Principles of grazing management for cattle (Proceedings)

In the current economy, one thing that has become abundantly clear is that things are going to change. One of the changes occurring rapidly is the increased interest in allowing the animal (cattle) to harvest the forage growing in the pasture. Historically, the cattle industry (both beef and dairy) has relied on forages harvested by man being transported to the cow to provide a large portion of nutrients required for production. When this forage was not adequate to meet the demands of production, cheap grain was located and added to the diet to meet any deficiencies that might occur. However with increasing fuel, fertilizer, and grain prices accompanied with decreases in prices received for beef and dairy products, profit margins are becoming narrower for all operations. As a result, many producers are attempting to minimize these expenses by maximizing the amount of forage harvested by the cow. This form of pasture management is commonly referred to as rotational grazing, intensive grazing, controlled grazing, management intensive grazing, or rotational stocking. Unless we received the training prior to veterinary school, we (veterinarians) are poorly equipped to deal with some of the issues and questions that commonly occur. This paper is designed to serve as a resource for some of those common questions and issues.

Stocking Density or Stocking Rate

One of the most common questions asked by producers is how many cows can I put on this pasture. The correct answer is "It depends". Factors to consider include stage of production, season of year, type of pasture, weather conditions, nutrient requirements, and many others. Stocking rate is defined as the number of animals or live-weight present on grazing unit for entire season or year. Stocking density is defined as number of animal units present within a pasture of known area for a specified period of time. For example we assume 100 cows (1200 pounds each) on a 100 acre pasture with continuous grazing, the stocking rate is 1 cow per acre or 1200 pounds per acre. However if the pasture is divided into 10 equal segments and all 100 cows are in one 10 acre pasture, the stocking density is 10 cows per acre or 120,000 pounds per acre. Although an unlikely example for extended periods of time, it is important to make this distinction when giving recommendations to producers. According to Jim Gerrish, stocking rate should be thought of in seasonal terms while stocking density should be thought of in immediate terms.

Other definitions important to consider when discussing grazing include:

Rotational length (grazing cycle) is the time required to make one complete cycle of the pasture in a grazing unit.

Grazing period is defined as the time that a group of animals occupy a pasture.

Rest period is defined as period between the end of one grazing and the start of the next grazing on the same pasture. Rest period can also be defined as grazing cycle minus the grazing period.

Forage Production and Utilization

Forage production is influenced by many factors including temperature, moisture, sunlight, availability of nutrients in soil, and degree to which plants are defoliated (either by grazing or harvesting). When a large proportion of leaf area is removed, regrowth is usually slow due to a low rate of photosynthesis, and the need to use carbohydrate stores to satisfy both metabolic functions and initiate leaf growth. This is where the art of grazing takes place. It is important that enough residual leaves are left on the plant to allow photosynthesis to occur and regrow the plant (usually 3 to 4 inches depending on plant species and season of year). However if excess growth is left on the plant, the resulting leaves will serve as canopy shading the lower levels of leaves with resulting decrease in photosynthesis and plant growth.

Leaf area index (LAI) and tiller density are greatly affected by grazing management. LAI is defined as surface area of leaf tissue per unit area of ground. Pasture yield will increase as leaf area increases until leaves intercept about 90% of light. Unfortunately most leaves begin to die after 30 to 60 days (depending on plant species), and these dead leaves are less palatable to cattle and have lower nutritional value in most instances.

Estimating Available Forage

There are numerous methods available for determining actual forage yield. However, many experienced graziers actually develop an "eye" for determining the amount of forage or number of grazing days available in the paddock. The most accurate but most labor intensive is the clip and weigh technique. This involves taking 10 to 20 quadrants of known size within the allotted paddock. All forage within that quadrant is clipped and weighed. Dry matter is then determined for these samples and an estimate can be made of the amount of forage available in the paddock. Unfortunately by the time the weighing is completed, the pasture has continued to change while waiting on the results. This technique is mostly used in research settings.

Other alternatives for estimating forage availability include weighted disks or rising plates in which the sward is compressed and measurement is taken. Electronic probes are also available that estimate forage mass. These techniques help to account for the variation in stand density that is commonly seen in pastures. These instruments can range from $20 to $1,000 or more.

