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Population dynamics of three winter annual grasses

W. L. Stump and P. Westra
1994 Res. Prog. Rep. West Soc. Weed Sci.

The winter annual grasses jointed goatgrass (AEGCY), downy brome (BROTE), and volunteer rye (SECCE) infest more than 1.2 million acres of Colorado winter wheat. These weeds cost Colorado wheat producers in excess of $20 million annually in lost production. Since there are no selective control measures for these grasses in winter wheat, rotations with alternative dryland crops are being implemented by some growers. The objective of this study was to determine the effects of various crop rotations on the population dynamics of these three weedy grasses.


In the fall of 1991, blocks of jointed goatgrass, downy brome, and volunteer rye were seeded at a rate of 600,000 seeds per acre for each species in a split block design on a dryland site near Platteville, Colorado. Superimposed over these blocks are four different crop rotation regimes utilizing winter wheat, proso millet, and sunflower in various combinations. The combinations allow for one, two and three years out of wheat production. All rotations were started with winter wheat to allow for establishment of the grasses. Population dynamics were measured by seedling emergence.


Volunteer rye established best, followed by jointed goatgrass and downy brome (Table 1, Spring 92). After one season following wheat, all grasses experienced an increase in emerged populations. Both the fall and spring cohorts were important contributing factors in the total emerged population. Since the fall of 1992 and the following spring was either a fallow period or prior to sowing of the summer crops, all the emerged populations were chemically controlled and no new seed had entered the system. In 1993, fall emergence of jointed goatgrass was still increasing while downy brome and volunteer rye had slightly decreased from the previous fall. This was probably due to dormancy mechanisms present in jointed goatgrass that buffer it better to changes in the system. The effects of the previous crop environment on grass emergence is shown in Table 2. Emergence for all grasses was greatest in the fallow plots. This was probably due to increased moisture availability. Proso millet had less of a stimulatory effect on emergence than sunflower. A possible reason for this is that proso millet typically depletes the upper soil moisture profile, decreasing grass emergence. The study will be monitored for three more years.


Table 1. Weed emergence over time.


Grass counts per m2
Spring 92 

Fall 92 

Spring 93 

Fall 93* 













*Average from all rotation treatments.


Table 2. Effects of cropping environment on winter annual grass emergence.


Fall emergence 1993 – counts per m2
FallowSunflowerProso millet 312287187 352219 531287217