As jointed goatgrass begins to head, it is easy to distinguish from wheat, downy brome, or volunteer rye. The head of jointed goatgrass, also called the spike, is a long narrow cylinder. This spike is made up of a number of spikelets, also called joints (Figure 4).
Each spikelet (joint) is about one-half inch long and contains from one to three seeds. At maturity, which normally occurs 2-3 weeks before wheat is harvested, part of the jointed goatgrass spike will often break off and the spikelets will fall to the ground. Spikelets that remain attached to the jointed goatgrass stem, and are harvested with the wheat, will look like short pieces of straw mixed with wheat in the combine bin. Wheat straw is hollow, however, while jointed goatgrass spikelets (joints) are closed on both ends.
Occasionally, wheat and jointed goatgrass will cross and produce a hybrid plant. When mature, this hybrid will share the appearance of both the parent plants (Figure 5). Seeds from these hybrid plants are almost always sterile.
A key to managing jointed goatgrass is understanding the survival of jointed goatgrass seeds in the soil. After one year, 80 percent or more of jointed goatgrass seeds may still be viable. After two years, 20 percent may be viable, and after 3 years, 5 percent may be viable. This is in sharp contrast to downy brome, which may have less than 20 percent seed viability after 1 year.
Seed survival is a major reason that jointed goatgrass is difficult to control in wheat. Because 20 percent or more of jointed goatgrass seed can survive 2 years or more, even after a year of fallow there may be a significant amount of viable jointed goatgrass seed in the soil. This reservoir of viable weed seed is called the soil seed bank. Some of this seed then germinates and establishes new jointed goatgrass plants in the next wheat crop. These new plants produce even more seed, further infesting future wheat crops.
As part of this multi-year seed survival, jointed goatgrass has staggered seed dormancy. As discussed earlier, jointed goatgrass seeds are contained in spikelets (joints). Each spikelet contains from one to three seeds. Even under optimum conditions, not all seeds within a spikelet may germinate at one time.
Jointed goatgrass can germinate on the soil surface, or it can emerge from as deep as 4 inches in the soil. In comparison, downy brome will rarely emerge from depths greater than 2.5 inches. Shallow tillage to incorporate jointed goatgrass spikelets into the soil may stimulate germination, but the results vary. Again, this is in contrast to downy brome where even a small amount of seed-soil contact will greatly increase germination.
Typically, jointed goatgrass emerges from September through early November, goes dormant over the winter, and heads out to produce seed in spring. A secondary flush often emerges in early spring. Jointed goatgrass plants that emerge in early spring can produce heads that may contain viable seed, even if the time for vernalization was very short. This makes late spring-planted crops such as corn, sunflower, or proso millet more effective in a rotation to control jointed goatgrass than early-spring planted crops such as oats or spring wheat. Jointed goatgrass generally heads after downy brome, but before wheat.
One other difference between jointed goatgrass and wheat is that anthesis (the period during which flowers in the emerged head are pollinated and become viable seeds) is much longer for jointed goatgrass than for winter wheat. Anthesis in both plants is identifiable when anthers (small, yellow cylinders of pollen) are visible on the heads. A long anthesis gives jointed goatgrass a better opportunity for favorable weather during anthesis and helps ensure seed production.