly digestible non-structural carbohydrate fractions. On-farm samples received by FARME Institute for digestibility evaluation have shown an exponential increase in rapidly digestible NSC/starch fractions over the past two to three growing seasons.
It is not uncommon to find corn silage with nearly complete cell content digestion after 3 hrs of ruminal digestion. In the wrong ration and/or management system, such forages can result in severe acidosis problems.
Evaluating whether herd production is suffering due to poor fiber digestibility is more difficult. Indigestible fiber does not hydrate quickly in the rumen. Hydration of forage particles is required to increase forage particle specific gravity to the point that the particles can pass out of the rumen. In addition, indigestible fiber may be resistant to particle size reduction during rumination, since digestive processes have not weakened the cell wall structure. The result is that indigestible fiber can be retained in the rumen for longer periods than more digestible fiber sources. Increased fiber retention times can increase ruminal fill, possibly enough to limit intake. Michigan State researchers have reported a 0.5 lb/head/day reduction in dry matter (DM) intake for each percentage unit reduction in total ration NDF digestion (Allen and Oba, 1996).
At FARME Institute our observations on corn silage based rations are that a macro in situ NDF digestion value of 20% or less after 30 hrs of digestion nearly always reduces the herd average DM intake by 1-2 lbs/head/day. Poor fiber digestion can also reduce efficiency of milk production. Low fiber digestion reduces the energy content of the ration. Normal forage testing will yield estimated values that are too high. If physical fill does not preclude it, cows may increase intake to meet energy needs. The result is lower than normal milk:feed ratios and poor profitability. In the longer term, forage tests that result in overestimated ration energy will reduce herd fertility by prolonging periods of negative energy balance. This will result in under-conditioned cows at dry-off. Poor fiber digestion may be due to a number of causes including acidosis, forages harvested at advanced maturities and improper hybrid selection. Regardless of the cause, poor fiber digestion generally results in either reduced intake or low milk production with a normal or high intake. Because there are a number of possible causes for these problems, an actual digestibility analysis is advised to confirm the problem.
Managing poorly digestible forages
Management may reduce the risk for low fiber digestibility in forages. Management practices to maximize fiber digestibility include the following.
- Avoid using ‘dual purpose’ corn hybrids from grain-breeding programs
- Avoid harvesting overly mature forages.
- Store over mature forages separately. - Manage grass-legume mixtures according to the predominant species.
- This may affect legume health. - Forages grown during periods of excessive soil moisture will have lower than normal fiber digestion; plan accordingly.
- This effect is compounded by high temperatures.
While it is easy to be an ‘armchair’ forage manager, the reality for most dairy operations is that low digestibility forages will be a problem from time to time. Ignoring the problem is not an option; a solution is required! Correcting management problems will improve next year’s forages.
Meanwhile, the existing forages must be used in current dairy diets. As farm advisers, we must also face the reality that forage quality problems may not be the most urgent problem on the farm. Suggested management changes to improve forage quality may be delayed for several years.
Several options exist to deal with the problem of poor forage fiber digestion. The most common solution is to dilute the low digestibility forage with another feedstuff(s). To make this solution work, identifying the poorly digested forage is necessary. In addition, the producer must have an adequate inventory of high digestibility forages to replace the problem forage. The high digestibility forages must be stored in a way that they can be fed as needed. Since many farms do not have adequate forage inventories and/or storage to effectively replace poorly digested forages with more highly digested sources, a related solution is to use a high by-product fiber diet. In this case, forage fiber is replaced with more digestible by-product fiber sources such as soy hulls, beet pulp, and citrus pulp. This approach has been very successful in a number of cases, although it requires careful ration balancing and management. Depending on feed prices and other ration constraints, using enough by-product fiber to correct a fiber digestion problem may not be economically feasible.
A major problem with using other feedstuffs to replace poorly digested forages is that the poor quality forage is still on the farm. Sooner or later (and probably sooner), it must be fed to something. Using an enzyme product can help solve poor fiber digestibility problems while allowing forage inventories to be used as planned. In addition, using an enzyme product may be more cost effective and/or more manageable than replacing poorly digested forages with either by-product fiber sources or highly digestible forages.
Using enzymes to improve fiber digestion
Why would we expect fibrolytic enzymes to be beneficial in rations when fiber digestion is impaired? Unquestionably, the rumen and its microbes exist to digest fiber. However, the most active fiber bug in the rumen, Fibrobacter succinogenes, requires close association between substrate and bacterium for digestion to occur (Dehority, 1993). Enzymes require less open space to penetrate cell structure than do ruminal microbes. As a result, enzymatic action may open up the cell wall for subsequent invasion by ruminal microbes. Enzyme activity may well be different for commercial enzyme products than for rumen microbial enzymes.
