2.2. Study set-up and treaments
The study was initiated in 1998 to determine the effects of glypho- sate rate and use patterns on changes in weed species composition and population dynamics in GR corn and GR Soybean production under dryland no-till practices in northwestern Kansas.
Prior to this study, the plot area was managed as annual sorghum-forage production system where conventional tillage and herbicides were used to manage during fallow period before forage sorghum planting.
The exper iment was a two-factorial split set in a randomized complete block design with four replications.
Main plots were continuous corn(cc) and corn and soybean in rotation with both phases of the corn- soybean rotations(corn-soybean(CS) and soybean-corn(SC) grown each year.
These crop systems were evaluated because corn and soy- bean are the major GR crops grown in the USA and are grown either as a monoculture or as rotation systems(corn-soybean system).
Sub- plots were four herbicide treatments consisting of glyphosate only her bicide management and a combination of glyphosate and other herbi cide chemistry. Brief descriptions of the treatments are:
(1) Glyphosate at 0.42 kg a e ha ammonium sulfate(AMS) at 2% wiv applied in-crop twice each year(TRT 1);
(2) Glyphosate at 0.84 kg ha 1 AMS at 2% w/v applied twice in crop each year(TRT 2);
(3) Glyphosate at 0.84 kg ha 1 AMS at 2% w/v applied twice in crop every other year alternated with non-glyphosate herbicide treatment in both crops(TRT 3); and
(4) Non-glyphosate herbicide treatment for each crop in all years(TRT 4)
Glyphosate in its isopropylamine salt form was used each of the first 4-yr of the study and the K-salt form of glyphosate was used in each of the later years.
The non-glyphosate treatments for corn consisted ofrec- ommended use rates of acetolachlor plus isoxaflutole and atrazine ap- plied pre-emergence to crop and weeds followed in most years by fluroxypyr or mesotrione plus nicosulfuron with appropriate adjuvants applied post-emergence.
The non-glyphosate treatments for soybean during the first three years of the study consisted of recommended use rates of clomazone plus alachlor applied pre-emergence followed by clethodim with crop oil concentrate applied post-emergence.
During the next 11-yr, the non-glyphosate soybean treatment was changed to sulfentrazone plus pendimethalin applied pre-mergence followed by imazamox plus encapsulated alachlor and quizalofop with appropriate adjuvants applied post-emergence.
Details on the type and amounts of herbicides applied for each year of the study is provided in the Supple- mentary table.