Chemical Regulation of Alfalfa/Grass Mixtures with Imazethapyr

Growth regulators can improve forage grass quality, but they have not been evaluated for quality enhancement with grass-legume mixtures. A field experiment was conducted for 2 yr to investigate the effects of applications of imazethapyr [5-ethyl-2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl) nicotinic acid] on production and quality of alfalfa (Medicago sativa L.) /grass mixtures. Mixtures of alfalfa with either orchardgrass (Dactylis glomerata L.), ryegrass (Lolium perenne × L. multiflorum Lam.), reed canarygrass (Phalaris arundinaceae L.), smooth bromegrass (Bromus inermis Leyss.), or timothy (Phleum pratense L.) were established on a Hagerstown silt loam (fine, mixed, mesic Typic Hapludalf) and were treated with imazethapyr at application rates of 0, 25, 50, and 100 g a.i. ha⁻¹. Results showed first-harvest mean (mean of rates and grass species) grass crude protein (CP), in vitro dry matter disappearance (IVDMD), and neutral detergent fiber (NDF) of 128,643, and 622 g kg⁻¹, respectively compared with 108, 623, and 654 g kg⁻¹, respectively for controls. Alfalfa was unaffected by treatments. Changes in CP, IVDMD, and NDF concentrations with increasing imazethapyr levels showed significant linear and quadratic components for first harvest in 1986, but showed only a linear trend in 1987 due to reduced effectiveness of the 25 and 50 g ha⁻¹ treatments the second year. Second- and third-harvest quality trends were inconsistent both years. Controls averaged 21% grass at first harvest in 1986 and 37% in 1987, whereas imazethapyr lowered first harvest grass percentage to 13 and 22%, respectively for the 2 yr. In 1986, treated plots showed enhanced grass regrowth at second cutting, but the same was not observed in 1987. Consequently, treatments did not affect seasonal yield in 1986, but resulted in a mean reduction of 0.6 Mg ha⁻¹ in 1987. Where forage quality of alfalfa/grass hay is paramount, imazethapyr may be an effective management tool. Journal Series Number 7928 of the Pennsylvania Agric. Exp. Sta.