PD Poll Question
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|Friday, 30 November 2007 02:56|
Evaluation of manure is one of the simplest methods to evaluate site and extent of digestion and fermentation in cattle. For years, people have “toe-tested” manure to evaluate rations. In fact, there is good biological basis as to why manure looks the way it does.
Key elements that affect the texture and particle size of manure include adequacy of the amount of physically effective fiber consumed and the impact of the types of NFC on ruminal pH. Either of these factors can change the residence time or extent of fermentation of a feed in the rumen.
If a feed is not extensively fermented in the rumen, its protein, fats, and starches may be digested and absorbed in the small intestine. If not digested there, the proteins and carbohydrates may be fermented in the hindgut (cecum and large intestine). If the rumen is functioning properly, hindgut fermentation is minimized.
In high-producing cows with high dry matter intakes, the rate of passage of feed through the rumen is increased, so more undigested feed will likely reach the hindgut. However, there are relative degrees of hindgut fermentation, and high intakes should not be used to excuse clear symptoms of rumen dysfunction.
If the rumen is not functioning properly, such as during bouts of ruminal acidosis, hindgut fermentation can be extensive. Ruminal problems can often be traced to feeding management in need of improvement, misfeeding of highly digestible carbohydrates, underfeeding of effective fiber or all of the above. Symptoms associated with subclinical ruminal acidosis include:
•Reduction in ruminal pH
•Rumen hypermotility or stasis
•Reduced rumination (cud chewing)
•Great daily variation in feed intake (individual animals, may not be noticed in groups)
•Feces in the same feeding group varies from firm to diarrhea
•Feces foamy, contains gas bubbles
•Appearance of mucin/fibrin casts in feces
•Increase in fiber particle size (greater than 0.5 inch) in feces
•Appearance of undigested fiber/feed in feces
•Appearance of much undigested, ground (less than 0.25 inch) grain in feces
•Reduced feed efficiency
•Reduced production compared to what the ration is calculated to support
How to evaluate manure
Manure evaluation includes the assessment of manure appearance and particle size. Evaluate appearance by feeding group; animals that receive the same ration should have similar-looking manure unless they are sorting their feed. About 5 percent of the cows will have manure that differs from the majority of the animals in their group, and this can be accepted as normal.
Is the manure very stiff? Is there some diarrhea? Is the manure variable, foamy or containing lots of larger bubbles? Is mucous visible in the manure? (If you drag the tip of your boot across a cow pie and something moves after your boot has passed, it’s likely a mucin cast.) Is undigested feed apparent in the manure? Is it ground or whole grain?
When you evaluate the manure, examine the cows and feed for more information: the proportion of the cows that are ruminating, body condition, general appearance, cleanliness, presence of waterers, feedbunk conditions (feedbunk space, how well feed is mixed, etc.), feed sorting by the cows, cow comfort, etc. Also examine the individual feeds where they are stored to look for mold, spoilage, or other problems. These other observations may well explain why the manure looks the way it does.
For each group of cows, take 4 or 5 samples of feces from individual cow pies. Try to pick for variation in appearance representative of the group. Make sure the samples are not contaminated with feed. Eight-ounce sample cups with lids are very good for this purpose. Fill the cup completely and cap. Use a screen or kitchen strainer (do not return it to the kitchen) with 1/16-inch (1.66 mm) openings. This is a qualitative, on-farm evaluation, so getting very specific about mesh size is not crucial. A strainer 7 inches in diameter and 4 inches deep works well.
Transfer a manure sample into the strainer, using a steady stream of water to rinse the cup into the strainer. Rinse the sample gently but thoroughly until the water runs clear.
The sample can be transferred back to the sample cup so that all of the samples taken can be compared side by side. Does fiber in the sample appear to be coarse (more than 0.5 inches long, whole pieces of corn stalk)? Does any cottonseed present have the lint still on it? Does the feed retain its color (grass that’s still green, citrus that’s still orange, etc.)? Is there much (relative term) whole grain in the sample? Ground grain?
Manure evaluation is qualitative, so you can assess whether there appears to be too much or an acceptable amount of coarser fiber or undigested grain in the manure (see “In context” of this article). There is no common, on-farm way to evaluate the proportion of manure your samples represent, so do not try to overinterpret the information they offer.