A number of studies have shown positive responses to fibrolytic enzymes (Feng et al., 1992a,b; Beauchemin et al., 1995; Beauchemin et al., 1997; Feng et al., 1996; Lewis et al., 1996; Treacher et al., 1997; Krause et al.,1998; Zinn and Salinas, 1999). However, other controlled studies have not shown a benefit for fibrolytic enzymes (Sanchez et al., 1996). Responses in the field continue to be variable. The reasons behind these variable responses must be determined before fibrolytic enzyme use is maximized.
Many fibrolytic enzyme products are sensitive to application method. Free enzymes in the rumen can become expensive soluble protein sources.
Alltech’s Fibrozyme avoids this problem by glycosylation, which makes the enzyme resistant to proteolysis (Plou et al., 1999). For other enzyme products, formation of a stable enzyme-feed complex is required to avoid enzyme destruction in the rumen. Beauchemin et al. (1997) have reported much larger fibrolytic effects when enzymes were applied to dry rather than wet forages. Enzyme preparations that require this type of application may not be practical for all herds. Another concern for enzyme products that depend on a stable enzyme-feed complex to avoid ruminal proteolysis is the fact that the complexed enzyme must be bound to the poorly digested fiber.
Poor fiber digestion is not the only cause of low milk production and/or intake. Fiber composition can be used to estimate maturity at harvest; fiber digestibility decreases as forage maturity increases. Johnston (2000) found significant, but low, negative correlations between the content of NDF and ADF in corn silage and response to Fibrozyme. However, these data did not support the use of chemical composition alone to predict successful use of enzymes.
To determine whether poor fiber digestion is truly a problem, digestibility must be measured directly. Unfortunately, there is no standard method for digestibility testing and few commercial labs offering this service. Digestibility testing may employ either in vitro or in situ methods, ground or unground samples, and single or multiple time points. While the various digestibility test conditions have not been compared for their ability to predict animal response to fibrolytic enzymes, available information gives some direction.
Most commercially available digestibility tests employ in vitro systems that require forages to be finely ground prior to testing. Grinding forages is known to increase digestibility. In addition, grinding forages will reduce or eliminate digestion time delay (Ellis et al., 1994). Effects of grinding on extent of digestion increase as potential digestion of forages decreases. All of the above factors suggest that grinding forages for digestibility testing may reduce our ability to predict fibrolytic enzyme responses.
A single time-point digestion extent value is the only option at most forage testing laboratories. Even at FARME Institute, which offers a number of digestibility testing options, the single time-point digestion extent test is most frequently requested.
A single time-point digestion extent value will reflect the rate of digestion, provided digestion has not progressed to completion during the time period. For ground samples in vitro, this would mean a 12-16 hr digestion period.
For unground samples in situ, forage digestion will not be complete until after 48 hrs. However, using a single time-point to predict digestion rate also assumes that digestion occurs at a continuous, constant rate.
Unfortunately, fiber digestion is seldom a continuous, constant process. Many times forages with the same extent of digestion differ greatly in the pattern of that digestion. Using an in vitro protocol with ground forages, Johnston (2000) was unable to find a single, consistent time point that would predict Fibrozyme efficacy on corn silage.
Given the nature of forage fiber digestion and the findings of Johnston, we may conclude that a multi-point digestibility test is most appropriate for determining which forages are most likely to show improved digestibility with Fibrozyme. However, we must consider that complete predictability of an effect is less important in the field than a test that improves our odds of successfully using a product. Along the lines of this argument, Johnston (2000) has shown that digestion of corn silage (in vitro, ground) after 6 hrs of incubation has a high, negative correlation with Fibrozyme efficacy.
Dawson and Tricarico (1999) also reported that the Fibrozyme effect in in vitro systems is generally larger during the initial stages of digestion.
At FARME Institute we recently conducted research with Fibrozyme using producer silage samples of known digestibility potential and pattern.
Results are detailed in Tables 2 and 3. The test was conducted using our standard digestibility techniques which include using undried, unground silages that were minimally processed to simulate mastication. The sole deviation from our normal protocol was that samples were incubated in vitro, rather than in situ, to allow for comparison of Fibrozyme and no additional enzyme using the same rumen fluid donor.
