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Animal Science 4332

Ruminant Nutrition

Lecture 6. High Concentrate Diets

Feeding high concentrate diets to cattle require an adaptation since cattle evolved from grazing forages. The microbial population in the rumen of cattle is a diverse population adapted to ferment cellulose to volatile fatty acids as an end product. The adjustment of the microbial population to grains which are high in starch result in proliferation of an amylotytic bacterial population. The amylolytic population compete for products of starch and hemicellulose hydrolysis at a lower pH and produce larger amounts of propionate.

During adaptation to a high concentrate diet, the low pH exerts a pressure against organisms that cannot tolerate a low pH. As pH drops, amylolytic and acid-tolerant bacteria incease while cellulolytic microbes decrease. The optimal pH for ruminal amylase activity has been suggested to be 5.6. Absorption of VFA stabilise ruminal pH. VFA are absorbed rapidly in the non-dissociated form. The pKs for most VFA are about 4.1, thus drop in pH toward 4.1,the amount of each VFA in the non-dissociated form increase and rate of absorption increases. When ruminal pH is below 5.5, rumen and animal function are usually abnormal due to acidosis. At this pH, lactic acid concentration, rumen papillae are sloughed from the rumen surface, decreasing absorptive capacity. Breaks formed in the ruminal surface permit bacteria to enter the blood stream and cause liver and ruminal abscesses if prevalent pathogenic bacteria exist.

The cereal grains utilized in cattle diets differ in starch content and degradability. Corn, wheat, and sorghum have the greatest starch content, while wheat and barley have the greatest starch digestibility. Grain sorghum does not compete with corn but grows best in areas where corn cannot be grown. It is produced best in semi-arid regions of the world. It is used widely in Oklahoma and Texas for finishing cattle. Grain sorghum requires rigorous processing for optimal feeding value. The seed coat of grain sorghum is hard and it must be crushed mechanically prior to feeding it to cattle. Dry-rolling or steam flaking have been utilized to increase the feeding value of grain sorghum. Barley also is a cereal grain that grows well in cool weather, relatively drought-resistant. Barley must also be processed prior to feeding it to beef cattle.Dry-rolling and steam-flaking have been acceptable methods of processing barley. Barley has a lower feed value than corn ( 75 to 78 % vs 80 %, respictively ). Wheat is typically too expensive to be considered as a livestock feed. Wheat has a TDN of 80 % and CP of 10 to 12 %. The starch in wheat is rapidly fermented compared to other grains and can lead to lactic acidosis. When used in a finishing ration, it is suggested that wheat not replace more than one-third to one-half of the corn in diets fed to fattening cattle. Wheat may be ensiled as a high-moisture grain or fed in ground or rolled form. Processing of these cereal grains typically increase the ruminal digestion of starch. Those grain starches having a greater rate of ruminal fermentation are more likely to cause acute acidosis. One must also take precautions in adjusting cattle to these grains because they are more likely to cause acidosis.

Starch digestion occurs in the rumen through fermentation. The amount of starch fermented in the rumen varies with each starch as well as with different processing methods. Grain sorghum has a slow rate of fermentation as compared to wheat grain. Steam flaking of cereal grains is one of the most effective ways to increase the surface of the starch for rapid fermentation.

It has been investigated that on can control the rate of fermentation of cereal grains by combining those with a slow rate of fermentation with those with a fast rate of fermenation. Starch is fermented in the rumen Any starch that is not fermented in the rumen is hydorlyzed by alpha amylase in the smallintestine, followed by hydrolysis of oligosaccharidases in the brush border of the intestine.

Table 1. Effect of Processing of corn on the energy value and weight gain of beef cattle.

St. Flake

Dry Roll

W. Shell

H. Moisture

ADG, lb 3.0 2.99 2.98 2.98
Feed Intake 18.8 19.9 20.2 20
ME* 108 100 99 99

* ME computed as a % of dry rolled corn.

Steam flaking corn had the greatest impact on improving the nutritional value of corn for cattle.

Table 2. Guidelines for roughage level

Grain Type

Corn Silage

Alfalfa

% of ration dry matter
Dry whole corn <10% <5%
Dry rolled corn 20 to 25% 10 to 15%
High moisture corn 20 to 25% 10 to 15%

Acidosis in feedlot cattle

Acidosis results from a series of biochemical and physiological stresses caused by rapid production and absorption of ruminal organic acids and endotoxins when an animal overconsumes a meal of readily fermentable carbohydrates. It is not just a drop in rumen pH rather it is several events. These events are organic acid production, ruminal pH, reduced salivary flow, reduced rate of passage, starch fermentation, and reduced feed intake.

