Predictions of forage availability are needed early in the growing season to allow adequate time for Western South Dakota ranchers to make stocking or destocking decisions. Incorrect predictions of forage growth potential can lead to incorrect stocking rates, which can have negative environmental and economic consequences. Forage growth potential depends on many variables such as grazing management, prolonged drought, and soil type, but precipitation is the most important factor. A decision tool for determining stocking rates that takes into account precipitation levels would be valuable to South Dakota ranchers.
A recent study completed by the USDA-ARS and North Dakota State University tested whether precipitation in the spring months, annual precipitation, or growing-season precipitation (April through October) were better predictors of forage growth potential. The study used data from grazed and ungrazed mixed grass prairie accumulated over 6-17 year periods at three sites; Streeter, ND, Miles City, MT, and Cheyenne, WY. Four time periods during Spring were tested at each location 1) January through June, 2) April through May, 3) April through June, and 4) May through June. Forage growth potential was determined by measuring the peak standing crop.
Spring precipitation was found to be the best predictor of forage growth potential. Annual and growing-season precipitation levels were poor predictors of forage growth. For Montana, the best predictor of forage growth potential was precipitation during the months of April and May. Total precipitation during May and June was the best predictor of forage growth potential for North Dakota, and total precipitation during April, May, and June was the best predictor of forage growth potential for Wyoming. Grazing did not affect the relationship between precipitation and forage growth. For Wyoming and North Dakota, June precipitation was very important for determining forage growth potential. Ranch managers in these states should consider forecasting June precipitation when stocking decisions will be made prior to or in early June.
This report confirms previous work based on long-term data developed at the SDSU Cottonwood Research Station. For western South Dakota, April through June precipitation provided the strongest relationship with vegetation production for the growing season.
Source: Kyle Dalzell