Daren D. Redfearn, Assistant Professor, Southeast Research Station
The primary goal of any forage producer should be to produce high quality forages year-round. Actual forage quality is the combination of all of the elements that affect nutritive value and subsequent animal performance, including the forage’s intake potential, digestibility, and chemical composition. Forage species and growing environment and the interaction between these two are among the most important factors that influence forage quality. Environment is the complex interaction of temperature, moisture supply, and soil characteristics, such as nutrient supply. Environmental effects on forage quality can be positive or negative. Three factors definitely not favorable for optimal plant growth and forage quality are heat stress, drought, and inadequate soil nutrient supply. These problems are very common in Louisiana. Although we may have little control over these environmental influences, we do need to understand the overall effects of environment on forage quality.
If moisture is not limiting, growing temperature has the greatest influence on plant growth and composition. As growing temperature increases, plant growth rate increases. This increase in growth rate causes the plant to produce more stems than leaves and generally decreases digestibility. In addition, heat-stressed plants maintain lower levels of soluble carbohydrates compared with non-stressed plants. The effects of temperature stress and drought stress are often difficult to separate because they typically occur together. Drought stress occurs when plant water loss exceeds water absorption. When heat stress is minimal, drought stress actually improves plant digestibility. This occurs because non-structural carbohydrates accumulate during periods of water shortage. Plants under water stress also develop and reach maturity more slowly than nonstressed plants.
Soil fertility can have dramatic effects on forage quality. However, soil types vary in their supply and availability of nutrients. Identical fertilization of two forage species grown on different soils rarely results in identical forage growth and quality characteristics. Nitrogen is the most common fertilizer applied to forages and results in rapid plant growth, if water is not limiting. Forage digestibility will be improved, if fertilization increases the proportion of leaves relative to stems. Nitrogen fertilization can have dramatic effects on forage economic value when the forage’s nitrogen (crude protein) level is increased, because less supplemental protein will be needed in the feeding program. Low forage nitrogen, as is common in mature forages, can limit animal performance, if supplemental protein is not fed. This occurs because low ammonia nitrogen levels in the cow’s rumen can greatly limit the ability of the rumen’s microorganisms to digest fiber. Forage mineral deficiencies can also negatively effect rumen function and animal performance; however, the impact is usually not as great as observed with low forage nitrogen.
The stage of plant maturity at harvest influences forage quality more than any other single factor influencing the development (leaf-to-stem ratio) and chemical composition (protein, fiber, and minerals) of the plant. These forage quality factors ultimately establish the amount of dry matter that can be consumed and the performance level that can be reached by the animal. As forage plants mature, the ratio of leaves to stems decreases. Although the nutritive value of leaf blades decreases progressively from the top to the bottom of the plant, leaves will still be, on the average, much higher in quality than stems. Based on this, any management practice that increases the leaf-to-stem ratio should result in higher forage quality. This would include both proper timing of grazing and also management practices that maximize hay and (or) haylage yield and quality.
Because the environment cannot easily be controlled, the ability to determine the optimum time to harvest a forage is extremely important. Other than timely harvest and preservation of the forage, few alternatives exist for maintaining forage quality for extended periods of time. In Louisiana, inclement weather frequently interferes with forage harvesting schedules. Substantial forage quality losses can occur with uncooperative weather. If harvesting is delayed, forages may become overly mature. Also, nutrient leaching losses may occur when cut forages are rained on for several days. The latter of these two scenarios is probably most desirable since the likelihood for additional opportunities to harvest high quality forage later in the growing season would increase.
Summary and Applications
Even if environmental conditions are good and the essential nutrients for proper forage growth are adequate, harvesting an overly mature forage will result in poor quality feed. It costs little more to produce high quality forage than it does a fair or poor quality forage. High quality forage helps meet the animal's nutritional requirements with less supplementation. Regardless of the forage species or growing environment, good harvest management is critical to consistently producing high quality forage.