August 22, 2023

Problems of Grazing Below Stocking Rate

Llewellyn L. Manske PhD Scientist of Rangeland Research Dickinson Research Extension Center 701-456-1118 llewellyn.manske@ndsu.edu

Grass plants produce double the leaf and stem biomass needed for photosynthesis and normal growth and development. This portion of the grass weight is expendable by the plant. The surplus standing leaf material accumulates rapidly and changes from an asset to a detriment if graminivores do not remove it by grazing.    When a grassland is managed by the biologically effective twice-over rotation strategy, the surplus carbohydrates produced by vegetative lead tillers are exudated through the roots into the rhizosphere and increase the biomass of the soil microbes. The larger biomass of microbes can then transform organic nitrogen into mineral nitrogen at quantities greater than 100 lbs/ac. When mineral nitrogen is available at 100 lbs/ac or greater, the internal grass growth mechanisms of compensatory physiological growth, vegetative reproduction by tillering, nutrient resource uptake, and water use efficiency mechanisms can be fully activated and are capable of increasing herbage biomass production. As the herbage biomass increases, the stocking rate should be increased at 10% per year until the stocking rate is 140% of the seasonlong rate, which, has been the biological potential of the twice-over system in the Northern Plains.    The increase in stocking rate must match the increase in grass herbage biomass. If the stocking rate remains at 80%-100% of the seasonlong rates and the herbage biomass increases to 140%, the surplus accumulating standing leaf biomass shade the lower leaves, increasing the rate of leaf senescence and reducing the rate of photosysnthesis, causing a decrease in the supply of carbohydrates that results in a reduction in growth of leaves and roots. Lack of grazing the surplus leaves reduces grass tiller densities by decreasing tiller development and increasing tiller mortality through shading.    After a few years, the shading increases, the composition of native grass species reduces, and the composition of shade-tolerant replacement species, like Kentucky bluegrass increases. The only way to prevent this unwanted grass species change from happening is by increasing the stocking rate as the rate of herbage biomass increases.    For More Information Go To:  https://www.ndsu.edu/agriculture/ag-hub/publications/grazing-handbook

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