Balancing Carbohydrates for Optimal Rumen

Function and Animal Health

Sandra R. Stokes

Texas A&M University, Route 2, Box 1,
Stephenville, TX, 76401, U.S.A.

Take Home Messages


Carbohydrates make up the largest nutrient component in dairy diets, accounting for greater than 65% of ration dry matter. Carbohydrates are present in the diet in two distinct forms: structural and nonstructural. Compatible combinations of carbohydrates are important in the diet to promote strong ruminal digestion (thus yielding high levels of energy production), support high yields of microbial protein synthesis, and maintain a stable fermentation. Consequences of a carbohydrate imbalance include inefficient ruminal digestion, body weight loss, and animal health complications (acidosis, displaced abomasums, and laminitis). Balancing dietary carbohydrates for maximum energy intake, while supplying adequate fiber for rumen health, is an art as much as a science. Much work has been done in this area over the past several years and nutritionists now have an idea of rate and extent of ruminal digestion of the common feedstuffs used in dairy rations. However, theses rates and extents are influenced considerably by field conditions such as feeding management. Feeding management factors affecting carbohydrate digestion include number of feedings per day, total mixed ration (TMR) or component feeding, ingredient inclusion, and forage base. To what extent these conditions influence digestion has not been clearly defined; therefore, many of the diet adjustments done in the field are guided by past experience or animal response. Research on individual feedstuffs and processing effects has evaluated ruminal responses in controlled settings; although much information is available on individual ingredients, the challenge for nutritionists lies in assimilating this information under the various field conditions that occur.


Structural Carbohydrates

The fibrous fraction contains cellulose, hemicellulose, and lignin. It is the support structure of the plant, thus the terms "structural carbohydrate" or "cell wall carbohydrate". The difficulty in characterizing this fraction lies in its complexity; fiber is a mixture of distinctly different chemical units with a high degree of variability in digestibility. Fiber was traditionally measured as crude fiber, but presently is further proportioned into neutral detergent fiber (NDF=cellulose, hemicellulose, lignin) and acid detergent fiber (ADF=cellulose and lignin). These values more accurately identify the structural fraction into those components that affect intake (NDF) and those that affect digestibility (ADF). However, challenges to using these values include the reality that not all NDF behaves the same in the cow and degree of lignification can influence fiber digestion in the rumen.

Nonstructural Carbohydrates

Generally, this component describes the non-NDF fraction and is composed primarily of sugar, starch, and pectin. The complexity within this component is equal to that of the fiber fraction, in that there can be a significant variation among feedstuffs in quantity of starch, sugar, and pectin which can yield considerable differences in ruminal fermentation.

Starches. Starches average between 20 and 40% of the diet. The starch content of cereal grains ranges from 45% (oats) to 72% for corn (18). Forages vary in starch content from very low levels for alfalfa and grasses to 35% for corn silage.

Sugars. The sugar content of dairy cattle diets is typically very low; however, it may be affected by inclusion of ingredients such as molasses, sugar beets, and whey. The passage rate of sugars is very high, due to their solubility; however, in spite of their high passage rate, their high rate of fermentation allows very little escape of unfermented sugars from the rumen.

Pectins. Although technically associated with the structural component of the plant, pectin is almost completely digested (90 to 100%) in the rumen (20).

The non-fibrous fraction is referred to in the popular press as nonfiber carbohydrate (NFC) and nonstructural carbohydrate (NSC), among others. Although the various terms are used interchangeably, one must be cautious in assessing these values in ingredients unless it is known how the values were derived. The nonstructural fraction are identified through two different methods; chemical analysis or by difference calculation. The chemical assay uses an enzymatic technique to quantify the level of starch and sugars in a feedstuff. This is a difficult and time-consuming technique and not presently offered by many commercial laboratories. A limitation associated with chemical analyses of NDF and NSC is that both exclude components such as pectins, gums, and fermentation acids of ensiled feeds. The latter can account for a significant part of feed ingredients such as beet pulp, alfalfa, soyhulls, and silages. Perhaps a more practical alternative is the difference equation, which uses NDF, crude protein, fat, and ash values to estimate the residual fraction (NSC = 100- (% NDF + % crude protein + % fat + % ash)). The exclusion of miscellaneous components (pectin, gums, etc.) may account for the considerable differences in NSC values sometimes observed when comparing those determined by enzymatic analysis to those determined by difference calculations. Table 1 illustrates the potential for these differences. An advantage to using calculated values of NSC is the inclusion of pectins, while a disadvantage would be that these values tend to overestimate ensiled feedstuffs, due to fermentation acids.

