Carbohydrate Nutrition for Lactating Dairy Cattle

 

 

Carbohydrates (CHO) are the major source of energy for rumen microorganisms and the single largest component (60-70%) of a dairy cow's diet. They represent the major component of net energy for support of maintenance and milk production. Diet carbohydrate concentration influences the composition of milk as precursors for lactose, fat, and protein.

 

There are two broad classifications of CHO: structural and nonstructural. The structural CHO consist of elements found in the plant cell wall. The nonstructural CHO are located inside the cells of plants and are usually more digestible than the structural CHO.

 

 

Figure 1 illustrates the organic composition of plant CHO. The structural or cell wall material contains cellulose, hemicellulose, lignin, pectic substances and Beta-glucans. The cell contents contain starches, sugars, and fructans and for ensiled feeds, organic acids.

 

The most common measures of fiber analysis are acid detergent fiber (ADF) and neutral detergent fiber (NDF). The structural components in the plant like cellulose, hemicellulose and lignin are measured by NDF. Acid detergent fiber measures lignin and cellulose.

 

 

 

Wet chemistry analysis for ADF and NDF are based on the differential solubility of plant components. The NDF concentration of a feed is measured by refluxing the sample in a buffered detergent solution (pH 7). During this process, water and detergent soluble compounds are removed, including sugars, lipids, some ash, non-protein nitrogen, and some protein. However, variable amounts of ash and protein can remain with the NDF. Ash contamination can contribute up to four percentage units to the NDF value. Ideally, both ADF and NDF should be expressed on an ash-free basis. Soil is often the culprit in mineral contamination.

 

There have been several modifications to the analytical procedure for NDF. The first was the inclusion of heat-stable amylase in the procedure to remove starch. The other was the use of sodium sulfite to minimize protein contamination. Currently the reference method adopted by the National Forage Testing Association and which reflects the NDF values listed in the 2001 NRC for dairy cattle is the amylase-sulfite procedure.

 

There is some controversy over the use of sulfite in the standard procedure for NDF determination. Adding sulfite to the NDF solution reduces crude protein contamination but does not quantitatively remove all of it. Using sodium sulfite in the NDF procedure is discouraged if the residues are to be assayed for neutral detergent insoluble protein. Sulfite also attacks lignin and should not be used in a sequential analysis for lignin or for subsequent in vitro digestion.

 

Whenever samples are sent out for analysis, scrutinize both ADF and NDF values. In addition to ash and protein inflating NDF, feeds with lipid contents greater than 10 percent can be a problem and interfere with the filtering process.

 

Forage labs report NDF in various ways, which helps the user better evaluate the forage analysis. When aNDF is reported, this implies an amylase and sodium sulfite treated NDF. When aNDFom is reported, this implies aNDF on an organic basis or ash-free, which is the removal of minerals, soil, and sand.

 

 

The in vitro NDF digestibility procedure is done in a test tube. Small amounts of dry, ground samples are incubated with ruminal fluid and buffer in a temperature-controlled system. Testing labs offer a variety of options for determining the NDF digestibility of forages. Most labs look at NDF digestibility after the sample has been incubated in rumen fluid for 30 hours. The assumption is that this represents the NDF digested in the rumen of higher producing cows with moderately fast rumen turnover. Currently the system provides an endpoint value for in vitro digestible NDF. However, in-vitro true digestibility can be measured for 24, 30, or 48 hours. For NDFDom (NDF digestibility on an ash free basis) and uNDFom (undigestible NDF on an ash free basis), some labs offer time points of 12, 72, and 120 hours or 30, 120, and 240 hours. Testing options vary by lab.

 

As with any procedure, there are several factors, which could affect results. They include the dilution of the ruminal inoculum, type of buffer used, particle size of the sample, type of mill used for grinding and type of diet the donor cow is fed.

 

It may be difficult to make any meaningful interpretation on one in vitro NDF digestibility value. To understand how these values influence animal performance in a particular farm situation almost requires several in vitro tests throughout the year and on a yearly basis. Digestibility of forage fiber components varies due to hybrids, maturity, temperature, moisture, fertilization, fermentation, and processing methods. One disadvantage of the in vitro procedure is that using a dry, ground sample may decrease the difference between samples or result in higher digestibilities than unground wet samples.

 

 

The more readily digestible CHO are not recovered in the NDF. The nonfiber CHO includes sugars, starches, and the other reserve CHO such as galactans and pectins. The lab analysis for starch is determining the amount of dextrose (glucose) and multiplying it by a factor to come up with the starch content.