Acute acidosis- animal is sick to the point of death or may have impaired physiological function, absorption. The events include low rumen pH, which damages ruminal eptithelial tissue and reduces absorptive capacity of the tissue. The reduced ruminal pH favors lactate producing bacteria and allows lactate to accumulate in ruminal fluid.

Subacute acidosis exists in cattle in the feedlot. With subacute acidosis, it manifests itself with reduced feed intake. The goal is try to control it rather than eliminate it. The potential for this condition to increase is great as the rate of starch digestion is elevated.

Factors disrupting normal feed intake cause acidosis: These factors include environment, diet composition, and grain processing,

Wheat and barley are more prone to lead to acidosis because they are rapidly fermented in the rumen.

If cattle are adjusted to these grains with minimal roughage as well as time to adjust to the grain load, it will cause acidosis.

Reduce Adicosis: Certain conditions may reduce the incidence of acidosis:

Shift site of digestion post-ruminally

Addition of roughage

Combination of fast degrading grains (dry rolled wheat and barley) with slow degrading grains

(dry rolled sorghum and dry whole corn, and dry rolled corn).

Rumensin- It reduces feed intake variation and modulates effects of acidosis.

Rumension is a family of ionophores (other ones include lasalocid and catalyst), which is transport ions, like Na, across membranes. Ionophores affect microorganisms that are sensitive to ion gradients. Ionophore senstivity is associated with lack of outer cell membrane (gram positive bacteria). They generally produce more H, ammonia, and lactate than do resistant, gram-negative microbes. When rumensin is fed, more metabolizable energy available from the feed as well as propionate. With less ammonia generated because decrease proteolysis and deamination, more peptides and amino acids available. With less lactic acid, ruminal pH will remain above 5.

Hepatic abscesses

Serious economic problem that is high (25-30 %).

Rumenitis- liver abscesses complex caused by development of ruminal acidosis from fermentation of high concentrate diet and accumulation of organic acids, lactic acid. The integrity of ruminal epithelial tissue is impaired and invasion of bacteria (like fusobacterium necrophorum) into the liver via portal blood. Feeding low level of antibiotics, tylan, chloratetracyclin, oxytetracyclin and bacitracin approved.

Tylan feeding at 10 g per ton improves gain and feed efficiency by reducing or eliminating F. necrophorum.

Fat feeding to beef cattle

The addition of fat to beef cattle diets has the potential as an energy source if the fat source is less expensive than carbohydrates per unit of energy. Historically, however, feed fats sell for more than 2.25 times the cost of cereal grains. The value of supplemental fat has been evaluated in numerous trials to determine its value as a replacement for corn grain energy. Most studies have established that 4 % fat will improve growth rate and feed efficiency of cattle. Gramlich et al., 1990 fed diets containing 0, 2, 4, 6, 8 % tallow as a % of diet dry matter. The 4 % level of tallow was optimal from the standpoint of improved gain and feed efficiency. Higher amounts of tallow actually lowered gains and feed efficiency. In the Southwest United States, the addition of fat at 5 % to finishing diets was practical. These diets are quite dry as a result of using steam-flaked milo and exposed to dry arid climate. In the Midwest, the response to diets with considerable moisture (corn silage, high-moisture corn diets) were minimal. These data suggest that the value of added fat is compromised for diets containing high moisture feeds such as corn silage and high-moisture grains.

It has been demonstrated that fat is an essential component of an animal's diet. Fatty acids composing the triglycerides have a number of effects in the body including skin integrity, cell membrane function, and reproduction. Many of the functions achieved by fatty acids, such as reproduction, require fatty acids with double bonds past the number nine carbon. These are called essential fatty acids. The only essential fatty acid required by animals is linoleic acid. If the diet contains sufficient linoleic fatty acid, cells can synthesize linolenic and arachodonic acid. Thus diets need to provide sufficient intakes of linoleic acid, or other polyunsaturated fatty acids to meet the requirements for essential fatty acids. These fatty acids, linoleic, arachodonic, and eiconsapentaenoic (20:5) are used to synthesize prostaglandins. These are , in turn, required to initiate and maintain the reproductive process. Although the mechanism is not fully known, it is inferred that synthesis rate is partially controlled by concentration of polyunsaturated fatty acids. Studies have shown that supplemental fat had a positive effect on reproductive performance in cows. Work at the University of Missouri demonstrated that feeding .6 lbs of soybean oil prior to calving can increase first-service conceptions. Other work (Bellows, 1999) demonstrated an increase in pregnancy rates in cows supplemented with 4.7 % safflower oil prior to calving ( last 60 days of gestation). Fat supplementation had greater effects on heifers than mature cows. It also had an effect on weaning weight of calves.

Link to Protein in Finishing Diets