Forage Yield and Quality of Brassicasp. Established Using Preemergence Herbicides1

Abstract

abstractBrassica sp. may serve as high energy forage crops at times when forage availability may be limited. Field studies were conducted in the Northeast to assess forage yield and quality of 'Sirius' turnips (Brassica rapa L.) and 'Perko' rape (B. napus L.) planted on a Hagerstown silt loam (fine, mixed, mesic Typic Hapludalf) soil in late May (1978) and late April (1979). Preemergence herbicides including metolachlor [2‐chloro‐N‐(2‐ethyl‐6‐methylphenyl)‐N‐(2‐methoxy‐ l‐methylethyl) acetamide], pendimethalin [Nl‐ethylpropyl)‐3,4 dimethyl‐2,6‐dinitrobenzeneamine], and nitrofen [2,4‐dichlorophenyl p‐nitrophenyl ether] were applied alone and in various combinations to determine effectiveness in reducing weed competition. Forage yields of turnips and rape 100 days after seeding were generally not affected by the herbicide treatments. However, associated weed yields in both turnips and rape were significantly reduced with all herbicide treatments applied. Pendimethalin applied at 2.2 kg ha−1was phytotoxic to both turnips and rape reducing yields 67 and 17%, respectively, as compared to the untreated control. Forage dry matter yields of turnips and rape with adequate weed control were as high as 5068 and 6711 kg ha−1, respectively. In 1978, turnip whole plant crude protein (CP) concentration was highest (227 g kg−1) where pendimethalin was applied at 2.2 kg ha−1. The 2nd year CP levels for turnip leaves were similar for all herbicide treatments, but root levels were highest where pendimethalin at 2.2 kg ha−1was used. The high rate of pendimethalin severely inhibited root development resulting in smaller roots with higher CP. Rape CP levels in 1978 were generally highest where herbicides were used, but in 1979, rape CP was not affected by herbicide application. Averaged across herbicide treatments, turnip (whole plant) and rape CP levels averaged 121 and 106 g kg−1, respectively. In vitro dry matter disappearance for turnips and rape were similar averaging 898 g kg−1and were not affected by herbicide application. Results indicate that appropriate preemergence herbicides will reduce early season weed competition and will enhance early production of turnips and rape. The high production potential and the high dry matter digestibility 100 days after seeding warrant consideration of turnips and rape for use as supplemental summer forage crops.