By ALICIA BOOR
Cottonwood Extension District
Fall is a wonderful season for agriculture and a time to, hopefully, reap the bounty of the year’s production. If producers have access to crop residue fields following grain harvest, then preparing to turn cattle out to graze such acres is one task that deserves to be near the top of the priority list. Grain sorghum represents an excellent opportunity to extend the grazing season. However, there are several important management considerations that need to be kept in mind with grazing sorghum residue.
Understandably, the absence of fences and water sources is one of the biggest limitations for residue grazing. Many acres are grown in rain-fed systems in which sourcing water from an irrigation well is not possible. Likewise, hauling water long distances is usually not practical. However, if water can be supplied and temporary fences put up, then grazing sorghum residues is usually economical. A single strand of electric fence can work quite well for keeping cows contained, while 2 or more strands may be needed for small calves.
Unlike actively growing forages, both the quantity and quality of residue available for grazing will be highest immediately following harvest and will steadily decline over time due to weathering, trampling, and consumption by cattle. Regarding forage quantity, field data indicate about 2.5 to 3.0 tons of residue per acre on average is produced following harvest. If accounting for 50% of the total residue on the field as available for grazing and of that amount 50% was utilized, then one acre would supply enough forage for one gestating cow for approximately 50 to 60 days.
High amounts of grain on the surface of the field pose a risk of acidosis, particularly for animals with experience grazing residue fields. Cattle selectively graze and will seek out and consume any grain present first. The degree to which grain in the field poses a risk is dependent on many factors, but levels greater than 10 to 15 bushels per acre generally require additional caution. Avoiding turning hungry animals directly on to fields by filling them up on forage, strip grazing to reduce the amount grain available per animal, supplementing low amounts of grain initially before turning them out, and feeding sodium bicarbonate as a rumen buffer are all strategies that may be used to decrease risk of acidosis.
Nitrate and prussic acid toxicity are both conditions that increase in prevalence due to plant stress and cause asphyxiation in the animal, yet are distinctly independent from each other. Nitrates accumulate in the base of the stalk or stem, and while issues associated with nitrate toxicity are generally less common in grazing situations as compared to when cattle are fed a diet, for this reason it is critical to manage stocking rate so that cattle are not forced to consume the lower 1/3rd of the stalk or stem. Prussic acid can occur in plant species within the broader sorghum family, and primarily accumulates in young, rapidly growing plant tissue. Therefore, avoiding grazing sorghum residue around a frost event and for the first 7 days following a killing freeze is a sound practice. Just as with harvested forages, representative samples of standing forage can be collected for analysis of nitrate and prussic acid to determine risk levels and guide management decisions.
Alicia Boor is an Agriculture and Natural Resources agent in the Cottonwood District (which includes Barton and Ellis counties) for K-State Research and Extension. You can contact her by e-mail at [email protected] or calling 620-793-1910.