Chemical Regulation of Growth and Forage Quality of Cool‐Season Grasses with the Imazethapyr

Abstract Forage quality in cool‐season grasses declines rapidly in the spring primarily due to the production of relatively indigestible stem tissue. This study was conducted to evaluate the ability of the plant growth regulator, imazethapyr [5‐ethyl‐2‐(4‐isopropyl‐4‐methyl‐5‐oxo‐2‐imidazolin‐2‐yl) nicotinic acid], to constrain losses in grass quality by reducing rates of neutral detergent fiber accretion and by maintaining levels of crude protein and in vitro dry matter disappearance. In 1986 and 1987 field plots of ‘Pennlate’ orchardgrass (Dactylis glomerata L.), ‘Bison’ ryegrass (Lolium perenne L. × L. multiflorum Lam.), ‘Palaton’ reed canarygrass (Phalaris arundinacea L.), ‘Saratoga’ smooth bromegrass (Bromus inermis Leyss.), and ‘Toro’ timothy (Phleum pratense L.) were treated with imazethapyr, at rates of 0, 25, 50, and 100 g a.i. ha−1. Plots were sampled at weekly intervals for 7 wk after imazethapyr application in the spring. Treated forage averaged between 36 and 71 g kg−1 lower in neutral detergent fiber, between 19 and 44 g kg−1 higher in vitro dry matter disappearance, and between 9 and 20 kg−1 higher in crude protein, depending on the level of imazethapyr applied. Improvements in quality were accompanied by corresponding dry matter yield reductions, which ranged between 0.8 and 2.8 Mg ha−1. Rates at which yield increased or quality decreased as the season progressed were reduced by higher rates of imazethapyr application. Imazethapyr appears to be an effective tool for manipulating the growth and nutritive quality of cool‐season grasses.