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PNW CONSERVATION TILLAGE HANDBOOK SERIES
Chapter 5 - Weed Control, No. 9, Winter 1988


Jointed Goatgrass Seed Longevity

Roger Veseth

Jointed goatgrass is a winter-annual grass weed that is an increasing problem in the winter wheat-producing areas of the Pacific Northwest. It is so closely related to winter wheat that no available herbicide will selectively control it in wheat without injuring the crop. Selective control in barley also is not possible. Thus, cultural practices are the only control methods available to wheat growers. Understanding jointed goatgrass seed longevity in the soil is essential in developing effective cultural controls.

Control of jointed goatgrass has become an important focus in the weed control research under STEEP and related conservation farming research projects. Three weed scientists who have field research underway on jointed goatgrass seed longevity in the Inland Northwest are Dorm Thin with the University of Idaho in Moscow and Alex Ogg and Frank Young with the USDA-Agricultural Research Service at Washington State University in Pullman.

Seed Production

The researchers point out that, besides the fact that it is closely related to wheat, several other factors make jointed goatgrass particularly difficult to control. The weed is a prolific seed producer. A single plant may produce as many as 100 spikes (seed heads) and 3,000 seeds. Jointed goatgrass matures earlier than winter wheat, and the seed segments (spikelets) shatter easily from the spike, ensuring a seed supply in the soil.

The segments also are difficult to remove from the harvested wheat. Wheat seed and harvest equipment contaminated with jointed goatgrass segments are believed to be partially responsible for the rapid spread of this weed.

The researchers also point out that even though jointed goatgrass is a winter annual, some early spring-germinated plants may produce seed. If sufficient freezing weather occurs after spring-germinated jointed goatgrass emerges, the plants may still vernalize and produce viable seed. If emergence occurs before seeding of the spring crop, the plants can be controlled with tillage or herbicides. If emergence occurs after early seeding of a spring crop, however, there is no control unless the crop is a non-cereal and grass herbicide options are then available.

Seed Survival

Sufficient time must be allowed between winter cereals in the crop rotation to reduce the jointed goatgrass seed supply in the soil. Since the life cycle of jointed goatgrass is similar to winter wheat and barley, the crop rotation must include only spring crops or fall non-cereal crops with grass weed herbicide options. Growing at least three successive spring crops has been suggested as one of the best control methods for the weed.

Spring crops allow the removal of fall and early-spring germinating jointed goatgrass by tillage or herbicides. Noncereal crops provide the advantage of being able to more easily identify the jointed goatgrass population in crop.

Research has shown that jointed goatgrass seed will seldom emerge from soil depths greater than about 2.5 inches. Moldboard plowing may bury a portion of the seeds below their potential emergence zone. Not all of the seeds will be deeply buried in the plowing operation, however. Also, seeds can be brought back to or near the soil surface by secondary tillage operations and by plowing in future years. Consequently, tillage alone is not a control option.

Research efforts are underway to further refine the crop rotation and herbicide/tillage control methods. One focus of the research has been in determining the seed dormancy and longevity in the soil under contrasting precipitation zones, tillage systems and other management options.

Research on Seed Longevity

A 5-year study by Thin and Young was initiated in August 1985 to characterize the dormancy and longevity of jointed goatgrass seed buried at varying depths in the soil. Two sites with contrasting precipitation and crop rotations were selected. A low-precipitation site (l O-inch annual precipitation) under a wheat-fallow rotation was established near Lind, WA. The annual cropping, high-precipitation (23-inch annual precipitation) site was Moscow, ID.

In August 1985, nylon mesh packets containing 50 jointed goatgrass segments were buried 2,4, 6, 8 and 12 inches deep at each location to simulate burial under various tillage practices. Packets were also placed on the soil surface. Fifteen sample dates are scheduled over the 5-year period with four replicate samples for each sample time. Seed packets are removed in September, November, March and June of each year to determine the seed germination percentage and total seed viability. Seeds that do not germinate under laboratory tests are sent to the Idaho State Seed Testing Lab to determine if the seeds are dormant or are no longer able to germinate. The percentages of germination plus dormant seed are added together to determine total seed viability. The original seed viability was near 100 percent. Field viability is calculated as a percentage of the viability of identical seed stored in the lab.

The viability of jointed goatgrass seed declined more rapidly and to a greater extent at the Moscow site than at Lind by November 1987. The researchers attributed the more rapid decrease in seed viability at Moscow to at least two factors. First, the higher soil water content increased in-field germination. Second, decay of segments and seeds is faster in soils that are wetter and contain a higher organic matter content because of the more intensive soil microbial activity.

When the seed viability was averaged over all depths, the difference between the Moscow and Lind sites are particularly evident. After 25 months, only about 1 percent of the jointed goatgrass seed remains viable at Moscow, compared to 14 percent at Lind.

Although there were some differences in viability for different depths of burial at earlier sample times, seed viability was similar at all depths in the November 1987 sampling at each location. This indicates that burial depth may not have a major impact on seed longevity in the soil. However, it is possible that yearly soil disturbance under actual field conditions may result in a difference in longevity compared to seed in the packets, which remained undisturbed at a constant depth.