 

There are some nuances in the starch content, especially with grasses. Grass forages contain very little if any starch. They contain sucrose and fructans, which can appear in the starch fraction. Sucrose is found in beet pulp and citrus pulp. For these feeds, the nonstructural CHO is primarily all sugar. For corn silage, grains, and most byproducts, starch is the primary nonstructural CHO. It is recommended for byproduct feeds to request analysis for starch, sugars, and fat as these can vary substantially depending on processing and the source.

 

The University of Florida has developed a system for partitioning the neutral detergent soluble carbohydrates or the CHO fractions excluding hemicellulose and cellulose (Figure 2). The system uses an extraction with 80 percent ethanol to separate low molecular weight sugars and organic acids from the starch and nonstarch polysaccharides. The sugars are measured directly in the ethanol extract and starch on the ethanol insoluble residue. The organic acids and nonstarch polysaccharides, which can be the most diverse fractions, are calculated by difference. Tables 1, 2 and 3 illustrate the CHO profiles for alfalfa, grass, and corn silages.

 

 

 

Fiber digestibility is usually defined as the proportion of consumed fiber that is not excreted in the feces. Fiber contains an indigestible fraction and one or more potentially digestible fractions, each of which is degraded at its own rate. The extent of fiber digestion depends on the size of the indigestible fraction and the competition between the rates of degradation and passage out of the rumen.

 

Ruminal fiber digestibility is affected by the passage rate of particulate matter out of the rumen. Rate of passage is affected primarily by intake. However, feed particle size, particle buoyancy, concentrations of dietary fiber and nonstructural CHO, and rate of digestion of the potentially digestible fiber fraction may affect passage rate.

 

There is a vast range in ruminal fiber digestibility between and among forage and non forage sources. Although fiber digestibility of forages is not constant for all animals and feeding conditions, much of the variation is due to composition and structural differences of the forage, harvest date, and height at harvest. The indigestible fraction of NDF is a major factor affecting the utilization of fiber CHO sources as it varies greatly and may exceed more than one half of the total NDF in the rumen.

 

As forages mature, the indigestible fraction of fiber increases and the rate of digestion of the potentially fermentable fraction decreases. In addition, environmental factors such as light intensity, day length, temperature, and soil moisture affect the relationship between fiber digestibility and maturity.

 

Particle buoyancy in the rumen may be another factor affecting digestibility. Particles are buoyant when they are actively fermenting. Carbon dioxide and methane gas produced during fermentation and associated with feed particles, make them float in the rumen. Buoyant particles become suspended in the fiber mat. As the fermentable fiber fraction of feed particles decreases, less gas is produced, and particles may become less buoyant and sink. Particles that have low concentrations of fermentable fiber that ferment quickly, such as alfalfa, might pass more quickly than particles that have more fermentable fiber, which ferment slowly, such as grasses.

 

Grasses generally have a lower indigestible NDF fraction than do legumes, which may give grass NDF higher digestibilities at longer ruminal retention times. Longer ruminal retention times of grasses due to greater buoyancy over time will tend to increase the digestibility of grass NDF, compared with legume NDF. Although grass NDF is generally more digestible than legume NDF, it may also be more filling and reduce intake because of an increased ruminal retention time. When intake is limited by ruminal fill of undigested feeds, legumes may allow higher intake than grasses as legume NDF ferments faster and probably sinks and passes from the rumen faster than grass NDF.

 

A practical feeding strategy can be applied from this information. Assuming a rumen retention time for NDF of 30 hours for early lactation cows and 48 hours for late lactation cows, the potential digestible NDF fraction of alfalfa may be nearly completely digested in the rumen of early lactation cows while that of grass is not. At shorter ruminal retention times, legumes may have greater dry matter digestibility because of their lower NDF contents and similar NDF digestibility compared with grasses.

 

Grass forages may have greater NDF digestibility when fed to cows managed for longer ruminal retention times. Mid to late lactation cows, because of their longer ruminal retention times, can utilize grass forages which may ferment more slowly, but have a higher potential digestibility.

 

Dry matter intake in early lactation cows can be limited by physical fill. Offering fiber sources that digest and pass from the rumen more quickly and decrease the ruminal fill appetite cessation effect may increase energy intake.

 

Corn silage is considered a grass, however, there is a narrower range of maturity when it is harvested and stored compared to grass hay-crop forage. This results in a tighter range in NDF digestibility and less variability. When corn silage is harvested at an over mature stage NDF digestibility will be lower. The greatest influence over fiber digestibility is the growing environment and climate during the vegetative growth stage. Oba et. al (1999) observed that a one unit increase in NDF digestibility was associated with a 0.37-pound increase in dry matter intake and a 0.55-pound increase in 4 percent fat corrected milk.

