There are two main photosynthetic pathways in grasses. In C3 (cool-season) grasses, the first measurable product of photosynthesis is a 3-carbon compound, 3-phosphoglyc-erate. In C4 (warm-season) grasses, 4-carbon intermediaries, such as malate or aspartate, are formed. Associated with C3 photorespiration is a leaf anatomy that has many more mesophyll cells and less lignified vascular bundles than that associated with C4 photorespiration. Thus, C3 grasses generally have a higher nutritive value than C4 grasses in terms of protein, sugars, and starches. Most C3 grasses can fix CO2 at temperatures near freezing, with net photosynthesis being maximized at 20 to 25°C and decreasing above 30°C. Photosynthesis at full solar radiation
is reduced. This means that C3 grasses are ideally suited for the temperate regions of the world. The C4 grasses have a higher biomass production per unit of water used than C3 grasses and are often deeper-rooted than C3 grasses. Because of their anatomy, C4 grasses tend to produce more biomass per unit of fertilizer (nitrogen, N) than C3 grasses, but this results in lower N content for C4 grasses. At full sun, photosynthesis of C4 grasses is nearly double that of C3 grasses. Photosynthesis of C4 plants is low at temperatures less than 100C, increasing to a maximum between 35 and 400C, and decreasing above 400C because of protein destabilization. These factors make C4 grasses well adapted to hot, dry climates.
Much of the world's land is poorly suited to growing legumes because of low pH or poorly drained soils, making the use of grasses under these conditions the most viable economic alternative. Perennial grass also can remove over twice the nitrogen/acre compared to corn, making perennial grasses attractive choices for nutrient management, regardless of soil conditions.1-3-1
From a nutritional standpoint, however, perennial grasses often are less preferred than perennial legumes. Perennial grasses generally contain more fiber than legumes, resulting in lower dry matter intake and consequently lower production if used as the major feed source in the diet.
Grass feeding value can be defined as its capacity to promote animal production or performance.1-4-1 Animal performance is a function of nutrient intake and availability, nutrient concentration, digestibility, and metabolic efficiency.1-5-1 Intake, digestibility, and efficiency of utilization are characteristics of forages that determine animal performance, with variation in intake accounting for 60 to 90% of the variation in digestible energy or dry matter intake.
Understanding biological mechanisms that affect forage digestion in relation to chemical analyses is critical for accurate prediction of animal response to a diet. Lignin is the chemical constituent most often identified as limiting fiber digestibility. Lignin is indigestible by ruminants or microbes, and inhibits the digestion of hemicellulose, probably accounting for its close association with digestibility. Notwithstanding this association, every species has a different relationship between lignin content of dry matter and dry matter digestibility.[9-
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