Grazing Management Of Summer Annual Grasses

DR. DIRK PHILIPP

FAYETTEVILLE, ARK.
   Summer annual grasses are considered a good choice for providing forage during the hot Arkansas summer months. Popular in our state are pearl millet (Pennisetum americanum) and sorghum-sudangrass (Sorghum bicolor) varieties. Both forages have outstanding yield potentials and deliver on them if correct planting strategies, fertilization and grazing management techniques are being followed. Summer annual grasses especially make sense on farms where fescue is the prevailing forage. Being C-4 plants, pearl millet and sorghumsudangrass have a high water use efficiency and respond well to nitrogen fertilizer.
   Pearl millet is an excellent forage that can also be safely used for horses. This grass is relatively tolerant of acidic soils, high humidity and diseases – all typical for Arkansas growing conditions. It does not present a danger from prussic acid as do the sorghum species. To achieve good productive stands, pearl millet should be established into a prepared seedbed to ensure good emergence and little competition from undesirable plants. As always, check regularly on soil pH levels and required fertilizer based on tests.
   Grazing of pearl millet can be initiated when the canopy reaches 1 to 1.5 feet in height. It should be noted that pearl millet is sensitive to close grazing. A stubble of 6 inches should be left in any case, primarily because otherwise the leaf area from which growth is reinitiated would be too small to generate sufficient energy via photosynthesis. If forage stands have been grazed unevenly, regrowth can be improved by clipping pastures to an even height, although in most cases this is probably not necessary. Stock pastures again when canopy height reaches about 1 to 1.5 feet or slightly higher. In our experience gained from a pilot study conducted at the Fayetteville location, summer annual grasses regrow very quickly, so it is likely that producers get a few grazing cycles out of the growing season.
   The number one problem of pearl millet is the possible high nitrate content. We have seen several instances in which high nitrate concentrations were evident in hayed pearl millet from fields that have been fertilized with poultry litter over the years. The lower stems usually accumulate nitrate at a higher rate than the leaves. Under stress, nitrate can build up in leaf and stem tissue. Stress factors include drought, freezing temperatures and over-fertilization. High N rates, especially on drought-prone sites can result in excessive nitrate accumulation.
   Sorghum-sudangrass has an even higher yield potential than pearl millet. The establishment is similar to other warm-season annuals – it is important to plant seeds into a prepared and settled seedbed and manage it appropriately to achieve high yields. Apply fertilizer according to soil test results, as always recommended. Seeding rates can be adjusted to achieve slightly higher leaf percentages as sorghum-sudangrass develops thicker stems than pearl millet.
   Minimum grazing height for sorghum-sudangrass is 24 inches. Similar to pearl millet, leave a stubble height of 6 inches to ensure good regrowth and do not restock pastures again until canopy heights are 18 to 24 inches.
   The apparent problem with sorghum-sudangrasses is prussic acid. This compound accumulates in the leaves, more so in short plants and during vegetative growth. Prussic acid accumulation is a response to stress by the plant, such as drought and freezing temperatures. Be especially careful with the last round of grazing in fall when plants are already short from repeated grazing and the first nights of frost occur. Defer grazing for at least a week to let the prussic acid concentration decline below nontoxic levels. In general, do not let horses graze sorghum-sudangrass because they can contract cystitis syndrome. Unlike nitrate, prussic acid concentrations decline during storage. If in doubt, test harvested forage for nitrate and prussic acid at a reputable laboratory. Monitor nitrate concentrations and prussic acid concentrations with the help of your county Extension office if in doubt to bring livestock safely through the summer months. ∆
   DR. DIRK PHILIPP: Assistant Professor, Department of Animal Science, University of Arkansas