Estimation of Ration Carbohydrates and Factors Affecting

Carbohydrate Digestion

Rumen digestion of carbohydrates can be affected by both chemical and physical factors. The amount of carbohydrate utilized is a function of both digestibility and site of digestion. Measures of carbohydrate digestion need to be determined, as does the extent of influence that factors such as grain source, grain processing, and site of digestion may have on carbohydrate digestion and utilization by the lactating animal.

Estimating Ration Carbohydrate Level

The total carbohydrate typically averages 65 to 70% of the dry matter. Considering the significance of this component to the dairy ration, an estimate of the digestibility of this component would be ideal for nutritionists as they balance rations. A crude calculation for dietary carbohydrate supply is the sum of NSC + NDF. While this estimates dietary carbohydrate, it does not account for the availability of this fraction to the animal. The best assessment of carbohydrate digestibility would be an in vivo measurement; however, these are expensive, time-consuming, and not widely commercially available. There are laboratory estimates of carbohydrate availability utilizing an enzyme incubation; again, these can be time-consuming and are not commonly available from commercial laboratories.

Table 1. Neutral detergent fiber and nonstructural carbohydrate analyses of selected feedstuffs1.
% DM % CP % NDF %NFC2 %NSC3

Corn silage

Sorghum silage

Alfalfa hay

















Corn, dry

Corn, HM

Barley, dry





























Bakery waste

Beet pulp

Brewers, wet

Brewers, dry

Distillers, corn

Gluten feed

Corn hominy

Wheat midds

Soy hulls














































Soybean meal, 44%

Canola meal











1 Adapted from Hoover and Miller (12). Analyses listed on a dry matter basis; DM = dry matter, CP = crude protein, NDF = neutral detergent fiber, NFC = nonfiber carbohydrate.

2 NFC = nonfiber carbohydrate determined by differential calculation.

3 NSC = nonfiber carbohydrate determined by chemical (enzyme) analysis.

While starch is the major NSC source, the energy contribution from structural carbohydrates can be (and should be) significant. Ultimately, total contribution from both the structural and nonstructural carbohydrate must be considered. In an attempt to have a ready estimate of carbohydrate digestibility for ration assessment, Nocek and Russell (19) developed the following equation to account for both structural and nonstructural carbohydrate digestion in the rumen:

where NDF = neutral detergent fiber and NDS = neutral detergent solubles (100-NDF).

Factors Affecting Carbohydrate Digestion

Grain source. Rate of fermentation of starch can vary considerably by type of grain. Research from the University of Arizona (11) compared ruminal starch degradability of corn, milo, wheat, barley, and oats. Ruminal availabilities of starch were used to rank the grains from fastest to slowest digestibility: oats, {wheat, barley, corn}, then milo (Table 2). The in situ trial data revealed greater than 90% of the total starch in oats disappeared within the first 2 hours of incubation. Whereas by hour 12, less than 66% of the starch in corn and milo had been degraded, suggesting a considerable difference in NSC makeup between grains.

Table 2. Starch content of grains1.
Item Corn Milo Wheat Barley Oats
Number of replications 23 23 23 23 23
Starch content2, % DM 75.7 71.3 70.3 64.3 58.1
Range of values 72-78 68-78 67-77 60-74 52-69
Starch degradation rate3, %/h 6.4c 3.1d 23.5a 8.8c 15.1b

1 Herrera-Saldana et al. (11).

2 Starch content determined by enzymatic assay.

3 Degradation rate measured in vitro.

a,b,c,d Values within rows with different superscripts differ (P<.01).

