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Biology of Jointed Goatgrass

Don W. Morishita
Assistant Professor, Weed Science
University of Idaho
1991 Goatgrass Symposium

Jointed goatgrass (Aegilops cylindrica) is a winter annual grass weed found in many agricultural areas of Pacific Northwest. It is genetically related to and very similar in appearance to winter wheat. In the seedling stage the coleoptile and first leaf are brownish-green. The leaf blades are sparsely hairy and the leaf sheath has a hairy margin. The ligule is membranous and the auricles are short and hairy. As the plant matures it can produce up to 135 tillers. It generally grows 15 to 30 inches tall. The seedhead is 2 to 4 inches long with 5 to 10 spikelets or joints per heads. Each spikelet can have 2 to 5 flowers with each spikelet 0.3 to 0.5 inches long with 1 to 2 and sometimes 3 seed per spikelet.

Jointed goatgrass is generally found in areas of 10 to 20 inches of annual rainfall and in elevations of 800 to 4,000 feet. It is most commonly found in winter wheat fields or other small grain cereal fields, fence rows, roadsides, waste areas, etc. Jointed goatgrass seeds usually germinate from mid-September to November. They also can germinate in late spring and still mature, if temperatures are low enough (37° F) to vernalize the imbibed seed or plants.


Jointed goatgrass and wheat are both C-3 plants. That is , they both utilize carbon dioxide through the same biochemical pathways. Jointed goatgrass and winter wheat have similar temperature optimums, maximum photosynthetic rates, and growth rates as well as other similar physiological characteristics. Comparing the growth rates as well as other similar physiological characteristics. Comparing the patterns of soil moisture use, jointed goatgrass will extract water from depths of 38 inches compared to winter wheat which can extract water to depths of 50 inches. The water use efficiency of winter wheat is about twice that of jointed goatgrass. Root and shoot growth studies have shown that the growth rates of jointed goatgrass are similar to wheat. Lateral root growth, however, is less than wheat as is the root length density of jointed goatgrass to a depth of 43 inches. The shoot to root ratio of jointed goatgrass is less than wheat and the leaf area index is about two times less than wheat.


As far as the reproduction of jointed goatgrass, because it is an annual it reproduces only by seed. It must be vernalized in order to flower and vernalization can occur with seeds, as well as seedling plants. Studies have shown that increasing the vernalization period progressively decreases the number of days the plant takes to reach maturity.


In terms of producing seed, a jointed goatgrass plant can produce up to 100 spikes, 1,500 spikelets or joints and up to 3,000 seeds in a single plant. Research has shown that 75 to 80 percent of the spikelets contain 2 seeds and about 20 to 25 percent of the spikelets contain 1 seed. A very small percentage of spikelets contain 3 seeds. Studies conducted in Colorado and Kansas to measure the distribution of jointed goatgrass spikelets in the soil profile in fields where winter wheat was grown in a summer fallow rotation, 89 percent of the jointed goatgrass spikelets were found in the top 1.4 inches of soil.

Jointed goatgrass has been reported to germinate and emerge from depths as great as 5 inches in the soil. Other research has shown that jointed goatgrass emergence is limited to just 1 inch in sand, 2 inches in silt loam soils, 3 inches in loamy sands and has been documented to germinate from the depth of 4 inches in Montana soils. Emergence is reportedly favored in compacted soils such as combine wheel tracks.

Seed viability and dormancy studies have been conducted since 1985 near Moscow, Idaho and Lind, Washington. In these studies burial depth apparently has little influence on seed dormancy or longevity. Soil moisture plays a more important role in jointed goatgrass seed viability and dormancy. At the higher precipitation zone in Moscow, the viability of jointed goatgrass seed declined more rapidly and to a greater extent that it did at the low moisture site near Lind, Washington. The more rapid decrease in seed viability at Moscow was attributed to at least two factors. First, a higher soil water content increased in-field germination. Second, decay of seed segments is faster in soils that are wetter and have a more intensive soil microbial activity.


After 3 years of burial, no viable seed remained at the Moscow location and after 5 years about 0.1 percent of jointed goatgrass seed remained viable at the Lind, Washington location. Similar results were observed in the Colorado and Kansas studies where after 5 years only about 0.2 percent of the jointed goatgrass seeds survived in the top 2 inches of soil and only about 0.5 percent viable seeds remained at the 12 inch depth. In terms of seed dormancy the basal seeds of an individual spikelet have greater dormancy than the subterminal seeds. This indicates that jointed goatgrass seed exhibit a type 3 seed behavior, that is their persistence in the soil is intermediate. Jointed goatgrass seeds are polymorphic meaning that a portion of the seeds germinate early and the remainder of the seeds may persist for years.

The competitive effect of jointed goatgrass in winter wheat has been studied near Pendleton, Oregon. In a 3-year study jointed goatgrass densities of 5 to 8 plants per ft2 reduced winter wheat yields 25 to 32 percent. Replacement series experiments conducted near Pullman, Washington have shown that jointed goatgrass, winter wheat and downy brome compete for the same resources (nutrients, moisture and light).

Winter wheat and jointed goatgrass are more effective competitors than is downy brome and when comparing the relative competitiveness of the three species, wheat is more competitive than jointed goatgrass and both wheat and jointed goatgrass are more competitive than downy brome. Jointed goatgrass has a faster growth rate than downy brome. This faster growth rate plus the use of herbicides that are more effective for controlling downy brome than jointed goatgrass may result in a weed species shift from downy brome to jointed goatgrass in growers fields and may explain why jointed goatgrass is becoming more of a problem in the pacific Northwest. Additional jointed goatgrass competition studies conducted near Pullman have shown that under moisture stress of high temperatures jointed goatgrass is more competitive than wheat.