Under certain conditions, beef cattle may be exposed to forages and feeds containing toxic or harmful compounds. When consumed, these antiquality factors can result in reduced cattle growth, depressed reproductive performance, poor health, and even death, say Jane A. Parish and Justin D. Rhinehart, in a Missippi State University Extension Service publication.

Simple management practices often can be implemented to reduce the risk of diet-induced production losses. Identifying potential anti-quality problems in beef cattle diets, using proper treatments, and preventing future occurrences can help protect cattle health and profitability.

Fescue Toxicosis

Cause

Fescue toxicosis is the forage-related livestock disorder that impacts the largest number of cattle in Mississippi and causes the greatest economic losses. Most of the tall fescue acreage in Mississippi is in the northern and central regions of the state. Most tall fescue plants in Mississippi pastures are infected with a wild-type “toxic” endophyte (fungus). The wild-type endophyte produces ergot alkaloids, which are livestock toxins.

Clinical Signs

Consumption of toxic endophyte-infected tall fescue depresses body condition, reproduction, and milk production in cows and weaning weights in calves. Grazing toxic tall fescue pastures or consuming toxic tall fescue hay decreases forage intake, lowers average daily gain, and alters hormone concentrations. Cattle that develop rough hair coats exhibit heat stress during warm periods, and suffer losses of ear tips and tail switches during cool periods.

Endophyte-free tall fescue is commercially available and has been marketed as a solution to fescue toxicosis. Although endophyte-free tall fescue does not produce ergot alkaloids and provides good animal performance, removal of the endophyte results in reduced seedling vigor and lower plant persistence. Accelerated forage stand losses in endophyte-free tall fescue pastures, relative to toxic tall fescue pastures, are common without intensive management. Unlike toxic tall fescue, endophyte-free tall fescue can be easily overgrazed and will not tolerate poor management.

“Friendly” endophyte-infected tall fescue contains an endophyte that does not produce ergot alkaloids. It is also referred to as “novel” or “nontoxic” endophyteinfected tall fescue. Novel endophyte-infected tall fescue combines the plant persistence advantage of toxic tall fescue with the animal performance advantage of endophyte-free tall fescue. Plant persistence is higher in novel endophyte-infected tall fescue than in endophyte- free tall fescue, and cattle performance on novel endophyte-infected tall fescue is similar to performance on endophyte-free and higher than performance on toxic tall fescue.

Part of the success that producers have with maintaining productive stands of toxic tall fescue is attributable to the negative effect of the livestock toxins on forage intake. Forage intake is higher on novel endophyte- infected tall fescue than on toxic tall fescue. Therefore, novel endophyte-infected tall fescue stands require a higher level of grazing management than toxic tall fescue stands and should not be overgrazed. Proper establishment to minimize stand contamination with toxic tall fescue seed and proper management practices to minimize stand losses are also vital considerations when renovating with friendly tall fescue.

Treatment

Although there are several products advertised to alleviate fescue toxicosis, there is currently no drug, feed additive, or supplement proven in university research trials to effectively restore lost cattle growth performance on toxic tall fescue. Cattle should be removed from toxic tall fescue pasture and hay if possible.

Nitrate Poisoning

Cause

Nitrate toxicity can be a lethal problem for all classes of cattle, including stockers. Even when nitrates do not cause death, production losses such as reduced milk yield, lower weight gains, and reproductive problems can occur with moderate levels of nitrates in the diet. Generally, nitrates are present in grazing cattle diets at levels that are not normally toxic, but at high dietary levels, nitrates can cause nitrate poisoning. Nitrates normally found in forages are converted to nitrites, then to ammonia, and then to protein by bacteria in the rumen.

Nitrate poisoning in cattle results from excessive nitrate consumption from grazed forage, hay, silage, weeds, water, or other sources. Nitrates accumulate in the rumen when cattle rapidly ingest large amounts of plants containing high levels of nitrates. Although rare, cattle may experience nitrate poisoning from drinking water contaminated with nitrogen-based fertilizer. Nitrate is absorbed into red blood cells and combines with hemoglobin to produce methemoglobin, a type of hemoglobin that cannot carry oxygen in the blood. Lack of sufficient oxygen transport to tissues results in severe problems, including abortions and possibly death.

Clinical Signs

Signs of nitrate poisoning include bluish discoloration of the skin, bluish-brown mucous membranes, labored or rapid breathing, muscle tremors, lack of muscle control, staggering, weakness, diarrhea, frequent urination, dark- to chocolate-colored blood, rapid pulse, coma, and eventual suffocation. Necropsy results often reveal brown-colored and severely coagulated blood. Pregnant females that survive nitrate poisoning may abort due to lack of oxygen to the fetus. Abortions generally occur 10 to 14 days after exposure to excess nitrates.

Plant Factors

Certain forages and weeds pose higher risks of accumulating potentially dangerous levels of nitrates. Forages known to have the potential for accumulation of toxic nitrate levels include sudangrass, sorghumsudan hybrids, pearl millet, corn, wheat, oats, soybeans, tall fescue, and bermudagrass. Weeds that pose a threat include pigweed (carelessweed), smartweed, ragweed, lambsquarter, goldenrod, nightshades, bindweed, Canada thistle, and bull or horse (stinging) nettle. Pigweed and the warm-season annual grasses are typically the more likely culprits in most nitrate poisoning cases in Mississippi.

Abnormally high levels of nitrates in plants are caused by various stress factors such as moisture conditions, low temperatures, and soil conditions. Plants will take up very little nitrate from dry soils. Nitrates are often at very high levels in plants for several days following a rain after drought conditions. Frost and low temperatures interfere with normal plant growth and can cause accumulation of nitrates in plants. Frost can cause leaf damage and reduce photosynthetic activity. So, nitrates absorbed by the roots are not converted to plant protein but are accumulated in the stem and stalk. Deficiencies of essential nutrients such as phosphorus can also lead to plant stress and cause a buildup of nitrates.

Nitrate levels tend to decrease as plants mature. Young plants have higher concentrations of nitrates than more mature plants. Mature plants can still have excess nitrate levels if environmental and soil conditions are favorable for accumulation. Water nitrate levels should also be considered. Nitrate levels (unlike prussic acid levels) in stored forages do not significantly decrease over time, so storing hay containing high nitrate levels is not an effective method of preventing nitrate poisoning. In addition, ensiling is not considered an effective way to reduce nitrate levels in forages.

To read the full report with management tips for producers, link here.