A crude but quicker method is to use a grazing stick. This stick has graduated marks that suggest the proper time to graze and remove cattle based on the predominant forage type, and it also allows the user an opportunity to estimate the yield. Table 1 is an example of determining yields based on canopy height of the forage.

Table 1: AYield Estimates for Three Pasture Types of Various Heights

Grazing Methods

The type of grazing method decided upon by the producer is decided based on their goals and objectives. Grazing methods can be easily divided into continuous or rotational stocking.

Continuous stocking is the simplest form of grazing management, and it is probably the most commonly used. Utilization of forages in this system is usually low. Spot grazing occurs as cattle identify the highly palatable species of plants. These plants are typically overgrazed unless the stocking density of livestock is changed as forage production changes. Unfortunately, this is often not the case and forage utilization per acre and production per acre is typically decreased.

Creep grazing is an alternative that allows younger animals with higher nutrient demands to pass into a paddock or pasture with higher quality forages while keeping more mature animals with lower demands in pastures of lower quality. Table 2 below shows the results of a study that allowed creep grazing of calves.

Table 2: Creep grazing of beef calves on pearl millet from June to September while cows remained on fescue

Strip grazing occurs when moveable electric fence is used ahead and/or behind a group of animals. This method results in high forage utilization. However, it may depress individual animal performance since forage selectivity by cattle is not possible. Labor requirements can be quite as high as well.

Limit grazing is another method of grazing in which cattle are allowed time access to high quality forage (usually high cost) forages for a set period of time. This system drastically reduces the waste associated with trampling of unconsumed forage (increasing forage utilization).

Deferred grazing or stockpiling is a commonly used method to reduce hay costs in the southeastern US by grazing forages during the winter that was not previously harvested in the fall. Tall fescue (KY 31) is one of the most common forage types that is used for stockpiling.

Rotational stocking is a system where each grazing area is divided into paddocks (from 2 to 40). In this system, one area is rested while the other area is grazed. In this system, quicker pasture rotations result in higher quality nutrition for the cattle grazing. The greatest advantage in this system is increased carrying capacity of land effectively reducing the amount of waste that occurs. If forage production is higher than the animals can consume, the excess forage can be harvested for future use when high quality pasture forage is unavailable.

The primary benefit of rotational grazing is greater utilization of the forage resulting in a longer grazing season with less stored forage being required. Other advantages include the following: calmer livestock due to increased handling, better persistence and productivity of pasture plants, less forage wasted by trampling, better distribution of dung and urine, and environmental improvements if cattle are fenced from ponds and streams.

Some of the concerns associated with rotational grazing include: unproductive or low quality forages show little benefit if used in rotational system, pasture layout determines how well forage can be utilized, overstocking can occur because there are periods of time when excess forage is present, too many paddocks may result in too long rest period with resultant poorer quality forage, and substantial investment in infrastructure (fencing, water, and labor) are usually required.

Forward grazing or leader/follower grazing is a type of rotational grazing in which animals with higher nutrient demands are grazed ahead of animals with lower nutrient demands. Animals in the leader group are allowed to graze the highest quality forage with a high level of availability, and then they are moved while the follower group is moved in to clean up the residual forage.

Common Grazing Formulas

Number of Paddocks = (Days of Rest/Days of Grazing) + 1

Acres required per paddock

AvgWt of Animals X DM consumed per animal as % Body Wta X # of animals X Day

Dry Matter Available to Grazeb X % of Dry Matter utilized by grazingc

Note: Grazing animals consume between 2 to 3% of body weight in dry matter per day

b Range of 150 to 300 pounds of dry matter for each inch of usable pasture is used

c Varies considerably based on grazing method but ranges between 40 to 70% utilization

Total Acres Required= Number of paddocks X acres required per paddock

Stocking Rate = Number of animals to be grazed / Total Acres Grazed

Stocking Density= Number of animals grazing / Paddock Size (Acres)

Common Health Issues Associated with Grazing

As interests increase and the amount of grazing becomes more prevalent, there will probably be an increase in the number of grazing associated problems that occur. Some of the more common health problems are nitrate (nitrite toxicity), grass tetany, bloat, and prussic acid poisoning.

