Evaluation of Weed Control Practices during Spring and Summer Alfalfa Establishment

Weed management during alfalfa (Medicago sativa L.) establishment is a major concern for producers in the northeast USA. The objectives of this research were to determine the yield, quality, and economic consequences of controlling weeds during the establishment of spring- and summer-seeded alfalfa. Alfalfa was band-seeded during spring (April and May) and late summer (August and September) into a conventionally tilled seedbed of Hagerstown silt loam (fine-loamy, mixed, mesic Typic Hapludalfs) at Landisville, PA, in 1991 and Murrill silt loam (fine-loamy, mixed, mesic Typic Hapludult) at Rock Springs, PA, in 1991 and 1992. Weed control treatments for the spring seedings included: (i) no herbicide or nurse crop (no weed control); (ii) oat (Avena sativa L.) nurse crop; (iii) benefin preplant incorporated; and (iv) sethoxydim and 2,4-DB postemergence. Weed control treatments for late-summer seedings were the same as those in the spring except that the nurse crop was replaced with a paraquat application to dormant alfalfa. Seeding alfalfa later, compared with earlier, in the spring tended to increase weed and decrease alfalfa dry matter (DM) yield but did not affect total DM yield or forage quality during the seeding year. For spring-seeded alfalfa, weed control treatment effects on alfalfa and weed DM yield during the seeding-year were sporadic, however, net economic return per acre was generally greatest when no weed control was used. Neither time of spring seeding nor weed control treatments produced consistent differences in yield, quality, or net economic return per acre after the seeding year. Delaying alfalfa seeding in the summer reduced first harvest alfalfa yield (704 lb/acre less) and season total alfalfa yield (1121 lb/acre less) compared with early seedings. Generally, weed control treatments in summer-seeded alfalfa did not improve alfalfa or total DM yield, forage quality, or net economic return per acre compared with no weed control. We conclude that when alfalfa is either spring or summer seeded, the weed control practices used in this study were not economically beneficial and may actually reduce net return per acre because of associated additional costs compared with seeding alfalfa with no companion crop or herbicide. Research QuestionConcerns over the perceived deleterious effects of weeds on alfalfa survival and forage quality cause many producers to employ a prophylactic weed control practice during alfalfa establishment. These practices not only add to the cost of forage production, but also can involve the use of chemicals, which are increasingly under environmental scrutiny. This study examines the effect of different weed control practices during alfalfa establishment on forage yield and quality and net economic return per acre. Literature SummaryWeed management during alfalfa establishment is important for improving forage yield and quality on the approximately 250 000 acres planted to alfalfa each year in the northeast USA. Numerous practices, including nurse crops, herbicides, and time of planting have been used for this purpose. Using nurse crops for weed control has provided inconsistent control and forage yield. When weed infestation is expected to be severe during alfalfa establishment, herbicide use can increase forage quality by reducing the amount of weeds in the harvested forage. However, this reduction in weeds can result in a net reduction in first-harvest and total seeding year yield, compared with establishing alfalfa without a herbicide. Many producers seed alfalfa in the summer (August) to reduce the infestation and competition from summer annual weeds. Although summer seeding alfalfa can reduce summer annual weed competition, it can increase winter annual weed competition. Additional expenses of a nurse crop or herbicides for weed control during alfalfa establishment may not provide a net economic gain. Our objectives were to determine the yield, quality, and economic responses of controlling weeds during the establishment of spring- and summer-seeded alfalfa. Study DescriptionAlfalfa was band seeded at 15 lb/acre into a tilled seedbed at two locations and at four times during the growing season using various weed control practices. Year and Locations: 1991 and 1992 near Rock Springs, PA, at the Russell E. Larson Agricultural Research Center 1991 near Landisville, PA, at the Southeastern Field Research Lab. Seeding dates and corresponding treatments: Spring (April and May) no weed control oat nurse crop benefin preplant incorporated sethoxydim + 2,4-DB postemergence Summer (August and September) no weed control paraquat as a dormant application benefin preplant incorporated sethoxydim + 2,4-DB postemergence Observations/Measurements: Visual estimates of weed severity and control, and alfalfa injury. Alfalfa, weed, and combined yield in seeding year and year after seeding. Quality of forage at each harvest. Applied QuestionsDo the weed control practices evaluated in this study increase forage yield and/or quality? For spring seeded alfalfa, different seeding dates and weed control practices did not result in consistent differences in yield or forage quality during the seeding year, first production year, or for both years combined. Earlier summer seedings had greater first-harvest and season total alfalfa yield in the year after seeding than later summer seedings. In general, when seeding alfalfa in the summer, weed control practices did not increase alfalfa or total forage yield, or forage quality compared with no weed control. We conclude that when alfalfa is either spring or summer seeded and weed infestations are light to moderate, use of a herbicide or nurse crop is not warranted on the basis of alfalfa yield or forage quality. Do the weed control practices evaluated in this study increase net economic return per acre? For spring seeded alfalfa, different seeding dates and weed control practices did not result in consistent differences in net economic return per acre during the seeding year, first production year, or both years combined. In general, early summer seedings had similar economic returns per acre in the year after seeding as late seedings. Weed control in summer seedings had similar economic returns per acre in the year after seeding as no weed control. We conclude that when alfalfa is either spring or summer seeded and weed infestations are light to moderate, weed control practices are not economically beneficial and may slightly reduce net return per acre because of additional costs compared with seeding alfalfa with no nurse crop or herbicide. RecommendationsWhile weed control practices may improve the appearance of an alfalfa field during establishment, they did not, under the conditions of this research, consistently increase alfalfa yield, total forage yield, forage quality, or net return per acre. Therefore, if weed infestation is not anticipated to be severe during alfalfa establishment (based on previous weed problems in the field), we do not recommend the use of a nurse crop or herbicides for weed control.