Fall is a wonderful season for agriculture and a time to reap, we hope, the bounty of the year’s production.
If you have access to crop residue fields after harvest, make preparing to turn cattle out to graze those acres a top priority.
Grain sorghum offers an excellent opportunity to extend the grazing season. However, with our warm fall, light frosts and no hard freeze yet, there are several important management considerations to keep in mind when grazing sorghum residue.
Nitrate and prussic acid toxicity both increase with plant stress and can cause asphyxiation in the animal, yet they are independent of each other. Nitrates accumulate in the base of the stalk or stem.
Although nitrate toxicity is less common in grazing than when cattle are fed harvested forage, it is critical to manage stocking rate so cattle are not forced to eat 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 after a killing freeze is a sound practice. Just as with harvested forages, representative samples of standing forage can be collected for nitrate and prussic acid analysis to determine risk levels and guide management decisions.
High amounts of grain on the surface of the field pose a risk of acidosis, particularly for animals with no experience grazing residue fields.
Cattle selectively graze and will seek out and eat any grain present first. The degree of risk depends on many factors, but levels of more than 10 to 15 bushels per acre generally require additional caution.
Strategies to decrease the risk of acidosis include not turning hungry animals directly onto fields, filling them up on forage first, strip grazing to reduce the amount of grain available per animal, supplementing small amounts of grain before turnout and feeding sodium bicarbonate as a rumen buffer.
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, grazing sorghum residue 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.
Unlike actively growing forages, both the quantity and quality of residue available for grazing will be highest immediately after harvest and will steadily decline because of weathering, trampling and consumption by cattle.
Field data indicate that harvest leaves an average of about 2.5 to 3 tons of residue per acre. If 50% of the total residue on the field is available for grazing and cattle use 50% of that, one acre would supply enough forage for one gestating cow for about 50 to 60 days.
Alicia Boor is the agriculture and natural resources agent with K-State Research and Extension’s Cottonwood District. Contact her by email at aboor@ksu.eduor call 620-793-1910.
