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Classes of Forage - Grasses

Classes of Forage - Grasses
The forages consumed by animals can be classified into one of the following categories: grasses, legumes, forbes, or browse. Grasses and legumes are the primary forages grown for animal consumption. Habitats such as native rangelands and pastures provide a combination of all four classes of forage; grasses, legumes, forbes, and browse. Forbes are defined as primarily broadleaf, nonwoody plants. Browse are woodier plants; the herbaceous and softer components of the browse plants are consumed primarily by selective grazers. Forages may be classified as annuals, biennials, or perennials. Annuals are plants surviving one year or less. Biennials survive two years. Perennials survive for a minimum of three years. As primary cultivated forages are grasses and legumes, the following is a general discussion of grasses and legumes. For more information regarding specific species, please refer to references cited in the text. For electronic resources, click on the following link to be directed to a website that provides subsequent links to a number of forage information websites: http://extension.agron.iastate.edu/aged/FEEL/InfoFolder/Forages/Links/ForageLinks.htm.

The following links are a few links to start on:


Grasses are plant species of the Gramineae family. In addition to grasses’ roles as a feedstuff, grasses also have a number of favorable agronomic properties. Grasses reduce soil nitrogen, contribute organic matter to the soil, improve soil texture, increase water holding capacity of the soil, and decrease soil erosion.


Figure 8.2 illustrates the basic structure of a grass plant; the image is a composite of several grasses.

Figure 8.2 - Basic structure of a grass plant; the image is a composite of several grasses
In general terms, the nutritional value of grasses is different than the nutritional value of legumes. In comparison to legumes, grasses are lower in crude protein, digestible energy, calcium, and magnesium. Grass forage varieties have been developed to grow in a range of conditions and perform a range of functions. Grasses are selected based on the climatic and soil conditions of the region and the specific function of the grass. Climatic and soil conditions include factors such as amount and timing of moisture, solar energy, heat, and supplemental water, and soil composition, fertility, and pH. To receive an accurate measure of soil fertility and pH, it is recommended to perform soil samples on a regular schedule. For grasses, critical nutrients include nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur and additional microminerals. Nitrogen is a critical nutrient for grass forages. Soil nitrogen, in the form of nitrate and ammonia, are the sources of nitrogen for grass forages. Limiting nitrogen will restrict both nutritional value and yield of the grass forage. Forage species are also selected to fulfill the nutrient and dry-matter requirements for a specific class of animals. Nutritional value and yield varies by species and stage of maturity at harvest. Forage species are also selected based on timing of availability. The optimal growing conditions vary by species and depending on harvesting method, whether it is grazing, greenchopping, silage-making, or hay-making, species are selected so the forage is available at the appropriate time. It is important to note, a successful forage system may include a combination of grass and/or legume species. Subsequent to establishment, appropriate management is also required for maintenance of a successful forage system. Management decisions regarding maintenance of a forage system include topics such as selection, application rate, and timing of fertilizer and irrigation, method and timing of harvest, and management of the forage species in the field. The next two lectures will discuss forage system management strategies.


Depending on the classification scheme, grasses may be classified as cool-season, warm-season, temperate, tropical, or a combination of the above terms. The term temperate may refer to either only cool-season grasses or include both cool- and warm-season grasses. In the textbook, the list of temperate grasses includes both cool- and warm-season grasses. In this course, grasses will be referred to as cool-season, warm-season, or tropical grasses.


Cool-season grasses grow best in cool, moist climates. Cool-season grasses are most productive in the spring and fall.


Figure 8.3 illustrates the general growth patterns of cool- and warm-season grasses.

Figure 8.3 - Growth patterns of grasses


Generally, cool-season grasses are winter hardy. Drought tolerance varies. Cool-season grasses are classified as C3 grasses. Examples of perennial cool-season grasses include the brome grasses, Kentucky bluegrass, tall fescue, orchardgrass, perennial ryegrass, reed canarygrass, and timothy. For more information on the specific cool-season grasses, click on the following link: http://agguide.agronomy.psu.edu/sect8/sec85a.htm or refer to the links listed earlier in the lecture.