 

A dairy cow needs rumen fiber to contribute to the fiber mat, but that fiber mat also needs to be digested to provide energy for milk production and allow the cow to consume more feed. The goal is to produce forages that are highly digestible. However, a ration that is too digestible can create problems. If rate of passage through the rumen is too fast, then cows may not be meeting their energy requirement. If forages are low in digestible NDF then dry matter intake may be compromised and energy needs not met. Digestibility coupled with the physical form of the ration and feeding management practices can either be assets or liabilities depending on the situation.

 

 

Starch and sugar make up the NSC component. Soluble sugars ferment very quickly in the rumen. When sugars are contained within plant cell walls, they must be retained in the rumen a sufficient length of time to be extensively fermented.

 

Starch digestibility has the largest impact on the rumen and the dairy cow production. Starch comprises the majority of the NSC in many feedstuffs. The rate and extent of starch digestibility and fermentation are influenced by several factors such as type of grain and processing method. Starch digestibility can be ranked from fastest to slowest as follows: oats> wheat> barley> corn> milo. Processing methods such as fine grinding, steam flaking, and ensiling can alter ruminal availability of starch. Most grain processing methods increase both rate and extent of starch fermentation and ruminal digestibility. Decreasing the particle size of a starch source, e.g., ground corn, increases both rate of digestion and rate of passage. These can have counteractive effects on ruminal digestion.

 

Ensiling time for corn silage can influence starch digestibility. Starch digestibility can increase seven percent on average when corn is ensiled for 30 or 45 days with a continued gradual increase in digestibility as ensiling time increases past 45 days. Starch digestibility can vary widely depending on the corn hybrid. It is recommended to test starch digestibility routinely and when feeding out from different storage structures.

 

An animal's physiological state and level of intake affect rate of passage. Fine grinding may have less effect on ruminal starch digestibility at higher levels of intake, such as early to mid-lactation cows, compared to animals in late lactation. There are several reasons why animal performance may vary due to NSC levels in dairy cattle diets. Rapidly digestible starch may affect ruminal fiber digestion, which can decrease the differences between diets relative to total CHO digestions. The level in which byproducts replaces forage fiber in the diet can affect rumination and saliva production. Other factors include the starch digestion site and the processing method used to alter the rate and extent of starch digestion.

 

Formulating rations for NDF successfully means adjusting levels on a herd-to-herd basis and not using one constant value for all programs. Some of the problems of extrapolating research data and applying it to real world experience are that research studies vary in many factors, such as stage of lactation, fiber sources, and manner of data expression. It makes it difficult to determine a true relationship between NDF, dry matter intake, and milk production to make any consistent recommendations. Some other reasons for the variability in NDF requirement include:

More research is needed to understand the carbohydrate requirements of the dairy cow. Rations improperly balanced or managed for CHO can have profound effects on rumen health and animal performance. Not only is the chemical nature of the fiber important, but also the physical form of the fiber is just as critical.

 

Feeding management in respect to how a ration is implemented can substantially impact rumen function and animal performance. Improper particle length in the diet (e.g., sorting), inappropriate mixing procedures, inadequate housing facilities (e.g., cow comfort issues), management practices that decrease feeding time, and feeding inconsistent forage and grain quality are just a few problems that can affect CHO digestibility.

 

 

Numerous factors influence the amount of forage NDF and total NDF that is formulated in rations. CHO nutrition requires more than meeting a certain NDF or starch value. Other considerations include starch source, processing methods, particle size, physical and effective fiber, buffer inclusion levels, and feeding management practices.

 

 

The optimal concentration of starch and sugar in dairy cattle diets is not well defined. When balancing rations for starch, an acceptable range in lactating cow diets would be 20 to 30 percent on a dry matter basis. Sugar levels can range from base levels of three up to nine percent on a dry matter basis. There are several factors to consider in addition to just the level in the diet. These include forage particle size, frequency of grain feeding, site of starch digestion, fiber digestibility, use of by-product feeds, grain processing method, and dry matter intake.

 

The following are guidelines for starch levels in lactating cow diets on a dry matter basis.

 

 

 

Formulating rations with an "ideal" forage NDF or total NDF level is challenging. Table 4 provides some guidelines for formulating forage and total NDF. If low fiber levels are formulated, other factors should be considered such as certain nutrition, management, and environment issues. These can have a tremendous impact on animal performance. It is not just NDF but also the level of starch and its digestibility rate that are important. The minimum level of NDF should be adjusted so that the maximum recommended amount of starch is not exceeded.

