Weed Management in Summer Annual Grass Crops

Quintin Johnson, Extension Associate, Weed Science; quintin@udel.edu and Richard Taylor, Extension Agronomist; rtaylor@udel.edu

The use of summer annual grasses for supplemental forage is gaining popularity. The advantage of the summer annual grasses is that they produce abundant forage during July and August, the period when cool-season grasses decline in productivity called the “summer slump”. When planted in late May or June, these grasses establish quickly, grow rapidly, have moderate to high drought tolerance, and can typically be grazed within 5 to 8 weeks of planting.

There are both common and species dependent disadvantages to the summer annual grasses. Common to all species is the fact that they usually are killed by the fall frost and will not survive the first fall freeze. The average date of the first fall frost is mid- to late-October for much of the region. By this time, cool-season grass production has increased again with the onset on cooler temperatures and fall precipitation levels. So although the death of the warm-season summer annuals can be considered a disadvantage, it is not a serious problem since pasture productivity usually has recovered by the time these summer grasses die.

Another common disadvantage is the extra production costs entailed with using annual species. The seed must be purchased each year, equipment must be rented or kept on hand to plant the annuals, and some form of annual tillage and/or vegetation control must occur. Seed costs, however, are relative and a review of many warm-season grass species as of the date of this article shows that seed costs range from $0.50 to $1.25 per pound and suggested seeding rates generally between 20 and 50 lb/acre. At a 30 lb/acre seeding rate and $0.75 per pound seed costs, annual seed cost would be under $25/acre contrasted with a novel-endophyte tall fescue which would cost about $150/acre but would be a long-term investment. Another mitigating factor to the cost issue is that many growers own or have access to brillion seeders that are old enough not to be considered a large capital expense. Annual tillage or seedbed preparation and the opportunity cost associated with the land area used for the annual crop will be the biggest expenses for summer annuals versus perennial cool-season grasses.

With certain precautions outlined below, cattle can graze or be fed forage sorghum, sudangrass, sorghum-sudan hybrids, foxtail millet, hybrid pearl millet, or crabgrass. Horses can graze hybrid pearl millet or crabgrass, and teff can be used for hay then grazed before frost.

Sorghum, sudangrass, and sorghum-sudangrass hybrids contain dhurrin which can break down and release prussic acid (HCN, cyanogenic compounds). Levels can be high enough in young, drought-stressed, wilted, injured, or frosted plants to cause cyanide poisoning. Millets do not contain dhurrin or prussic acid. The concentration of dhurrin varies by species with the most in sorghums and the least in sudangrass. Grazers should not graze these species if they are less than 18 inches tall and should delay grazing until young or re-growing plants are 24 to 30 inches tall. Do not graze drought-stressed plants until they recover after sufficient rainfall (usually 4 – 5 days) or until the plants fully recover from other stresses. Frosted plants should not be grazed until the leaves are dead and completely dried down, usually about a week to ten days following a killing frost. Cyanide dissipates from properly cured hay and properly ensiled forage, making them safe to feed.

All summer annual grasses can accumulate high levels of nitrates when fertilization is followed by stress (usually drought). The potential for nitrate poisoning can be reduced by moderating nitrogen fertilizer rates, allowing stressed plants to fully recover, testing for nitrate levels before grazing, and providing supplemental low-nitrate forage when moderate to high nitrate levels are suspected. Nitrate levels are not reduced in cured hay or green chopped forage, and are only partially reduced in ensiled forage. Another management option if the nitrate levels are not excessively high is to graze the tops of the forage and leave all the lower stubble. Nitrates are highest in the stem material and are highest closest to the soil surface. If harvesting fertilized drought-stress warm-season grasses for hay, avoid harvesting the lower stems. Millet accumulates as much or more nitrate than the sorghums. Horses have been observed grazing entire hybrid pearl millet plants to within an inch or two of the soil surface, so top grazing may not work in this situation.

Weed management begins with a clean seedbed. For conventionally tilled seedbeds, tillage should occur as close to planting as possible. In no-tillage seedings, follow the label concerning how much time to allow between the use of a nonselective herbicide such as glyphosate and seeding. When possible, choose fields with few to no perennial weeds, and avoid fields with a high number of grassy weeds.

As with any cool-season forage crop, successful establishment begins with properly amended soils (pH and fertility), proper seeding rate and depth, and appropriate seeding equipment. A critical component for establishment is waiting until the soil temperature warms enough to ensure rapid seedling emergence from the seedbed. When moisture is adequate, summer annual forage grasses emerge quickly, grow fast, and compete well with weeds. There often is no need for additional weed control. However, chemical weed control can be warranted when establishment is slow, weed populations are high, potentially toxic weeds are present, or high quality (weed-free) hay or forage is desired.

Unfortunately, herbicide options for summer annual grasses are limited. Some plant growth regulator (PGR) herbicides are labeled for use with these annual forages. Another limitation is that PGR herbicides are not recommended during the hot summer months due to their potential to injure sensitive plants with physical or vapor drift. Non-PGR herbicide options are listed in the table below. Pay particular attention to rotational crop restrictions for the preemergence herbicides (refer to label for crops not listed). Atrazine is typically not recommended due to its long rotation to other forage crops, and should only be used if corn or sorghum will be planted in the following spring. Check herbicide labels for weeds controlled. The postemergence herbicides in this table typically control only small annual broadleaf weeds. The herbicide labels will list the maximum weed size or growth stage at which the herbicide will be effective. Often the best herbicide is that weed-free seedbed and a rapid vigorous seedling. 

Table 1. Herbicide options for weed control in summer annual grasses

^a Check label for adjuvant recommendations.

^bNo adjuvant is recommended.

^cRequires the use of Concep-treated seed.

^dCoarase soils 1.0 to 1.33 pt/acre, medium soils 1.33 to 1.5 pt/acre, and fined soils 1.33 to 1.67 pt/acre.

^eNext year if applied before June 10 or the second year after application if applied after June 10.