In addition to the mineral- and vitamin-related disorders or conditions already discussed in this chapter, a number of other nutrition-related disorders can affect beef cattle. Most of these can be classified as resulting from impaired digestive function or a nutrient imbalance. The two most prevalent that result from impaired digestive function are bloat and ruminal acidosis, and the two most prevalent that result from a nutrient imbalance are hypomagnesemia and urinary calculi.
Bloat in Beef Cattle
Bloat results from an animal’s inability to release gases produced during ruminal fermentation. This inability is most often due to the nutrient content, form, and consistency of the animal’s diet; in some cases, however, it may be due to anatomical abnormalities. The most common clinical sign of bloat is abdominal distention observed on the left-hand side of the animal. In severe cases, this distention may continue to the extent that it inhibits the diaphragm from expanding the animal’s lungs. When such cases are left untreated, bloat often results in death of the animal through suffocation. Bloat is typically categorized as being either frothy or gaseous.
Frothy bloat generally occurs on pasture and is due to the nutrient content of the animal’s diet. Frothy bloat is most prevalent in cattle that consume too much protein that is soluble in the rumen, as is common with lush green alfalfa, clover, or small-grain forages such as wheat pasture.
Once solubilized in the aqueous environment of the rumen, protein interacts with other nutrients to form many small bubbles, or a froth, that traps gases. Because the gases are trapped in the small bubbles, which have a relatively high surface tension and are therefore difficult to rupture, the animal is not capable of releasing them through eructation (belching).
If identified, frothy bloat can generally be treated by the administration of compounds that act as a surfactant (eg, mineral oil or poloxalene) through an esophageal tube. These compounds decrease the surface tension of the liquid that makes up the outer shell of the bubbles and release the gas that was trapped inside.
Frothy bloat can most easily be prevented by providing other feedstuffs that decrease or dilute the amount of soluble protein entering the rumen. In addition, ionophores and poloxalene are quite effective at preventing frothy bloat when supplemented before the onset of the condition.
Gaseous bloat generally occurs in a dry-lot setting, and it is usually a function of the physical form of the animal’s diet. Diets that are too finely ground and do not contain enough coarse roughage, or physically effective fiber, do not encourage rumination or normal rumen peristalsis. As a result, the ruminal contents are not mixed as well as they should be, and the frequency of eructation may consequently be decreased. Without eructation, cattle are unable to release the gas that is trapped in the rumen.
Gaseous bloat can often be treated by passing a tube through the mouth and into the rumen. It is prevented by feeding a ration consisting of ingredients with a larger particle size and containing a higher amount of roughage or physically effective fiber.
Feeding management also contributes to the risk of gaseous bloat. Cattle should not be allowed the opportunity to overeat. Feeding times and amounts should be consistent, and any changes in the amount fed or ingredient composition of the diet should be made gradually. Full dietary transitions should be implemented over a period of 3–6 weeks, and the amount fed should not be increased more frequently than once every second or third day after a previous increase. In addition, some cases of gaseous bloat can be prevented by feeding ionophores.
Some animals are much more prone to bloat than others; these susceptible animals are often referred to as “chronic bloaters.” As a general rule, the frequency of bloat in chronic bloaters increases over time; therefore, chronic bloaters should be culled.
Ruminal Acidosis in Beef Cattle
Ruminal acidosis, which can occur both with and without bloat, results from an overaccumulation of acids in the rumen. Although a normal by-product of ruminal fermentation, acid production can at times exceed the animal’s capacity for buffering and absorption. In such cases the population of microorganisms in the rumen shifts, further complicating the problem by promoting additional acid production.
Left untreated, acidosis can compromise the integrity of the rumen lining. This compromised barrier may allow rumen microorganisms to enter blood flow, potentially leading to systemic infection. Acidosis can also lead to laminitis and consequent problems with hoof integrity and conformation.
Like bloat, acidosis is generally due to dietary nutrient composition, feed form, or abrupt dietary changes. However, the cattle most prone to developing acidosis are those that undergo abrupt dietary changes, particularly from rations composed of primarily roughages and fiber to diets consisting primarily of grains and starch.