 

Seed dormancy is another factor that can influence the length of time that seed remains viable in the soil. This biological survival mechanism helps prevent all of the available seed from germinating in one season under favorable conditions, thus ensuring some future seed supply. Thin and Young found a similar decline in percent dormancy averaged over the burial depth at the Moscow and Lind sites. Some cycling in and out of dormancy occurred seasonally with changes in temperature, soil water content and other factors. By November 1987, percent dormancy had dropped to zero at both locations. Some dormancy may have occurred again over winter after sampling.

From data collected through the November 1987 sampling on the 5-year study, Thin and Young developed a preliminary predictive equation for jointed goatgrass viability in the soil over time, For Moscow conditions, an estimated 1,150 days or about 3 years may be required to reduce the percent viability to less than 0.1 percent. This verifies current recommendations that 3 years of spring crop are needed to effectively reduce seed reserves in the soil.

At Lind, however, the equation predicts at least 6 years before percent viability declines to less than 0,1 percent. The rate of seed germination and decomposition in the field are less than at Moscow because Lind receives less than half the annual precipitation. Consequently, control of jointed goatgrass with crop rotation may take twice as long at Lind than at Moscow.

Recrop Field Study

The research findings by Thin and Young, which indicates that viable jointed goatgrass seed can persist in the soil for 3 to 6 years depending upon precipitation, is supported by preliminary results of an ongoing field experiment near LaCrosse, WA. Average annual precipitation at the LaCrosse site is about 15 inches. The field study of jointed goatgrass seed longevity was initiated by Ogg and Young in spring 1985. Seed longevity is being monitored under continuous recrop spring barley and two tillage systems: (1) fall moldboard plowing and conventional seedbed preparation in the spring and (2) no-till using the USDA III research no-till drill with large off-set double disk openers (plot-sized Yielder Drill). Seed production is being prevented by eliminating jointed goatgrass plants through tillage or herbicides before planting and hand pulling any surviving plants in the barley crops.

The 1984 crop preceding the first spring barley planting was winter wheat planted on summer fallow under conventional tillage in a wheat-fallow rotation. There was a dense population of jointed goatgrass in the winter wheat. The moldboard plow was not used regularly in the field before the experiment.

To estimate the number of jointed goatgrass seeds remaining in the soil over time, the researchers took 12 soil cores, 1 inch in diameter, in each plot treatment and sieved for seed in 3-inch increments to 12 inches. Viable jointed goatgrass seeds were still found in the moldboard plow plots in fall 1986 after the second spring barley crop but not in the following February sampling. In the no-till plots, the reverse was true. The researchers point out that because of the small size and number of soil samples, low seed populations are probably not accurately determined.

Plot evaluations for live jointed goatgrass plants were made in March 1987 before replanting the third year of spring barley. Although jointed goatgrass plants were found on the no-till plots, none were found in the moldboard plow plots. The researchers point out that fall moldboard plowing probably killed or prevented emergence of fall-germinating jointed goatgrass. No herbicides were used in the fall on the no-till plots. A preplant herbicide was used to control the jointed goatgrass and other weeds before no-till planting. May population counts in crop revealed that only a few plants survived the herbicide or emerged after herbicide application in the no-till plots. No plants were found on the moldboard plow plots in the May plant count.

From the preliminary results of this field study, Ogg and Young conclude that jointed goatgrass seed can persist in the soil at least through 2 years of spring cropping at the LaCrosse location. If winter wheat were planted in the fall after the second year of spring crops, jointed goatgrass would again have become a problem, particularly under no-till. Consequently, spring crops would have to be grown for at least 3 years, and possibly longer, to effectively reduce the jointed goatgrass seed reserve in the soil.

Although jointed goatgrass plants can be largely eliminated by a preplant herbicide application before planting spring crops under no-till, it appears that the reduction in soil seed reserves may take more time under no-till than moldboard plowing. The impact of reduced tillage systems, which are intermediate between moldboard plowing and no-till, on jointed goatgrass seed reduction in the soil is unknown. The researchers estimate that the results would be similar to the moldboard plowing treatment used in this experiment.

Ogg and Young stress that jointed goatgrass plant counts in spring 1988 will be needed to more accurately assess the effects of rotation and tillage on control of jointed goatgrass in this study. Soil sampling for jointed goatgrass seeds was not continued in fall 1987 because the researchers felt it was not accurate in measuring low seed populations.

Conclusions

The researchers stress that the results presented here are from ongoing studies and final conclusions may be different. To date, it appears that at least 3 years of preventing seed production through crop rotation, tillage and/or herbicides will be required to effectively reduce jointed goatgrass seed carryover in the soil in areas receiving about 23 inches of annual precipitation. In areas receiving about 10 inches annual precipitation, more than 3 years, and potentially up to 6 years of preventing seed production may be required.

Under LaCrosse conditions, the reduction of jointed goatgrass seed carryover under continuous spring barley was greater with moldboard plow-conventional tillage than with no-till. The impact of an intermediate tillage system, such as minimum or mulch tillage is unknown, The impact of tillage in areas receiving higher precipitation amounts also is uncertain.

The researchers plan to continue their efforts in evaluating seed longevity in the soil and the impacts of potential management options on control of this serious weed problem in the Northwest.

     
 

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