Figure Animal in early stages of bloat. Note that the animal's. crownvetch, arrowleaf, J-21

Transcription

1 TOXIC SUBSTANCES or mineral imbalances in forages and weeds consumed by livestock may result in reduced productivity, visible symptoms of ill health, or even death of grazing animals. In the South, one of the most common ailments is fescue toxicity (see Chapter 23). Several other disorders are also potential problems for southern livestock producers. Bloat Bloat can be a serious problem in cattle grazing pastures dominated by certain legumes. Other ruminant animals can be affected, but appear to be less susceptible than cattle. Bloat is caused by formation of a stable foam in the rumen which prevents eructation (belching) of gases produced by microbial fermentation of forage. As a result, the gases normally lost by eructation are retained. The pressure in the left side of the rumen then increases, resulting in the eructation mechanism being inhibit.ed. When this occurs, the animal's oxygen supply is reduced or cut off, eventually causing it to suffocate. Animals affected will swell rapidly, and in acute cases, death can sometimes occur within an hour. Individual animals differ in sus.ceptibility to bloat (Figure 21.1). Persian, ladino, or white clovers, and alfalfa are examples of plants which have bloat potential. Certain legumes, including birdsfoot trefoil, sericea lespedeza, annuallespedeza, crownvetch, arrowleaf, and berseem clovers rarely or never cause bloat. This is probably because leaf tannins within these plants act as protein precipitants which aid in breaking up the stable foam in the rumen. LikeWise, tropical legumes such as kudzu, cowpea, perennial peanut, and alyceclover rarely or ~ever cause bloat. Occasionally, bloat occurs on lush pastures of ryegrass or small grains, usually in spring. Feedlot bloat in beef cattle raised on highgrain diets that mayor may not contain legume forage occurs rather infrequerltly. Control. Hungry animals should not be turned into a lush legume or winter annual grass pasture. Dry Figure Animal in early stages of bloat. Note that the animal's left side is especially distended. J-21

2 hay should be fed before allowing animals to graze such a pasture. Limited hay feeding during the initial days of grazing can also reduce the incidence of bloat. Cattle placed on legume pastures should be checked frequently and removed quickly if bloat develops. The first signs of bloat will be swelling of an animal's left side. Bloat hazard is greatly reduced when at least 50 percent of a pasture is grass. Providing ready access to salt and water when grazing a legume pasture is also helpful. Bloat potential is greatest during ra pid growth periods in spring, declining during the summer months. In most of the South, the period between mid-march and mid-may is when bloat is most likely to occur. An effective method of bloat prevention on legume pastures is providing salt-molasses blocks containing surlactants, a detergenttype compound that reduces development of stable foam in.the rumen. Although surfactant blocks are relatively expensive, they can prevent animal deaths. Generally, the need for surfactant blocks is greatest in early spring and they may not be needed as the grazing season progresses. Feeding an ionophore (several types are commerdally available) can also reduce the potential for bloat. Nitrate Poisoning Cause. Nitrate poisoning in livestock is primarily caused by the consumption of pasture or hay containing high levels of nitrate-nitrogen (NO3-N.). During periods of low soil moisture or low humidity, nitrates can accumulate in plants heavily fertilized with N. Hay cut during or just after a drought period is suspect, especially if N was applied just prior to hay harvest. Shading by other plant species, cloudy weather, and frost may also increase nitrate levels in plants. The application of N fertilizers during cool, wet, cloudy weather may also result in nitrate poisoning. Nitrates in h~ are stable and can cause deaths months after harvest. Some forage plants are more likely to accumulate nitrates than others. Plants known to have considerabje potential for accumulation of toxic levels of nitrates are: ~udangrass, sorghum-sudan hybrids, pearl millet, corn, wheat, and oats. Certain weeds may also accumulate toxic levels of nitrates and thus pose a threat, especially in hay. Examples are pigweed, smartweed, ragweed, lambsquarter, goldenrod, nightshades, bindweed, Canada thistle, and stinging nettle. In addition, the application of 2,4-0 herbicide can increase nitrate levels in plants. Nitrates present in forages may be reported by laboratories as nitrate-nitrogen or as potassium nitrate (KNO3). Plants containing more than 1.5 percent nitrate (15,000 ppm) are considered toxic to many classes of livestock (see Appendix 14). The reason for the toxicity is that nitrates are reduced to nitrites in the digestive tract. Nitrites then oxidize the iron in blood hemoglobin and prevent adequate oxygen transport. Animal symptoms are labored breathing, muscle tremors, and a staggering gait after which the animal collapses, gasps for breath and dies quickly (Figure 21.2). The membranes (\f the eyes and mou th are bluish, indicating a lack of oxygen. The blood is chocolate-brown but turns a brighter red when exposed to air. r ii J-22 SOUTHERN FORAGES