 

The maximum amount of dietary NDF that can be formulated in diets is dependent upon fiber source, amount, physical characteristics of the forage, and the physiological state of the cow. The upper limit of NDF in the ration is a function of the cow's energy requirement, the minimum amount of NSC required to maintain normal rumen function and the potential negative effect of high NDF on feed intake.

 

 

There are several indicators to monitor that may reflect rations improperly balanced or implemented for CHO. These include milk fat percentage, rumination and cud-chewing, dry matter intakes, metabolic problems (e.g., ketosis), lameness (laminitis), ruminal acidosis, and fecal consistency.

 

 

The maximum NDF concentration in the ration is also the minimum level of starch/sugars needed for good ruminal fermentation and to avoid negative effects on dry matter intake related to rumen fill. The maximum forage NDF is 1.1% of body weight. The maximum total NDF is 1.3 to 1.4% of body weight. At this increased level, forage NDF can be as low as 0.75% of body weight. For example, the maximum total NDF for a 1300-pound cow would be between 16.9 and 18.2 pounds. As a percent of the total ration dry matter, the total NDF could be as high as 38 to 42%, however, this would be for herds milking below a 70-pound average. Herds averaging greater than 80 pounds of milk typically have a total NDF ranging between 28 to 32% of the total ration dry matter.

 

Feeding situations where NDF may be excessive and starch deficient are when feeding overly mature forages, especially grasses, and/or total mixed rations containing excessively long particle size (e.g., more than 20% of the particles greater than 0.75 inches using the Penn State Particle Size Separator). The problem that occurs is a longer retention time in the rumen, which can restrict dry matter intake. This would be of concern to early lactation and high producing cows.

 

In the past several years, some researchers have studied the effects of feeding lower starch diets to lactating cows to minimize the inclusion of corn or other grains to improve farm profitability. Dann et al. (2014) evaluated lower starch diets in lactating Holstein cows and measured production and ruminal responses. Dietary starch ranged between 17.7 and 24.6 percent on a dry matter basis. There was no measurable effect on short term feed intake, milk component production or ruminal fermentation. Although these results look promising, more work is needed to evaluate the long-term effects on animal performance and rumen health.

 

Feeding situations that can confound low starch diets are corn grain coarsely processed and/or the inclusion of a lot of high-fiber byproduct feeds. These scenarios illustrate the importance of evaluating both the chemical and physical properties of the ration.

 

 

Cow health is negatively affected by low NDF and high starch rations. A general term used to describe this scenario is ruminal acidosis. Indicators that respond quickly to this feeding scenario are ruminal pH, milk fat percent, and chewing activity. Long-term effects include laminitis, ketosis, abomasal displacement, and liver abscesses.

 

Stauder et al. (2020) evaluated the effect of decreasing the forages in the diet on chewing activity, sorting, and rumen health. Cows were started on a 60 percent forage and 40 percent concentrate total mixed ration for 2 weeks and then were switched to a 40 percent forage and 60 percent concentrate ration for 4 weeks. Time spent eating and rumination decreased when cows were switched to the low forage ration. First lactation cows selected for more for longer particles compared to older cows. Even with the greater rumination time and sorting for longer particles, first lactation animals had a greater subacute ruminal acidosis index (spent on average 4.6 hours/day longer below a pH of 5.8 compared to older cows). First lactation cows may be more susceptible to ruminal acidosis than older cows fed the same diets and feeding management strategies geared specifically to those animals may be warranted.

 

 

There is a strong positive relationship between the fat content of milk and ruminal pH. A sudden drop in milk fat percent may indicate low ruminal pH. High producing cows milking over 80 pounds and early lactation cows are sensitive to rumen acidosis. However, their milk fat percent is less sensitive to changes in ruminal pH because they may be also mobilizing body fat, especially during the early lactation period. Due to an energy deficit diet or other improper management practices (e.g., overstocking pens) during early stages of lactation, body fat mobilization can lead to increases in milk fat. The milk fat to milk protein ratio can be used as an indicator for energy deficiency, subclinical ketosis, and ruminal pH. Using the Dairy Herd Improvement Association 202 Summary Report (stage of lactation profile table), some metrics for determining if a herd problem is occurring using the first 40 days in milk and all lactation groups include a milk fat test less than 3.4% or greater than 4.6% and a fat to protein ratio less than 1.35. Other nutritionally related reasons for low fat tests include the amount and type of fat being fed and cows freshening at a low body condition (body condition score less than 3 on a 5-point scale).

