Effects of Weed Density on Herbicide Absorption and Bioactivity

The effects of weed density on the phytotoxicity of atrazine (2 - chloro -4- (ethylamino) -6 - (isopropylamino) -s - triazinel to white mustard (Brassica hirta Moench) and alachlor (2 - chloro - 2',6' - diethyl - N-(methoxymethyl)acetanilidel to foxtail millet (Setaria italica (L.) Beauv.J were determined in the greenhouse. The fresh weight of both plants was reduced by the herbicide treatments, but as the weed density (plants per pot) increased, the herbicide rate required to cause equivalent fresh weight reductions per pot also increased. White mustard and foxtail millet grown in pots absorbed less 1 4C -atrazine or 1 4 C-alachlor per plant from the soil at high weed density than low, regardless of herbicide rate. Foxtail millet seedlings grown in petri dishes absorbed less 4C-alachlor per plant from aqueous solution as the number of seedlings per petri dish increased. Data from field experiments in 1977 and 1979 indicated that more alachlor or metolachlor (2-chloro- N - (2 - ethyl - 6 - methylphenyl) - N - (2 - methoxy - 1 - methylethyl)acet - amidel was required for equal control of a high than low density of forage sorghum (Sorghum bicolor (L.) Moench 'Rox Orange') inter- seeded in maize (Zea mays L.). Rox Orange sorghum was used to simulate shattercane (Sorghum bicolor (L.) Moenchi, a problem weed in Nebraska. At the higher Rox Orange sorghum seeding rate used, both alachlor and metolachlor treatments decreased Rox Orange sorghum plant populations but not yield. Additional index words. Rox Orange, herbicide competition, alachlor, metolachlor, EPTC.