Although corn silage is more frequently identified in our lab as bringing indigestible fiber to dairy rations, both corn silage and hay crop samples were evaluated. Fibrozyme completely eliminated NDF digestion lags for both forages. These results were similar to those reported by Johnston (2000) with ground corn silage in an in vitro system. In the corn silage samples only, Fibrozyme reduced indigestible fiber fractions. This finding differs from those of Johnston (2000) when using dried and ground corn silage.
From the results of our trial and others (Johnston, 2000; Dawson and Tricarico, 1999), we can begin to identify digestibility characteristics that will increase the odds of success when using Fibrozyme. Fibrozyme is most likely to work with forages that have either poor initial fiber digestion (3-6 hrs) and/or those with a significant lag time. Fibrozyme can work with corn silages that have a high indigestible fiber content. Fibrozyme may also be beneficial with forages that have a slow overall digestion rate.
Failure of a fibrolytic enzyme to produce a production response may occur even when the enzyme increases ruminal fiber digestion. Fibrolysis increases carbohydrate available for fermentation in the rumen. The subsequent carbohydrate fermentation results in volatile fatty acids that will increase the ruminal acid load. Use of fibrolytic enzymes without considering the fermentation pattern of the entire ration can result in either transient or chronic ruminal acid overload. Unfortunately, most current ration balancers and evaluators do not provide information on ruminal acid levels nor do they provide maximal allowable levels. If the use of an enzyme product results in signs of acid overload, the ration must be adjusted. Rations should be balanced conservatively for nonstructural carbohydrate, starch and rapidly digested starch fractions. Buffers may be beneficial, since fibrolytic enzymes appear to increase initial fiber digestibility more than total digestibility (Dawson and Tricarico, 1999). In some cases, it may be necessary to balance rations according to guidelines for high by-product fiber diets.
Table 2. Effect of Fibrozyme on corn silage digestion in vitro.
Table 3. Effect of Fibrozyme on hay crop digestion in vitro.
If an enzyme product successfully increases either rate or extent of fiber digestion, responses will be rapid. When attempting to correct a fiber digestibility problem, the expected response (milk and/or DM intake) should be monitored closely. If a response does not occur within 2-3 weeks, efficacy and management of the enzyme product should be re-evaluated.
Finally, the fact that all fibrolytic enzyme products are not the same must be considered. Fibrolytic enzymes are typically cellulases, which degrade cellulose, or xylanases that act on hemicellulose (Beauchemin et al., 1997). Both cellulase and xylanase are generic terms for groups of specific enzyme activities. As a result, two products with identical labels for enzyme level can differ in effects on ruminal fiber digestion. In field applications, this may mean that failure of one enzyme product to produce a response does not mean other products will be equally ineffective. Since barriers to fiber digestion will vary from forage to forage, a particular enzyme product may not work in all situations.
Conclusions
Poor fiber digestion can reduce milk production and/or dry matter intake of dairy cows, resulting in serious financial losses. Fibrolytic enzymes can allow producers to feed out forages with low fiber digestion while correcting digestibility problems.
References
Author: JOANNE SICILIANO-JONESAllen, M. and M. Oba. 1996. Increasing fiber digestibility may increase energy density, dry matter intake. Feedstuffs, Nov. 18, p. 123.
Beauchemin, K.A., L.M. Rode and V.J.H. Sewalt. 1995. Fibrolytic enzymes increase fiber digestibility and growth rate of steers fed dry forages. Can. J. Anim. Sci. 75:641.
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Dawson, K.A. and J.M. Tricarico. 1999. The use of exogenous fibrolytic enzymes in ruminants. In: Biotechnology in the Feed Industry, Proc. of the 15th Annual Symposium (T.P. Lyons and K.A. Jacques, eds).
Nottingham University Press, Nottingham, UK. pp. 303-312.
Dehority, B.A. 1993. Microbial ecology of cell wall fermentation. In: Forage Cell Wall Structure and Digestibility (H.G. Jung et al., eds). ASA-CSSASSSA, Madison, WI. p. 425.
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Zinn, R.A. and J. Salinas. 1999. Influence of Fibrozyme on digestive function and growth performance of feedlot steers fed a 78% concentrate growing diet. In: Biotechnology in the Feed Industry, Proceedings of the 15th Annual Symposium (T.P. Lyons and K.A. Jacques, eds). Nottingham University Press, Nottingham, UK. pp. 313-319.
F.A.R.M.E. Institute, Inc. Homer, NY
Good article about poor forage digestibility in dairy rations Joanne! Thanks.
J. Goodson