Particle size and undigested material in feces
Large fiber particles or noticeable ground grain in the feces suggest feed is not being retained in the rumen for a sufficient period to be reduced in size through rumination or microbial fermentation. The depression in ruminal digestion may be related to low pH. An inadequate ruminal fiber mat may not effectively retain larger particles in the rumen.
Both of these situations can be related to inadequate intake of physically effective fiber (peNDF). The peNDF is fiber in the ration that enhances rumination and rumen motility. Generally, when adequate peNDF is consumed, fecal particle size is smaller and ground grain is less apparent, than when fiber requirements are not met.
Sorting of feed by cows is a very common reason peNDF needs are not filled. Providing palatable sources of forage and processing them (chopped to 1- to 2-inch lengths) so they can be blended into a moist total mix that cows cannot readily sort can help to prevent sorting.
Effectiveness of fiber is not only related to particle size, but to a variety of factors that affect rate of digestion. For example, grass NDF tends to ferment more slowly than does that in legume forages. Additionally, the particles from grass tend to be more needle-shaped and those from legumes to be more cuboidal. In my experience, grass has tended to be a more effective peNDF source than legume forages possibly because the fiber is retained in the rumen for a longer period of time. One- to 3-inch- long pieces of very tender or pliable grasses can sometimes be found in the feces; they seem to be able to bend and escape the rumen.
The peNDF has to be in the rumen to be effective. A greater amount of NDF from a more rapidly fermented peNDF source would have to be fed to provide the same amount of peNDF as from a more slowly fermenting source. Take as an example that a small amount of chopped straw included in a ration can quickly resolve problems due to peNDF inadequacy of the ration.
Alfalfa can be an excellent feed, but it can be a poor choice as a major source of effective fiber. The need to provide adequate peNDF to allow for proper rumen function and ration digestion is a balancing act with providing adequate nutrients. It is best done with high quality forages and feeds in adequate quantities.
Undigested feed in feces is indicative of an overall reduction in digestibility of the ration. Both fiber and starch can escape digestion. Long pieces of fiber from forage, or even cottonseed with the lint intact can pass undigested through the gastrointestinal tract if they are not retained in the rumen for digestion. The visible particles of ground grain in feces may contain 6 to 18 percent starch.
Much whole or coarsely ground grain in manure usually indicates problems with silage harvest methods or inadequate grinding of dry grain. Finer grinding of the dry grain can help to reduce appearance of grain in the manure. Problems with silage usually need to be addressed the following year.
Reduced digestion of feed represents a loss of ration nutrients. Consequently, the predicted protein and energy supplies for the ration overestimate what the cow actually receives. High-producing cows with high dry matter intakes may also show an increased passage of undigested feed, but they should not show evidence of ruminal acidosis.
Mucin/fibrin casts or gas bubbles in feces
When feed is fermented in the rumen, the organic acids are absorbed across the rumen wall, the gas (carbon dioxide and methane) is eructated (belched) out by the cow and the microbial cells pass to the small intestine for digestion and absorption. When fermentable substrates pass to the hindgut (cecum and large intestine) they are fermented there by bacteria. The microbial protein produced is not absorbed, but passes out with the manure.
Gas produced from hindgut fermentation can appear as bubbles in the manure, sometimes to the point the feces have the texture of shaving cream. The organic acids can be absorbed by the gut. However, a major difference between the hindgut and the rumen is the potential for the fermentation to be buffered. Where rumination and mixing with saliva provide buffers to reduce the extent of pH decline in the rumen, a system of that magnitude does not exist for the hindgut.
When a great deal of fermentable carbohydrate reaches the hindgut, its fermentation to organic acids may result in injury to the gut. The increased acidity may result in a damage to and sloughing of the surface cells (epithelium) in the large intestine. When the damage is sufficiently severe, the intestine secretes mucous or fibrin to protect the injury. Depending upon the severity of the damage, the gut can repair itself in a few hours to a day. The mucin/fibrin casts found in the feces often have the tubular form of the gut; they are evidence intestinal damage has occurred.