Several studies have utilized various grain sources and combinations thereof to manipulate the rate of NSC digestion in the rumen (5, 9, 10, 22, 25). While ruminal starch digestibility is greater in barley than corn in most studies, there is no steady concurrent response in microbial protein synthesis. In addition, some studies have shown an effect on dry matter intakes, in that corn-based diets supported higher dry matter intakes and greater milk production than barley-based diets (2, 3, 17, 22).

Grain processing. Processing methods used to alter digestibility of grains are numerous, including both dry (grinding, cracking, popping, extruding, micronizing, roasting, and pelleting) and wet (soaking, steam processing, exploding, pressure cooking, and high moisture fermentation) treatments. The cost of these processes and to what extent they affect digestibility varies considerably (Table 3).

Table 3. Estimated cost for processing grains and average improvement in feed efficiency 1.

Processing Method Cost/ton Improvement in feed efficiency
Grain sorghum Corn
Grinding 2.00 10% ?
Dry rolling 1.50 10% ?
Steam flaking 4.50 12-14% 6-7%
Pressure flaking 6.00 "2 "
Extruding 20.00 " "
Roasting 7.00 " "
Popping 4.50 " "
Exploding 7.00 " "
Micronizing 3.00 " "
Reconstitution 3.00 " "

1 Richardson (27).

2 Improvement in feed efficiency with these processes are similar to steam flaking.

Typically this work has been targeted toward increasing the digestibilities of grains with lower ruminal availabilities (milo) and much of it began with and has been done as feedlot research. Grinding disfigures the crystalline regions of the starch granules by disrupting the pericarp of the grain kernel and the protein matrix surrounding the endosperm (26) and a reduction in particle size has been shown to increase the rate of starch fermentation (8, 14). Lykos and Varga (14) noted an increase in NSC ruminal degradability of 45% as mean particle size of corn decreased 4308 to 686 m. Processing methods that initiate the gelatinization of starch, such as steam flaking and micronizing, usually increase ruminal starch fermentation. Consistent increases in ruminal starch digestion have been observed for steam flaking corn as compared to dry whole or ground corn (4, 21, 29). Zinn (29) evaluated the influence of steam processing on barley for feedlot cattle and reported a 79% increase in starch solubilization with steam-rolled barley when compared to dry-rolled barley at similar densities (Table 4). He further evaluated the effect of decreasing flake density and reported an increase in starch solubility of 143% as flake density was decreased by 50% (from .39 kg/L to .19 kg/L). This study suggested the responses in ruminal starch availability to flake density did not appear to be due simply to supplying more surface area with decreasing flake density as all grain processing treatments were ground to a similar screen size prior to enzyme incubation for analysis. Another trial conducted at the University of Arizona with lactating cows (4) would suggest similar results to steam flaking. These workers reported a 5% increase in milk yield as dietary inclusion of steam flaked sorghum increased from 15% to 45% and a linear response in milk protein (3.04% and 3.10% for diets containing 15 and 45% steam flaked sorghum, respectively). This trial also reported an 8.4% increase in milk yield as flake density decreased from 360 g/L to 283 g/L.

Table 4. Influence of grain processing on characteristics of corn and barley1.

Item Steam-flaked corn Dry-rolled barley Coarse flake Thin


DM, % 80.9 89.0 84.2 85.5
Density, kg/L .31 .39 .39 .19
Thickness, mm 1.67 -- 1.35 .90
Starch, % 66.2 55.0 55.0 55.0
Starch reactivity, % total starch2 17.3 8.4 15.0 36.4

1 Zinn (29).

2 Amyloglucosidase reactivity, a measure of starch solubilization. Grains were ground to pass through a 20-mesh screen before enzymatic digestion.

