Air October 29, 2003

Thank you Gary, and good morning everyone. We finally had a pretty wide spread frost across the region. Not necessarily a hard freeze - which I describe as down to 26 degrees, but enough of a frost to start making a difference. So, from the record books the last frost ocurred on October 25th. This is one week later than average but well within the expected range of the first frost date. Our last 32 degree reading last spring was on April 9 so we had a 199 day growing season, almost two weeks longer than average. I'm not going to try to characterize anything more of the growing season at this point in time other than saying that it upheld the tradition of lots of extremes!

The frosty weather will have nipped some of the sorghum and sudan fields. It is at this point that we have to watch the prussic acid situation very closely. Normally five days after a freeze is when the sorghum shoots will be safe to graze. Prussic acid is a precursor of hydrogen cyanide and what it means is that if cattle are in a hot field, you can lose a lot of head in a hurry! I'm afraid, based on what I've seen so far, that we will not have actually had a killing freeze just yet. We will have nipped some upper leaves, but a lot of those lower leaves were still looking green a day or two after the frost. What we know is that to really shut down sorghum we need a temperature of 28 degrees or lower for 2 hours or longer. I don't think we had a serious slow down for the milo, but enough of a frost to raise some concerns over prussic acid. Amazingly, and perhaps contrary to what I've seen, soybean leaves are not damaged until air temperatures reach 26 degrees. But all it takes is 5 minutes at that temperature, or lower, to damage the leaves. I do think that the frost, although maybe not down to 26 degrees, has helped knock a few more leaves off and hasten the drying process. Alfalfa is another story. Have we had enough cold weather to stop alfalfa growing? I don't think so. But alfalfa is a hard crop to figure out. Day length is going to slow it down. Naturally cooler weather will slow it down and eventually stop it. Lack of moisture will certainly stop it for the season. The frost was enough to start knocking off leaves, but it may still try to grow. But if you want to try to get that last little bit off - I won't chastise you for cutting it now!

This is Chuck Otte, Geary County Extension Agent, with Ag Outlook 2003.

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