Fall is a wonderful season for agriculture and a time to, hopefully, reap the bounty of the year’s production. Grain harvest, weaning calves and pregnancy checking are all in full swing; combine all that with fall sports activities, and there’s no shortage of things to do.
If cattle producers have access to crop residue fields following grain harvest, 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 or milo residue represents an excellent opportunity to extend the grazing season into the winter, thereby reducing the need to feed harvested forages. However, there are several important management considerations that need to be kept in mind with grazing sorghum residue.
Fence and water
Understandably, the absence of field fences and water sources is one of the biggest limitations for residue grazing. Most areas where grain sorghum is grown are rain-fed production systems in which sourcing water for livestock from an irrigation well is not possible. Likewise, hauling water long distances is usually not practical, either. However, if water can be supplied and temporary fences put up, grazing sorghum residues is usually economical. A single strand of electric fence can work quite well for keeping cows contained, while two or more strands may be needed for small calves, particularly near high-traffic areas.
Forage quantity and quality
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 of higher-quality and more palatable plant components. Data from Kansas State University suggests leaves of the sorghum plant can range from 6% to over 11% crude protein (CP) and 50% to 57% total digestible nutrients (TDN) on a dry matter (DM) basis. Likewise, the stem can range from 5% to 8% CP and 47% to 52% TDN (DM basis).
Regarding forage quantity, field data indicate about 2.5 to 3 tons of residue DM 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 1 acre would supply enough forage for one gestating cow for approximately 50 to 60 days. Use body condition to guide the nutrition program for mature cows and bulls when grazing sorghum residue, particularly as the grazing season advances. Spring-calving cows in a condition score of at least 5 at the start of the grazing period will likely have little response to supplementation, provided overgrazing does not occur. However, supplementation will likely be needed later in the grazing period to offset declining forage quality as calving nears.
Excess down grain
High amounts of grain on the surface of the field from either plant lodging or spilling when loading trucks pose a risk of acidosis (founder), particularly for animals with experience grazing residue fields. It is important to note that the sorghum kernel has a slower rate and lower extent of digestion than corn. Cattle selectively graze and will seek out and consume any grain present before leaf and stalk material. 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 and management. Avoiding turning hungry animals directly on to fields by filling them up on forage, strip grazing to reduce the amount of grain available per animal, supplementing low amounts (less than or equal to 0.5% of bodyweight) 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 when grazing fields with excess down grain.
Toxic compounds
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 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 cattle are not forced to consume the lower third 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 seven days following a killing freeze is a sound practice because prussic acid concentrations generally increase as temperatures near freezing and then dissipate upon rupture of the plant cells once the plant has been completely frozen and is no longer actively growing.
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.
Grain sorghum residue represents an opportunity to extend the grazing season well into the fall and early winter. Keep in mind that forage availability and quality changes throughout grazing and the nutrition program for a cow herd will depend on the relationship between these changes and stage of production.
