Abstract
We conducted this study to evaluate the effects of post-harvest tillage on jointed goatgrass (JGG) emergence and interference in winter wheat over a 5-year period in a conventional dryland wheat-fallow crop rotation. The study consisted of two trials located at the Columbia Basin Agricultural Research Center in Moro, Oregon. The first trial was established in September of 1998 and the second in September of 1999. Both trials were established on winter cereal stubble on adjacent sites with no previous history of JGG infestation. Main plots consisted of six different post-harvest tillage timings on wheat stubble. Treatments consisted of 1) early post-harvest disking, 2) disking in late fall, 3) disking in early spring, 4) disking in late spring, 5) disking at all tillage timings, or 6) no tillage. Main plots were split into two sub-plots consisting of a low or high JGG density achieved by seeding JGG spikelets into standing cereal stubble at the start of each trial. Post-harvest disking had little effect on percent crop residue cover. As expected, the high JGG density sub-treatments had greater JGG densities, spike counts, and biomass each year than did the low JGG density sub-treatments in both trials. The high JGG density sub-treatment reduced wheat yield and JGG dockage in all years of both trials. Post-harvest tillage had little effect on JGG on the first trial site, but in the second trial site in the presence of high JGG populations, there was a significant reduction in JGG density, spike counts, and biomass in wheat when post-harvest tillage was done after wheat harvest in the fall. Post-harvest tillage did not influence wheat yield in either trial, but JGG dockage was reduced by tillage soon after wheat harvest, or later in the fall. Since reductions in JGG from fall disking were not observed in both trials, this leads to a general conclusion that post-harvest tillage effects on JGG populations can be beneficial but inconsistent between years. The yearly variation in environmental factors, especially growing season precipitation, had a more significant influence on JGG populations than did post-harvest tillage in these studies.
Key words: Jointed goatgrass, residue management, tillage, winter wheat, yield.
Introduction
Jointed goatgrass (JGG) continues to be a serious weed problem in winter wheat in the western United States. It does not only cause yield losses by competing with the crop for soil moisture, but JGG spikelets can contaminate the harvested grain, causing dockage losses at the grain elevator and further financial losses to the grower. Due to the genetic similarity between JGG and wheat, selective chemical control is difficult to acheive in the growing wheat crop. Rotation of winter wheat with broadleaf crops can facilitate long-term JGG control because selective grass herbicides can be utilized. However, relying on chemical control alone is not a good long-term solution because weed resistance to herbicides can become a problem. Another JGG management approach is to grow spring seeded crops in rotations with winter wheat. Spring seeded broadleaf or cereal crops can help reduce JGG problems because JGG typically germinates in the fall and can be controlled prior to planting spring seeded crops. However, in areas with limited rainfall, annual or spring cropping is often not considered to be a profitable alternative. In addition, recent research has shown that JGG can complete its life cycle and produce seed in spring cereal crops. In a winter wheat-fallow rotation, control of JGG during the fallow period is essential to help minimize increases in JGG infestations. The study described in this report was designed to determine if a shallow tillage after wheat harvest would result in reductions of JGG during the following wheat crop.
Methods
A study was conducted in dryland wheat at the Columbia Basin Agricultural Research Center, near Moro, Oregon. The study consisted of two trials; the first was established in September 1998, and the second in September 1999 in close proximity to the first trial. Both trials were arranged in a randomized complete block, split-plot design with five replications. Main treatments consisted of six post-harvest stubble tillage timings that included: 1) post-harvest disking soon after wheat harvest, 2) disking of wheat stubble in late fall, 3) disking in early spring, 4) disking in late spring, 5) disking at all tillage timings, and 6) no tillage. Tillage operations consisted of disking standing grain stubble twice with a John Deere 620 tandem disk with 20-inch discs at 7.5-inch spacing. Main plots were 40 by 50 ft in size. Sub-plots consisted of a target JGG density of 5 (low density) or 75 (high density) JGG joints per m2 seeded into standing cereal stubble using a drop-type seed spreader on September 4, 1998 in the first trial and on September 13, 1999 in the second trial. Sub-plots were 20 by 50 ft in size.
After the last post-harvest stubble disking treatments were performed in the spring, operations to establish conventional summer fallow consisted of a glyphosate application to all treatments, chisel plowing, cultivation, and rodweeding. No glyphosate application was needed on wheat stubble in the fall on any treatment, in any year, due to dry autumn conditions that prevailed at the study sites. Percent residue cover on the first study was estimated on December 9, 1999. Percent residue cover was not estimated further in either study. Jointed goatgrass plant counts were taken in wheat stubble each spring before final fallow tillage operations were performed by counting JGG plants per m2 in three locations along a diagonal transect in each plot. Wheat and JGG plant counts, spike counts and biomass per m2 estimates were obtained prior to harvest during each year that plots were planted to winter wheat. Winter wheat was harvested each crop year using a small plot combine. Jointed goatgrass dockages were estimated by taking a 750-ml sample of harvested grain from each plot, weighing the sample, separating goatgrass spikelets from the wheat, and weighing them. Percent dockage was estimated using the following formula: (weight of JGG spikelets ÷ weight of wheat) x 100 = percent dockage.
Results and Discussion
Post-harvest tillage (light disking of wheat stubble) had a relatively minor effect on percent crop residue cover (data not shown). As expected, the high JGG density sub-treatments had greater JGG densities, spike counts, and biomass each year than did the low JGG density sub-treatments in both trial sites (Tables 1, 2, 3). The high JGG density also had reduced wheat yield and increased JGG dockage in both trial sites (Table 4).
Post-harvest tillage treatments had no significant influence on JGG at the first trial site. However, in the second study site, JGG plant density, spike counts, and biomass were significantly reduced in both years of wheat crop when post-harvest tillage was performed after wheat harvest in August or in October (Tables 1, 2, 3). No differences in crop yield were observed from the fall tillage treatments. However, in the second trial site, at the high JGG density, JGG dockage was reduced when post-harvest tillage was performed after wheat harvest in August or in October (Table 4).
Since the fall tillage effects on JGG populations were not consistently observed in both trials, it leads to a general conclusion that effects of post-harvest tillage on JGG populations and subsequent wheat yield and dockage can be generally beneficial but not consistently so. The yearly variation in environmental factors, especially growing season precipitation, had a more significant influence on JGG populations than did post-harvest tillage in these studies. In addition, since post-harvest tillage did not have a negative impact on the JGG problem in these trials, it may have value as a residue management tool in certain production systems.
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