Digestive site. The major site of starch digestion in the dairy cow is the rumen. Typically, dairy rations provide large quantities of dietary starch, some of which may bypass ruminal degradation and become available for digestion in the post-ruminal tract. McAllister et al. (16) suggested that increasing the supply of starch post-ruminally to the small intestine may improve the feed efficiency by reducing the loss of energy through methane or heat. In a review of literature involving starch digestion in the small intestine, Owens et al. (23) reported that starch digestion in the rumen is approximately 70% as efficient as starch digestion in the small intestine of steers. This review also concluded that absorption continues to increase as starch supply increased and refuted the suggestion that extent of digestion of starch in the small intestine has a ceiling. This would support the suggestion of an advantage to increasing the availability of starch present in the small intestine. However, this may be offset by decreases in efficiency of starch digestion and absorption of glucose in the small intestine with increasing post-ruminal starch passage. In a more recent review of the literature, Nocek and Tamminga (20) concluded that even though digestible dietary starch is presented to the intestine, there is no net glucose absorption at the portal vein and peripheral glucose concentrations appear highly regulated. Furthermore, their review of the lactation studies did not suggest any clear benefit of postruminal starch digestion to lactation performance. Processing grains affects both rate of digestion and rate of passage and, ultimately, site of digestion (Table 5). Ewing and co-workers (7) evaluated particle breakdown and consequent passage from the rumen and reported that a reduction of particle size would result in increased rates of passage from the rumen. Effects of reduction in particle size is a complicated and dynamic study, subject to particle density, specific gravity, and passage rates. Smaller particles sink and are passed out of the rumen, while larger particles tend to rise within the ruminal strata and become further exposed to additional mastication and microbial attack.

Table 5. Site of starch digestibility of corn as affected by processing method1.

Processing method % of starch in the diets
Rumen Small intestine Large intestine Total tract
Whole 58.9 17.0 2.8 91.7
Cracked 68.9 12.9 8.2 87.6
Rolled 71.8 16.1 4.9 93.2
Ground 77.7 13.7 4.3 93.5
Ensiled 86.0 5.5 1.0 94.6
Steam flaked 82.8 15.6 1.3 97.8

1 Adapted from Owens (23).

Balancing Rations to Promote a Stable Fermentation

The importance of ruminal fermentation to lactation has been reviewed (Hoover and Stokes, 1991). Strong ruminal fiber digestion is imperative as some level of fiber is necessary to maintain ruminal health; however, equally important is the supply of available energy to meet lactation demands. To optimize energy intake in early or peak lactation, high levels of NSC are typically included in the diets thereby limiting the inclusion level of fiber. The association between fiber and NSC is a complicated and dynamic relationship. Nonstructural carbohydrates tend to have an effect opposite that of fiber on ruminal pH and fermentation end products. Therefore a balance of these two components is necessary to achieve stable rumen fermentation. Several groups have evaluated the relationship of NSC and NDF and the effects of this relationship on lactation performance (6, 19, 24). Poore et al. (24) investigated the effect of the ratio of forage NDF to ruminally degradable starch (FNDF:RDS) by calculating these ratios from the literature reported. These workers reported that when the FNDF:RDS was low for barley-based diets relative to corn-based diets (.8:1 vs. 1:1), dry matter intake, milk fat percentage, and fat-corrected milk production were generally lower with barley based diets. In comparison, when this ratio exceeded 1:1 for both corn and barley diets, milk fat percentage and milk yield were less dramatically affected. Thus, they proposed that a FNDF:RDS ratio of at least 1 will allow acceptable performance from lactating cows. These values generally agree with those of Eastridge (6; >.5 forage NDF:NSC) and Nocek and Russell (19; NSC:NDF between .9 and 1.2).

