The clover populations and yields of a Kentucky bluegrass sod as affected by nitrogen fertilization, clipping treatments, and irrigation

Clover populations and yields of herbage as affected by two levels of soil moisture, two levels of nitrogen fertilization, and four clipping treatments in all possible combinations were determined on a Kentucky bluegrass sod during 1944 and 1945. The moisture levels were natural rainfall with and without irrigation; the nitrogen levels were no nitrogen fertilization and a very high level of nitrogen fertilization (40 pounds of nitrogen per acre in the early spring and repeated after each clipping); and the clipping treatments included (a) clipping to 1/2 inch when the herbage reached a height of 4 inches, (b) clipping to 1 inch when 4 to 5 inches high, (c) clipping to 2 inches when 5 inches high, and (d) clipping to 1/2 inch when 3 inches high in the early spring and thereafter clipping to 1 inch when 4 to 5 inches high. All plots were limed and well fertilized with phosphate and potash. On plots without nitrogen fertilization or irrigation the closer clipping treatments (1/2 inch and 1 inch) resulted in excellent stands of clover. Clipping to a height of 2 inches produced a more dense sod of grass with considerably less clover. High rates of nitrogen fertilization, without irrigation, greatly decreased the stand of clover and on the plots clipped to 2 inches clover was practically eliminated. With the more severe clipping treatments considerable amounts of clover were maintained even with heavy nitrogen fertilization. On the irrigated plots, clipping to 1/2 inch or 1 inch resulted in excellent stands of clover even on plots that received as much as 360 pounds of nitrogen per acre per year. Fairly good stands of clover were maintained at the 2-inch height of clipping. The highest yields of dry matter were obtained on the plots clipped to 1/2 inches, whereas the lowest yields were obtained on plots clipped to 2 inches. It is concluded that clover populations are determined by the ability of the clover to compete with grass for space, light, moisture, and nutrients. Changes in these factors or of such factors as management practices, winter injury, severe drought, or disease epidemics may greatly change the botanical composition of the pasture. Management practices, particularly as they affect the carbohydrate reserves of the grass, are especially important. It is emphasized that the level of carbohydrate reserves which is best for the grass may not be the best for the pasture. In the present experiment close clipping with ample time for recovery between clipping dates was very effective in maintaining clover. The results obtained in these experiments indicate the need for studies under grazing conditions to determine the possibilities of increasing the clover content and thereby the yields of pastures.