Nitrate poisoning occurs when animals consume forage containing high levels of nitrate-nitrogen. Nitrates accumulate in plants heavily fertilized with nitrogen, following a drought, shading by other species, during cloudy weather, or following a frost. Certain plants such as sudangrass, sorghum-sudan hybrids, pearl millet, corn, wheat, oats, pigweed, smartweed, and many others are known to accumulate nitrates. Also, the addition of some herbicides (especially 2, 4-D) can increase nitrate levels in plants.

Nitrates tend to accumulate in the stem of plants rather than the leaves. Therefore if high nitrates are suspected, forage can be harvested with cutters set at higher levels to prevent the inclusion of the lower stem into the hay or silage. Also, ensiling of forages is believed to reduce nitrate levels approximately 50%.

Animals that are accustomed to nitrates can tolerate higher levels than naïve animals. The rate at which nitrate is converted to nitrite depends on the rate of adaptation of rumen microorganisms to nitrate, the rate and amount of nitrate ingested, and the amount of carbohydrate available in the rumen.

In addition to nitrates in the forage, it is important to consider nitrate levels in the water. Water nitrate can be compared to forage nitrate equivalent by dividing by 500. Nitrate in water appears to be more toxic than nitrates in forage because of the rapid availability. Therefore, most recommendations are to avoid water with higher than 100 ppm nitrate-nitrogen.

Nitrate toxicity occurs when nitrate is reduced to nitrite in the rumen (usually due to insufficient carbohydrates being present). The nitrite then converts hemoglobin (Fe2+) ) to methemoglobin (Fe3+ ) effectively preventing binding or transportation of oxygen. Clinical signs of hypoxia occur when > 30% methemoglobin is present. Clinical cases respond well to methylene blue (4 to 15 mg/kg of a 2 to 4 % solution) IV. Withdrawal times set by Food Animal Residue Avoidance Databank are 4 days after last treatment for milk and 14 days after last treatment for meat.6 Table 3 lists recommendations for nitrate levels in forages.

Table 3: choiceManagement considerations for use in feeding forages with various levels of nitrates

In addition to the variations in the levels that create issues, many labs report the results in different units. Therefore, information provided in Table 4 will be helpful in converting to common units of nitrate levels into information that can be easily interpreted.

Table 4: NitrateConversion Factors for Nitrate and Nitrite Compounds

The best prevention in rotational grazing system is to not over apply nitrogen fertilizer. Nitrate levels in hay are best determined prior to feeding. Additionally, a commercially available product containing Propionibacterium acidipropionici strain P5 (Bova-Pro ®). The recommendations on label are to administer bolus 7 to 10 days prior to consumption of high nitrate concentrations in either feed or water. Repeat treatment every 180 days. Additional information can be obtained at http://www.agtechproducts.com/beef.htm.

Grass Tetany(hypomagnesemia) is another common disease syndrome seen in grazing situations. Grass tetany occurs when there is a magnesium ion deficiency in blood or cerebrospinal fluid. Magnesium uptake is under no direct hormonal control, and the body stores are not easily labile. Therefore, magnesium requirements must be met by daily intake (approximately 20 g/day). Any adverse weather conditions that impacts normal food intake may precipitate clinical signs.

Clinical signs of grass tetany include loss of normal neuromuscular function, tetanic muscle spasms, and eventually clonic convulsions. Diagnosis is based on magnesium blood levels less than 1.2 mg/dl or vitreous humor samples in post-mortem cases less than 1.4 mg/dl. In addition, urine magnesium levels are usually less than 1 mg/dl can be diagnostic. Low magnesium to calcium ratios potentiate acetylcholine release, and alterations of this ratio may contribute to muscle tetany and eventual cardiopulmonary arrest.4

Grazing cattle are usually grazing lush rapidly growing grass in the late fall or early spring. There are many things that influence uptake of magnesium by the rumen including moisture content of forage, excess potassium in forage, or other minerals (particularly aluminum). A ratio has been determined that indicates whether a particular forage may be more prone to inducing hypomagnesemia. This ratio is % potassium (K) divided by (percent magnesium plus percent calcium). If this ratio is greater than 2.2, the forage is prone to inducing grass tetany.

