Potassium and Harvest Time Affect Production and Nutritive Value of Alfalfa.

Alfalfa (Medicago sativa L.) is the third most valuable crop in the United States. Unfortunately, yield of alfalfa has not increased during the past three decades despite release of many cultivars with high yield traits. Potassium (K) fertilization and harvest time have beneficial effects on increasing alfalfa production. However, limited information is available of K and its interaction with harvest time on alfalfa production and nutritive value. A field study was initiated at the University of Wyoming James C. Hageman Sustainable Agriculture Research and Extension Center at Lingle to determine the effect of K on forage accumulation and nutritive value of alfalfa at two harvest times. Treatments were two cultivars (“Hi-Gest 360”; “AFX 457”), four K rates (0, 56, 112, and 168 kg K₂O ha^-1), and two harvest times (early harvest, late bud to early (10%) bloom; late harvest, 7 days after early harvest). Results indicate that K and cultivar affected forage accumulation (P < 0.05) however, harvest time did not (P > 0.05). Total forage accumulation was the highest in Hi-Gest 360 (8372 kg ha^-1) and AFX 457 (8283 kg ha^-1) at 168 and 112 kg K₂O ha^-1, respectively. This implies that moderate and high levels of K are required for high yield in AFX 457 and Hi-Gest 360, respectively. Forage nutritive value was not affected by K application (P > 0.05) however, it was affected by harvest time and number of harvest (P < 0.05). For example, early harvest had higher crude protein (CP, 275 g kg^-1), total digestible nutrient (TDN, 743 g kg^-1), and in vitro dry matter digestibility (IVDMD, 761 g kg^-1) than late harvest (CP, 268 g kg^-1; TDN, 735 g kg^-1; and IVDMD, 756 g kg^-1). Preliminary results show that K application and harvest time can impact production and nutritive value of alfalfa.