Warm-season grasses grow best in the summer. Warm-season grasses are tolerant of periods of high temperatures and drought. Generally, they are not winter hardy. Compared to cool-season grasses, warm-season grasses provide higher yields and poorer-quality forage. The various warm-season grasses are classified as both C3 and C4 grasses. Examples of warm-season grasses include bahia, bermudagrass, bluestems, Indiangrass, carpetgrass, johnsongrass, and switchgrass. For more information on the specific warm-season grasses, refer to the links earlier in the lecture.


Additional types of forage grasses include forage cereals such as corn, oats, sorghum, and wheat. The grasses provide forage in the form of pasture, silage, and hay. For cattle, corn is commonly grown to produce silage. In addition to the forage harvested for silage, the field residues may be grazed by cattle.


Figure 8.4 illustrates cattle grazing corn field residues.

Figure 8.4 - Cattle grazing corn field residues

Corn silage will be discussed in more detail later in the lecture. Varieties of forage sorghums provide forage in the form of pasture, silage, and hay. One potential problem with forage sorghums are the presence of the antinutritional cyanogenic glycosides. In the Midwest, it is common to graze beef cattle on winter wheat forage. Three potential issues with grazing wheat forage are bloat, grass tetany, and nitrate poisoning.


Three primary issues to consider when feeding grass forages are the potential for nitrate poisoning, grass tetany, and endophyte toxicity. Nitrate toxicity or nitrate poisoning occurs when animals consume excessive levels of nitrate. Nitrate toxicity primarily affects cattle, but also may affect sheep and horses. In the GI tract, the microorganisms convert nitrate to nitrite. Excessive nitrite is absorbed into the bloodstream. In the bloodstream, nitrite combines with hemoglobin to form methemoglobin. Hemoglobin transports oxygen in the bloodstream. Methemoglobin does not transport oxygen in the bloodstream.


Figure 8.5 illustrates the pathways of nitrate in the ruminant.


Figure 8.5 - Pathways of nitrate in the ruminant


Signs of nitrate toxicity include respiratory distress, bluish mucous membranes, convulsions, and death and an internal sign is the blood will be brown. The amount required for toxicity varies. To prevent nitrate poisoning, avoid excess nitrate in forages. Nitrates accumulate in forages when the nitrogen supplied to the plant exceeds the nitrogen metabolized by the plant. To prevent nitrate toxicity consider factors such as fertilization rate and growing conditions. Analyze nitrate levels of questionable forages. Ensiling questionable forages will reduce the content in forages. In addition, provide adequate energy in the ration. The most common treatment is the administration of methylene blue. Grass tetany, grass staggers, or hypomagnesemia is a complex nutritional disorder caused by extremely low blood magnesium levels in the animal. Generally, grass tetany occurs in ruminants consuming lush grass forages. Immature grass forages have low levels of magnesium. In addition, factors such as high potassium, high nitrogen, and low calcium will promote the development of grass tetany. Typical signs of grass tetany include nervousness, incoordination, convulsions, and death. A urine sample is an accurate guide for the magnesium status of the animal. Treatment of grass tetany requires administration of magnesium and/or calcium to the animal. To prevent further complications, administration should be slow and the animal should not be stimulated during treatment. To prevent grass tetany, provide a supplemental source of magnesium to at risk animals. Supplementation may either be through feeding additional magnesium in the mineral mix, providing a supplemental feedstuff high in magnesium, or magnesium administered to the plants to increase magnesium content in the forage. As legumes are higher in magnesium, planting legumes with grass forages will increase the magnesium level of the diet. In addition, monitor calcium, nitrogen, and potassium levels of the diet. Endophyte toxicity is an issue for ruminants and horses consuming grass forages; the grass forages of concern are tall fescue and ryegrass. Endophyte toxicity is primarily associated with turf versus forage varieties of the grasses. Endophytes are fungi. Endophytes produce toxins detrimental to the health of animals. Laboratory analyses are required to identify the presence of endophytes. Signs of endophyte toxicity in tall fescue include hyperthermia, decrease in production, problems associated with reproduction, and tissue death associated with the feet. Signs of endophyte toxicity in ryegrass include problems associated with muscle control, especially when the animal becomes stimulated. To prevent endophyte toxicity, only feed low-endophyte or endophyte-free forage varieties of the grass forages.

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