Treatment for grass tetany consists of treatment with 200 to 300 mls of a 20% magnesium solution given slowly IV. Another alternative treatment in emergency situations if IV therapy is not possible is the use of a magnesium rich enema (60 g of MgCl2 6H2O mixed in 250 to 500 mls of warm water. In this author's experience, the addition of calcium containing solutions is usually beneficial.

The best method of prevention is the inclusion of a 12 to 14% magnesium free choice loose, trace-mineralized supplement for grazing cattle. Intake of these particular minerals should be approximately 4 oz per head per day. A mineral that is labeled for 4 oz per day should be consumed at the rate of approximately 2 pounds per cow per week.

Bloat is a serious problem in grazing pastures dominated by certain legumes.2 Bloat is caused by formation of stable foam in the rumen that prevents eructation. The ingesta in the septae between adjacent bubbles forms a complex structure that prevents coalescence.4 Frothy bloat is higher in chloroplast membrane fragments, soluble protein, and very fine particles than non frothy ruminal fluid.4 The plugging of ingesta at the cardia and failure of reflex relaxation at cardia are believed to result in bloat. Persian, ladino or white clovers, and alfalfa are examples of plants that have bloat potential. Bloat is best controlled by not placing hungry animals into lush pasture. The general recommendation is to feed dry high before moving to these pastures (especially if heavy due or frost has occurred).2 Surfactants added to minerals and ionophores have been shown to reduce bloat cases.2

Prussic acid(hydrogen cyanide) toxicity occurs when damage to the plant (chewing, crushing, wilting, or frozen) causes contact between -glycosides, releasing free cyanide.4 The tight affinity of cyanide for ferric (Fe3+ ) iron in the cytochrome prevents electron transfer.4 Cyanide poisoning in livestock is most commonly associated with Johnson grass (Sorghum halpense), Sudan grass (Sorghum vulgare), and other forage sorghums.2 As glycosides accumulate they inhibit nitrite reductase instead favoring the conversion of nitrate to cyanogenic glycosides rather than amino acids.5

The young rapidly growing portion of the plant and the seeds contain the highest concentrations of cyanogenic glycosides.5 Ensiling will reduce cyanide levels approximately 50%.5

Sudden death is the predominant clinical sign associated with prussic acid poisoning. Levels of cyanide in liver or blood exceeding 1 ppm are indicative of cyanide toxicity.5 A recommended treatment for cyanide toxicity is a mixture of 1 ml of 20% sodium nitrite and 3 ml of 20% sodium thiosulfate given at a dose per 100 pounds of body weight.5 FARAD has recommended a 24 hour slaughter withdrawal and a 48 hour milk withdrawal for the use of sodium nitrite and sodium thiosulfate.6

Prussic acid toxicity is best prevented by allowing sorghums to reach at least 2 feet in height prior to grazing. Additionally animals should be prevented from grazing sorghums during early regrowth following frost, drought, or after recent harvest.

Summary

Grazing livestock creates many new challenges and opportunities for the interested veterinarian. As awareness for "natural" products increases, grazing livestock becomes the basis for all comparison. The first step for most veterinarians is to become acquainted with the forages available in your area. The extension service and university forage specialist are invaluable resources when entering into this field of production management.

References

Gerrish, J. Management-Intensive Grazing. The Grassroots of Grass Farming. 2004.

Ball DM, Hoveland CS, Lacefield GD. Southern Forages. 4th Edition. 2007.

Mueller, JP, Green, JT "Grazing Management Principles". Grassland Management Bulletin 9. North Carolina State University.

Smith B. Large Animal Internal Medicine. 2nd Edition. 1996

Knight, AP and Walter, RG. A guide to plant poisoning of animals in North America. 2001

Haskell SR, Payne M, Webb A, et al. FARAD Digest. "Antidotes in food animal practice". JAVMA, Vol 226 (6), 2005.

Chamblee DS and Green JT. Production and utilization of forages in North Carolina. North Carolina State University Tech Bulletin 305. 1995.