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Allowing dairy cows to graze forages versus harvesting and storing the forages can be a very economical means of providing some or all of a dairy cow's forage needs. For a grazing herd, the amount of grazed forage consumed can vary greatly depending on herd size, pasture acreage, growing conditions, or the time of year. Grazing forages for the milking dairy herd has been practiced for years, but some of the forage management practices have changed over time. One of the biggest realizations is that pasture systems must be managed closely, and the movement of the cows must constantly evolve and change to match the availability of forages. For profitable milk production, proper management of the pasture is essential. Pastures must be managed so that lactating cows consume extremely high-quality (highly digestible, low to moderate concentrations of fiber, and high concentrations of protein). This goal can be accomplished primarily by maintaining the pasture so the plants are consumed when they are immature. The species of pasture plants is secondary to forage maturity with respect to nutritional value. Proper plant maturity can be obtained by managing the rotation of fields or paddocks using temporary fencing. Outlined in this article are some of the key components in managing a grazing system for the milking dairy herd.
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Manage the forage rotation to allow the forage plants time to rest and regrow. In the summertime, rest periods need to be approximately 28 to 35 days between grazing periods. Grasses regrow from the tillers that are close to the soil surface; thus, it is important for 3 to 4 inches of growth to remain after grazing orchardgrass or fescue plants. Closer grazing increases the time for regrowth; decreases survivability of the plants, especially during drought conditions; and decreases the nutritional value of the plants. In contrast to grasses, alfalfa and red clover plants regrow from the carbohydrate stores in the plants’ roots; thus, a lower residual grazing height is possible. In stands in which you want to favor the growth of the legumes, graze the plants lower. If you want to favor the growth of the grasses, graze the plants higher.
Graze young lush plants in their vegetative state. Vegetative plants contain less fiber, and the fiber is more digestible than more mature plants. Vegetative plants also contain more starch, sugars, energy, and protein, which will reduce the amount of supplemental concentrate needed to maintain high milk yields. More mature plants have more lignin and are less digestible in the rumen. These factors not only reduce the energy concentration of the forage but also reduce dry matter intake, which will reduce milk yield and increase the amount of supplemental concentrate needed.
Dairy cows graze about 8 hours daily, with the heaviest grazing periods in the early morning and later in the evening. Dairy cows do selectively graze forage types (legumes are preferred over grasses). In addition, the first bite of forage is from the top of the plant containing the highest concentration of nutrients. The next bite is the middle of the plant where the nutrition is somewhat less than the top. The lower part of plants contains more stem and thus more fiber than the upper parts of both legume and grass plants (Table 1). Requiring animals to consume most of the plant in a single grazing pass results in more even consumption of nutrients found in the plant. Strip grazing using temporary fencing allows the producer to achieve this objective. Remember with dairy cows, we want them to take half of the available forage and leave half behind.
Table 1. Concentration of nutrients (dry matter basis) in orchardgrass harvested in October from the Northeast U.S. (data from Karen Hoffman, eOrganic eXtension Webinar 9/16/2010).
|Total plant height was 9 inches
||Crude Protein (%)
In any feeding system, maintaining dry matter intake in dairy cows is critical. Dry matter intake on grazed forages is determined by bite rate, time spent grazing, and bite size. Bite size is the most variable of these three factors. Cattle usually are capable of 50 to 70 bites of forage per minute, spend approximately 8 hours grazing each day, and average a bite size of 0.2 ounces per bite. Bite size is directly related to stand density and forage height. The take-home message is that cows need to be able to consume a mouth full of feed from pasture plants to optimize forage intake.
Forage programs should be designed such that dairy cows have high-quality forage to graze at all times. Stored forages should be used when high-quality grazed forages are not available in amounts that match the dairy herd’s nutrient needs. In the summertime, cool-season grasses, such as orchardgrass and fescue, do not grow well when temperatures exceed 70°F. Alfalfa, sudangrass, pearl millet, and warm-season perennials will grow during summertime temperatures and in full sun radiation. Brown midrib varieties of sudangrass improve digestibility because they contain less lignin. With grazing legumes, caution needs to be taken to prevent bloat. With the sudangrasses, grazing should be avoided until the plants are 24 inches tall to reduce the risk of prussic acid poisoning. Supplemental minerals with magnesium included are important for reducing the risk of grass tetany.
Remember to provide plenty of cool, clean water in every grazing area. Limiting water intake will decrease milk production quickly. Dairy cows producing 50 lb/day of milk drink about 25 gallons/day of water when the ambient temperature is 60°F. When the temperature increases to 90°F, water intake increases by approximately 5 gallons daily. Make sure water is always close to the cows (within 800 ft).
During the daylight hours, provide plenty of shade or allow the dairy cows to return to the barn. Rotation of shade areas is important to prevent environmental mastitis. A study in British Columbia showed that, given a choice, dairy cows preferred the barn during the day and the pasture area at night.
Pasture plants contain protein that is highly degradable in the rumen, and these plants are low in sugars and starch. In diets with high amounts of forage from pasture, oftentimes milk urea nitrogen (MUN) is higher than with conventional diets. Adding starch-based concentrates (such as corn or hominy) and sugar sources (e.g., molasses) will allow rumen bacteria to capture more of the plant protein, which can increase milk yields and milk protein concentrations and decrease MUN levels. Adding protein sources high in rumen undegradable protein (RUP), such as dried distiller’s grains, has not resulted in a milk production response.
The bottom line is that pasture can be an excellent and profitable feed for dairy cows if managed correctly and supplemented properly.
Dairy Extension Specialist, University of Kentucky