Last week, I visited about toxic nitrate levels in forages for livestock. Today's topic is prussic acid toxicity that can potentially be lethal as well.

The most common plant species associated with prussic acid toxicity are sorghums, sudangrasses and Johnsongrass. Potential for toxicity varies greatly among varieties and hybrids. Grain sorghum overall has the greatest potential for prussic acid poisoning greater than forage sorghum or sudangrass. Millets have an extremely low potential for prussic acid poisoning.

Plant age/condition is a factor as prussic acid concentrations are highest in young, rapidly growing plants. Concentrations are higher in young leaves compared to older leaves and stems. Sorghum growth following drought or frost often contains lethal levels of prussic acid. Any environmental condition creating stress elevates prussic acid levels. Anything that damages plant leaves also increases levels.

Environmental conditions. Drought and frost result in elevated prussic acid levels. Often levels will remain elevated after drought. Waiting four to five days after a frost will allow prussic acid to dissipate.

Fertility. High soil nitrogen levels, in combination with low phosphorus and potassium levels, leads to greater likelihood of prussic acid accumulation. Split N applications help prevent this from occurring.

Animal condition. Hungry and/or stressed animals are especially susceptible to prussic acid toxicity. Ruminants are more susceptible than non-ruminants.

Harvest technique. Concentrations are higher in fresh forage than in silage or hay. Prussic acid is given a chance to dissipate when forage has a chance to cure or ferment.

Toxicity levels are determined only by a forage test from a qualified lab. The following describes ranges of prussic acid levels and their potential effect on cattle. Prussic acid (HCN) levels in forage on a dry weight basis and potential harmful effects on cattle are:

HCN level of zero to 500 ppm is safe and should pose no problems; 600 to 1,000, potential toxicity (should not be the only feed source); more than 1,000, dangerous to cattle and often fatal.

For more information, see "Nitrate and Prussic Acid Toxicity in Forage," K-State publication MF-1018; www.oznet.ksu.edu/library/crpsl2/MF1018.PDF or stop by the Finney County Extension Office.

Power raking and core-aeration

September is the optimum time to power rake or core-aerate tall fescue and Kentucky bluegrass lawns. These grasses should be coming out of their summer doldrums and beginning to grow more vigorously. This is a good time to reconsider what we are trying to accomplish with these practices.

Power raking is primarily a thatch control operation. It can be excessively damaging to the turf if not done carefully. For lawns with one-half inch of thatch or less, I don't recommend power raking. For those who are unsure what thatch is, it is a springy layer of light-brown organic matter that resembles peat moss and is located above the soil but below the grass foliage.

Core-aeration is a much better practice for most lawns. By removing cores of soil, core-aeration relieves compaction, hastens thatch decomposition, and improves water, nutrient and oxygen movement into the soil profile. This operation should be performed when the soil is just moist enough so that it crumbles easily when worked between the fingers. Enough passes should be made so that the holes are spaced about two to three inches apart. Ideally, the holes should penetrate two-and-a-half to three inches deep.

The cores can be left on the lawn to decompose naturally (a process that usually takes two or three weeks but is often viewed as unsightly), or they can be broken up with a vertical mower set just low enough to nick the cores, and then dragged with a section of chain-link fence or a steel doormat. The intermingling of soil and thatch is beneficial to the lawn.

Give cool-season grasses a boost

September is almost here and that means it is prime time to fertilize tall fescue or Kentucky bluegrass lawns. If you could only fertilize cool-season grasses once a year, this would be the best time to do it. These grasses are entering their fall growth cycle as days shorten and temperatures moderate. Cool-season grasses naturally thicken up in the fall by tillering (forming new shoots at the base of existing plants) and, for bluegrass, spreading by underground stems called rhizomes.

So yes, just as you read or hear the media ads, September is the most important time to fertilize these grasses. Apply one to one-and-a-half pounds of actual nitrogen per 1,000 square feet. The settings recommended on lawn fertilizer bags usually result in about one pound of nitrogen per 1,000 square feet. We recommend a quick-release source of nitrogen at this time. Most fertilizers sold in garden centers and department stores contain either quick-release nitrogen or a mixture of quick- and slow-release. Usually, only lawn fertilizers recommended for summer use contain slow-release nitrogen. Any of the others should be quick-release and fine to use at this time of year.

The second most important fertilization of cool-season grasses also occurs during the fall. A November fertilizer application will help the grass green up earlier next spring and provide the nutrients needed until summer. It also should be quick-release applied at the rate of one pound actual nitrogen per 1,000 square feet.