Effects of Weed Control and N Fertilization at Establishment on the Growth and Nodulation of Alfalfa1

Abstract The establishment of productive alfalfa ( Medicago sativa L.) stands is greatly dependent on competition by other plant species and on the seedling development of the forage legume. The objective of this study was to determine the effects of weed control methods in combination with N fertilization rate on seedling growth and nodulation of alfalfa. ‘Saranac’ alfalfa was established on check (no weed control), handweeded, and herbicide 4‐(2,4‐ dichlorophenoxy)butyric acid + 2,2‐dichloropropionic acid (2,4‐DB ‐f‐ dalapon) plots with N applied at 0, 25, 50 or 100 kg/ha at emergence of the forage legume. Dry matter (DM) yields of alfalfa in the first cutting were significantly increased by N applications of 25 to 50 kg/ha over the check in two out of three experiments. Nitrogen applied at seeding did not influence the DM yields of the second cutting. On plots treated with 2,4‐DB and dalapon, the DM yields of alfalfa in the first cutting were 30 to 81% and 11 to 78% of those of check and handweeded plots, respectively. In the second cutting, DM yields of alfalfa were not affected by the method of weed control. Weed growth was significantly increased by applied N in one experiment only. Nodule size was reduced by N applications of 50 and 100 kg/ha. Similarly, the number of nodules per plant was decreased in one experiment. Percent total N of alfalfa tissue was increased by applied N in the first cutting of two experiments. In the second cutting, high N rates resulted in a lower N content of alfalfa tissue which coincided with poor nodulation. The N rate ✕method of weed control interactions for most criteria were not statistically significant.