Graduate Thesis Or Dissertation
 

The impact of three stocking intensities and four grazing treatments on the carbohydrate reserves of perennial ryegrass (Lolium perenne L.)

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  • A study was conducted during 1974 and 1975 to evaluate the impact of stocking intensity and season of grazing upon carbohydrate concentration, biomass of storage organs, and total carbohydrate per plant in the roots and crowns of perennial ryegrass (Lolium perenne L.) in improved rangelands in Western Oregon. The stocking intensities were 7.4 ewes per hectare (moderate), 9.9 ewes per hectare (heavy), and 12.4 ewes per hectare (overstocking). The grazing treatments were Complete Protection, Fall Protection, Fall Grazing, and Yearlong Grazing. Another study was established during the same period to relate the seasonal carbohydrate reserve cycle and plant growth to phenological stages of development. The influence of stocking intensity on carbohydrate concentration was significant for both roots and crowns. Effects of season of grazing on the concentration in these plant parts were also significant. Ryegrass plants under the moderate stocking intensity had a significantly higher concentration of total available carbohydrate (TAC) than those under each of the other two higher stocking rates. Fall grazing resulted in higher total available carbohydrate concentration than the other seasonal grazing treatments. Stocking intensity impact was significant on the crown biomass, but not on root biomass. Plants under moderate stocking intensity had significantly greater biomass than those under heavy and overstocking rates. Season of grazing as well as interaction among year and season of grazing had a significant effect on both crown and root biomass. Stocking intensity had a significant effect on total carbohydrate per plant. The TAC per perennial ryegrass plant was higher in moderately stocked pastures than in pastures with heavy or overstocking rates. A significant effect was shown for the season of grazing treatments on root TAC per plant, but not for the crowns. Perennial ryegrass displayed clearly defined seasonal trends in TAC concentration during the period of data collection. The crowns maintained greater TAC levels than the roots at all phenological stages. The carbohydrate reserve curve for different phases of development displayed the expected spring and winter depletion of reserves for meeting growth initiation and further development. It had its greatest slope during these phenological stages, concurrent with sharp reserve depletion. Fall regrowth did not result in a sharp decline in reserve level as did spring and winter growth. The management implications were discussed. Ryegrass showed a high tolerance to season of grazing. Grazing during the early phase of spring is recommended to be at moderate intensity, since the combination of high grazing pressure and early defoliation is detrimental to the carbohydrate reserves. Winter grazing at overstocking rates may be potentially harmful to perennial ryegrass. Moderate to heavy stocking intensities are recommended, if yearlong grazing is considered. Grazing at overstocking rates is possible through a program that considers deferment during one or two seasons of grazing.
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