3 A.14. Nitrate in feed on dry matter basis % ppm Comments ,500 Generally considered SAFE ,000 Generally safe when fed with a balanced ration. For pregnant animals limit to one-half of total dry ration. Make,certain water for livestock is low in nitrates. Prolonged feeding may result in Vitamin A deficiency. Do not feed with liquid fe('d or other non-protein nitrogen supplements. Be cautious with pregnant and young animals, , ,0 00 Limit to one-fcurth of ration. Should be well fortified with energy, minerals, and Vitamin A. May experience milk production lqss in 4 to 5 days, possible occurrence of reproduction problems. Ov~r 1.5 over 15,000 Toxic. Do not use in free-choice feeding program. Feed containing such levels may be ground and mixed if high nitrate feed is limited to 15% of total ration. t., Method of expression Chemical designation to nitrate to nitrate nitrogen to potassium nitrate Example: 1.0% nitrate x 0.23 = 0.23% nitrate nitrogen 1.0% potassium nitrate x 0.14 =.14% nitrate nitrogen ppm to percent, move the decimal point four places to the left, e.g. 4,400 ppm = 0.44%, 5,000 ppm = 0.5%. NarE:. The table above presents general guidelines which can be helpful in determining the nitrate toxicity potential of forages. However, the likelihood of toxicity can be affected by several factors including species and class of animal, degree of animal stress, animal health, extent of previous exposure to nitrates, and feeding management (see Chapter 21). In adliition, sampling errors can result in misleading results. Considerable variation exists among state-to-state guidelines with regard to nitrate levels which may cause animal feeding problems. Source: Materials compiled from several sources including: D. Miksch and G.D. Lacefield Kentucky Agric. Ext. Ser. Herd Health Memo and B.G. Ruffin, D.M. Ball, and H.A. Kjar Alabama Agric. Ext. Ser. Cir. ANR 112. J- ")'1

4 Figure Cow affected by nitrate toxicity on sorghum-sudan pasture. Control. Animals grazing heavily than 15 percent by weight of the N-fertilized pastures of suspect total ration. species during drought, or wet pas- Animals can tolerate low levels of tures during cool cloudy weather, nitrates, but problems quickly should be carefully watched for develop when threshold leyels in symptoms. Prompt medication the blood are exceeded. Use of large with a 4 percent solution of meth- round bales or stacks increases the ylene blue supplied intravenously danger of nitrate toxicity because using 100 cc per 1,000 Ib of body the animals are likely to have the weight can prevent death. However, opportunity to consume more hay it is. rare that the problem is diag- and thus get a larger total amount of nosed quickly enough to adminis- nitrates in their bodies within a ter this treatment. Supplemental short period of time. Also, large grain feeding can reduce risk; bales and stacks may have concenthrough a dilution effect. trated spots of nitrates. When Hay well-fertilized with N and nitrate poisoning occurs, it often produced during drought periods kills many' animals in a herd should be analyzed for nitrates in a because a large group of animals laboratory. Hay containing up to often have the same opportunity to 2,500 ppm nitrate is usually safe to gorge themselves on high nitrate feed. At levels of 2,500 to 5,000 forage. ppm, several feeding cautions are. suggested. Levels of 5,000 to 15,000 '. PrussIc AcId are considered dangerous and Cause. Naturally occurring gly- '~.quire feeding restrictions. Levels cosides may fonn prussic acid, also over 15,000 are considered toxic to. called hydrocyanic acid or HCN, most classes of livestock and should not be fed free choice. The danger which can build up to toxic levels in leaves of a number of plants includ - does not decrease with time.-toxic ing ]ohnsongrass, sorghum, sudhay may be ground and mixed if the angrass, sorghum-sudan hybrids, nitrate-containing hay is no more and wild cherry. Pearl millet does SOUTHERN FORAGES J-24

5 not produce prussic acid. Prussic acid is most likely to build up to dangerous l~vels immediately after a killing frost. Also, tender young growth occurring immediately after a long drought can be potentially toxic. Young, tender fast-growing plants are more likely to be toxic than older, more matufp plants. Herbicides, including 2,4-0, may temporarily increase prussic acid levels. Prussic acid causes death by interfering with the oxygen-transferring ability of the red blood cells, causing animals to suffocate. Symptoms include excessive salivation, rapid breathing, and muscle spasms, and may occur within 10 to 15 minutes after the animal consumes prussic acid-containing forage. Animals may stagger, collapse, and eventually die. Prussic acid and nitrate poisoning are not the same. Toxic levels of nitrates result from heavy N fertilization followed by severe drought stress. Unlike nitrates, prus~ic acid deter~orates with time. Forage with high levels of prussic acid which is ensiled is usually safe to feed after the ensiling process is completed within 3 weeks after silo fill. Hay which has dried enough to be safely baled (18 to 20 percent moisture) will not contain toxic levels of prussic acid. Standing plants killed by frost are normally safe after about one week. However, in some instances only plants in certain portions of a field are initially killed and subsequent frosts create danger spots in other areas. Control. A producer should know which forage crops have the potential for prussic acid buildup. Grazing should be avoided until at least a week after the end of a severe drought. Once frost occurs, grazing or feeding greenchop should be avoided for at least a week after the last green material has been frosted. When grazing potentially toxic pasture~1 it may be advisable to first turn one or two low-value animals into the pasture and observe them closely. Turning hungry cattle into potentially toxic pastures should be avoided..grass Tetany Cause. Grass tetany is associated with low levels of magnesium (Mg) in the blood of cattle and sheep grazing ryegrass, small grains, and cool season pelennial grasses in late winter and early spring. It is mostly confined to cows and ewes in the early stages of lactation and. often affects the highest-producing animals in a herd or flock. It results from animals grazing plants grown on soils low in available Mg,causing them to be deficient in this element, especially when lactation requires a substantial quantity of Mg. Wet c;oils, low in oxygen, may prevent plants from taking up sufficient Mg regardless of the soil Mg level. Grass tetany is more likely to occuronsoilslowinphosphorus(p) but high in potassium (K) and N because this combination tends to inhibit Mg uptake. This can be a problem with cool season grass forage fertilized with high rates of broiler litter. Generally, forage containing 0.2 percent Mg or more is unlikely to cause tetany. An animal going into teta~y initiall:.' is nervous, with muscle twitching, staggers when walking, and later goes down on its side, with muscle spasms and convulsions. If not treated, death will occur. Control. Pastures deficient in Mg should be limed with dolomitic limestone which contains this element. However, Mg fertilization J-25 C:r\1 IT1Jr,,~1 CrI".1r:~~