 

 

Rumen acidosis decreases the frequency of rumination so observing cows for chewing activity is often used as an indicator. The amount of time an animal spends ruminating can be affected by species, breed, physical and chemical characteristics of the diet, health condition, feed intake, and production level. About a third of the variation in rumination time in dairy cattle has been shown to be related to feed intake, more specifically intake of NDF and starch. The variation observed among healthy cows and the poor relationship between chewing time and ruminal pH, limits the usefulness of this indicator. However, a noticeable reduction in the number of cows ruminating one to two hours after consuming a meal can be used jointly with other indicators that a rumen acidosis problem exists. When visually observing rumination on a dairy farm, 60 percent or more of the herd at any time should be ruminating (chewing cud) and is considered a healthy rumen function. This information can be used at the population or individual level to monitor abrupt changes in animal rumination behavior related to their health or improper management practices.

 

 

Historical information on feed intakes is necessary to use this indicator effectively. Feed intake is farm specific and it is influenced by numerous factors including environmental temperature and humidity, stocking density, bottom surface of the feed bunks, cow comfort (e.g., improper stall size or inadequate bedding), feeding times, ration consistency, forage quality, TMR moisture, palatability of ration, particle size of the diet, milk production, cow physiological stage, cow nutritional status, cow lactation number, and body weight. Therefore, to distinguish between explainable drops in feed intake and those related to rumen acidosis, routine and continuous monitoring of dry matter intake is important. Implementing good feeding management practices and keeping records can minimize drastic changes in dry matter intake.

 

 

Manure consistency may increase in fluidity during periods of acidosis. This occurs when an increased amount of readily fermentable starch is consumed resulting in lactic acid passing to the lower digestive tract. Also, some starch may escape the rumen and get fermented to volatile fatty acids in the large intestine. When either of these scenarios occurs, osmolarity increases in the digestive tract causing an influx of water from the blood. However, there are other nutritional reasons why fecal consistency may change. The degree of change in consistency needs to be evaluated and determined if it is severe enough to be a problem. This metric should be used along with other indicators to determine if acidosis is a problem.

 

 

Laminitis is an aseptic inflammation of the dermal layers inside the foot. There is usually inflammation, which causes pain above the hoof and around the coronary band. A typical sign of an animal with laminitis is a slight lameness characterized by the animal shifting weight while walking or standing. Cows standing on toes on the edge of their stall are very typical of a stance to alleviate pain.

 

Rumen acidosis has been shown to be a key factor leading to laminitis. As the rumen pH decreases below 5, acid production is elevated in the rumen. Endotoxins release due to the microbes killed by the low ruminal pH, which is absorbed and can trigger histamine release. Histamine causes vasoconstriction at the hoof level, which in turn causes laminar destruction and hoof deterioration.

 

The main problem with using laminitis as an indicator of ruminal acidosis is the interval between the occurrence and the onset of visible signs (e.g., sole hemorrhaging, sole ulcers, or lesions) as it is related to the hoof growth rate, which is 0.20 inches per month. The thickness of the normal sole is about 0.40 inches. Therefore, the visible insults to the hoof or claw might be observed about two to three months after the internal insult occurred. In chronic and subclinical cases, observing hoof health and laminitis is typically a distant historical indicator of the nutrition program. It may have little relevance to the current feeding program or recent ration changes.

 

 

Carbohydrate nutrition is complex and requires a balanced approach between the structural and non-structural components when feeding dairy cattle. Both the chemical analysis and the physical form of the carbohydrate must be considered in ration formulation. Every farm will require a different approach based on forage quality, cereal grains fed, and the various by-product feeds used to complement the diet. Feeding management practices can greatly affect how CHOs will be utilized by the cow. Herd dynamics regarding days in milk, cow physiological stage, cow lactation must be considered when formulating the fiber and starch aspects of the ration. Imbalances of these diet critical components can result in negative impacts on animal performance and health that can have both short-term and long-term implications.

 

 

 

Note: WSC or water-soluble carbohydrates are simple sugars and fructans; ESC or ethanol simple carbohydrates are the simple sugars; NDFD or neutral detergent fiber digestibility; aNDFom is amylase and sodium sulfite treated NDF on an ash-free basis.

 

 

Click here to view Table 3 from PSU

Note: WSC or water-soluble carbohydrates are simple sugars and fructans; ESC or ethanol simple carbohydrates are the simple sugars; NDFD or neutral detergent fiber digestibility; aNDFom is amylase and sodium sulfite treated NDF on an ash-free basis.

 

 

 

*Higher minimum may be necessary if forage or ration is chopped very fine.