The diet of dairy cattle frequently include byproduct feedstuffs, many of which have moderate amounts of fiber, with greater potential NDF digestibility, as compared to traditional sources of corn silage and alfalfa hay (Table 6). Inclusion of these ingredients can result in diets with low actual forage levels (<40% of the diet), but levels of NDF exceeding 35% of the diet (Table 7). The struggle in this scenario is to balance the diet for "adequate effective fiber" and maintain rumen health. Including ingredients such as soyhulls and beet pulp to supply fiber in the ration also provides a corresponding increase in digestible carbohydrate; this is especially true when these ingredients replace poor quality forages. This is perhaps best exemplified by the responses observed with ingredients such as beet pulp and soyhulls, which are high in NDF, but utilized efficiently in the rumen. Beet pulp has been found to be comparable to barley as an energy source for ruminal fermentation and for support of lactation (1, 15). Research evaluating the replacement value of soyhulls as an effective fiber source (28) concluded that when high quality forage is limiting, dietary NDF from forage can be reduced to 45% with the inclusion of 25% soyhulls and 20% coarsely chopped hay.

Table 6. Comparative ruminal NDF digestibilities of various forages and byproducts1.
Feedstuff Average ruminal NDF digestibility, % Rate, %/h
Alfalfa hay 33-63 5-13
Alfalfa silage 31-41 8-17
Corn silage 25-35 3-5
Red clover hay 48-59 10
Oat hay 57 4-5
Corn cobs 28-56 3-4
Cottonseed hulls 33 4
Beet pulp 69 1-9
Brewers grains, dry 50 3-10
Corn gluten feed, dry 42-49 8-11
Distillers grains, dry 67-79 3-6
Soybean hulls 86-95 1-7
Corn 73 3-6
Barley 28 8-9

1 Adapted from Nocek and Russell (1988).

Diets should be balanced to maximize ruminally fermented carbohydrate, energy intake, and microbial protein production. As carbohydrate digestion drives microbial fermentation (13), this component should include a variety of sources ranging from fast to moderate digestibility to supply a continuous, steady supply of energy. Extensive ruminal fermentation results in a large supply of end products for absorption, thus economically contributing to the lactational demand for energy and protein. Dietary factors to consider in determining the balance between carbohydrate components include the presence of long forage particles, level of forage inclusion, fiber source, number of times fed per day, feeding management scheme (TMR or component feeding system), and use of buffers.

Table 7. Example ration containing fibrous byproducts.
Ingredient kg DM
Alfalfa hay 8.1
Cottonseed 2.5
Barley 3.2
Corn 3.2
Brewers grains 1.7
Beet pulp 1.8
Soy hulls 1.8
Soybean meal, 44% 1.1
Nutrient Analysis Level (DM basis)
CP, % 18.5
NSC, % 35.0
NDF, % 35.8
Forage, % 34.5
Intake NDF / Body weight 1.4

Guidelines for Ration Formulation

Establishing set values to be used as guidelines for dietary formulation are difficult; at best, ranges can be suggested. Specific formulas will be subject to individual farms, based on their forages, ingredients, and feeding management styles. Identifying the correct formula for an operation begins with an initial diet and then involves a series of adjustments based on animal responses. Dairy cows are unique in that they will give immediate responses to some changes (intake, milk production, milk fat yield); however, there can also be subtle ration problems that result in long-term implications. Diets containing excessive levels of NSC, very rapidly digestible carbohydrate, or inadequate digestible fiber will not sustain stable rumen fermentation and may promote erratic dry matter intake and other health problems (ketosis, laminitis, and displaced abomasums).

Based on studies with cows producing greater than 30 kg of milk, general guidelines for the various carbohydrate fraction concentration (% of total ration DM) have been used:
Component % DM
NDF 25-32
NSC (by difference calculation) 35-40
Starch (determined by enzyme analysis) 32-40

Furthermore, the following ranges of degradable fractions may be considered: 50 to 75% of the total starch as ruminally degradable; 50 to 65% of the total NDF as ruminally degradable NDF; and 50-60% of the total carbohydrate as ruminally degradable carbohydrate. It must also be realized that adequate supplies of ruminally degradable protein are necessary to optimize efficiency of carbohydrate digestion and maximize microbial protein production (13).


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