To prevent acidosis, cattle should be transitioned from roughage-based rations to grain-based rations gradually, over 3–6 weeks. Lack of the necessary amount of physically effective fiber, or the amount of fiber required to elicit normal rumen mixing and rumination, can also lead to acidosis. In these cases, acidosis occurs because the animal is not secreting enough bicarbonate through saliva to buffer the amount of acid produced by ruminal fermentation.
Rumination (cud chewing) increases the amount of bicarbonate that enters the rumen. In addition, providing feed at irregular times, or allowing an animal to become overly hungry and therefore to overeat, can quickly result in acidosis.
Common signs of acidosis include loss of appetite; lethargy; and feces that are gray or off-colored, have an abnormally loose consistency, and contain bubbles. The best known practices to prevent acidosis include making gradual dietary changes, ensuring that cattle consume enough roughage to sustain rumination, and feeding on a consistent schedule that does not allow cattle to become overly hungry. Feeding ionophores also helps prevent ruminal acidosis. For more information on ruminal acidosis, see Subacute Ruminal Acidosis in Cattle and Sheep Subacute Ruminal Acidosis in Cattle and Sheep Subacute ruminal acidosis is characterized by intermittent, self-correcting periods of low ruminal pH. Affected cows do not show clinical signs when ruminal pH is depressed. However, sequelae... read more .
As with bloat, because ruminal fermentation does not stop immediately after death but the animal's ability to absorb or buffer acids produced in the rumen does stop, ruminal pH begins to decline immediately after death of the animal. Therefore, postmortem ruminal pH serves little purpose in necropsies and should not be used as a major clinical finding to identify or support cause of death.
Hypomagnesemia in Beef Cattle
Although almost every nutrient imbalance that affects the productivity of beef cattle has a name, the most prevalent of these imbalances in grazing cattle is a condition known as hypomagnesemia. More commonly known as “grass tetany,” hypomagnesemia is due to a deficiency of magnesium in the animal’s system. This deficiency often occurs in early spring and late fall, or after a dry period or drought, when forages grow rapidly and contain relatively high concentrations of potassium and low concentrations of magnesium and calcium.
Grass tetany can also result from abrupt dietary changes, such as during drought and winter storm conditions. This form of grass tetany is often referred to as “winter tetany.”
Some common signs of grass tetany include incoordination, muscle twitching, staggering, and restricted breathing. Severe cases that are not treated within the first few hours often result in death.
Grass tetany can easily be prevented by giving cattle access to a supplement that contains a relatively high magnesium content before and throughout periods when cattle may be at risk for developing grass tetany. Appropriate concentrations of magnesium are most commonly achieved through free-choice mineral supplementation.
The losses avoided by preventing one fatal occurrence of grass tetany often more than pay for the entire annual mineral supplementation expenses for 20 or more mature cows. For more information on grass tetany, see Hypomagnesemic Tetany in Cattle and Sheep Hypomagnesemic Tetany in Cattle and Sheep An acute neurological condition due to low dietary intake of magnesium typically affecting lactating cattle grazing green pastures or cereal crops. Parenteral treatment with magnesium sulfate... read more .
Urinary Calculi in Beef Cattle
Another common nutrient imbalance that affects beef cattle is urinary calculi, which is often the result of an imbalance in the dietary ratio of calcium to phosphorus. Steers are more susceptible to the development of urinary calculi than are other cattle.
Like hypomagnesemia, urinary calculi can often be prevented through proper nutrition. The dietary ratio of calcium to phosphorus for beef cattle should ideally be ~2:1, but always at least > 1:1. Diets that contain calcium:phosphorus ratios of > 1:1 are at risk of resulting in the development of urinary calculi.
Grains and grain by-products often contain a greater amount of phosphorus than calcium; therefore, calcium may need to be added to the diet to correct an imbalance when feeding or supplementing cattle consuming high concentrations of these ingredients. Feed-grade limestone and calcium carbonate often are readily available, economical ingredients that have a relatively wide margin of safety and can be used as a source of added calcium in the diet.