Damage to the large intestine and increased concentrations of organic acids in the gut lumen may play a role in causing the diarrhea often seen with ruminal acidosis. Feeding spoiled or moldy feed can also cause diarrhea.
Reduced feed efficiency
If the site of digestion is shifted from the rumen to the hindgut due to a poorly functioning rumen, it is no wonder feed efficiency suffers. Compared to our usual predictions for digestion in the rumen or small intestine, the amounts of nutrients available to the cow are diminished.
The argument has been raised that increased grain and decreased forage are necessary to meet the energy requirements of the cow. However, if concentrate levels are increased to the point that fiber needs are not met, the analyzed or tabular TDN or net energy levels used to formulate the ration are meaningless.
In the pursuit of providing the cow with more energy, violation of the rules for formulating a balanced ration actually reduces the amount of energy the ration provides. This quote by Dr. Paul W. Moe, a USDA researcher who did much work in the area of net energy, explains the situation:
“...The net energy value of a single feedstuff, however, is not a constant but is influenced by such factors as the composition of the remaining portion of the diet, the level of the feed intake, the physiological state of the animal that consumes the feed, etc. This means that while a net energy value may represent the best estimate of the real energy value of a feed in a given situation, it should not be considered as a constant.”
“...The net energy value listed in a table usually represents an optimum value, that is the value of that feed when incorporated into a “normal” or “balanced” diet. The value may be considerably less than that if fed in excessive amounts or in a diet which has a nutrient deficiency.”
In this light, including excessive amounts of concentrates in an effort to increase ration energy levels is self-defeating.
Another cause of abnormal manure is heat stress. Changes in a cow’s behavior and acid/base balance during heat stress predispose her to ruminal acidosis. Heat stress alters a cow’s acid-base balance. As a cow pants and exhales carbon dioxide, it appears the total amount of buffering capacity within her system may be decreased as evidenced by increases in her blood pH.
In addition, changes in feeding behavior such as consuming feed in fewer meals (slug feeding) and decreased rumination may lead to decreases in ruminal pH even on rations containing adequate fiber.
In a study that tested the effect of ambient temperature on rumen environment, lactating Holstein cows were fed high roughage or high concentrate diets at ambient temperatures of 65°F (cool) or 85°F (hot) with relative humidities of 50 percent and 85 percent, respectively. Ruminal pH was lower at the higher temperature and on the higher concentrate ration. There was an interaction of diet and temperature. Ruminal ammonia and lactic acid concentrations were higher for the hot treatment.
Other studies have reported decreased ruminal pH at hotter versus cooler ambient temperatures. Ruminal changes appear to be responses to ambient, not ruminal temperatures.
In this light, the recommendation of adding more concentrate to rations in summer is not well advised. The rationale for decreasing forage and increasing grain during heat stress is to meet animal energy demands in the face of decreasing dry matter intake. If feeding more concentrate further depresses ruminal pH, little may be gained and more may be lost by compromising the cow’s health.
Fiber should be provided at levels to meet animal requirements under all conditions. Reports from commercial dairies suggest increasing forage or fiber levels with palatable feeds may reduce the negative effects of heat stress on production and health.
The most effective management for reducing the impact of heat stress on ruminal pH is to cool the cows. Fans, sprinklers, misters, cooling ponds or shade can be used in cooling systems.
So, what to do with the information from evaluating manure in a herd? Combine it with information on cow health (digestive upset, acidosis, laminitis, etc.), cow performance (milk and milkfat yields), rumination (at least 40 percent of cows not eating or sleeping should be chewing their cuds), cow observations (sorting the ration or not, comfortable or not), ration and feed evaluation, etc.
Manure evaluation describes the interaction of the cow and her ration. The story it tells adds to a body of evidence that something within the ration or in cow and feeding management does or does not need to be modified. If everything else looks fine, but the manure does not seem quite right, keep observing the cows to make certain they continue to do well, and question what you haven’t checked.
Transient problems like eating patterns changing with weather fronts, a passing problem with silage, etc. can also generate changes in the manure.
Manure evaluation offers a simple way to assess rumen function and how well and where a cow is digesting/fermenting her feed. It is a qualitative system. When used in context with other observations, it can offer confirmation and direction for ration and management changes. PD
—From Penn State Dairy and Animal Science website