Quickly summarizing, by the first of October it had more or less reached maturity, was starting to put out seed heads, and the better parts of the field were about 8 feet tall. We had a killing freeze (27 deg. F) on October 31, which should have killed it, and so it will be safe to graze about 7-10 days later.
With sorghum (and a number of other forages) there is a danger of prussic acid poisoning after a frost or a freeze. After a frost, when the plant isn't completely killed and it regrows, there is still a risk of prussic acid poisoning until the regrowth is around 24" tall. If I had a field that had frost damage, I would play it safe and wait until a killing freeze before I let cattle back into that field. After a freeze, when the plant is completely killed, it is supposed to be safe to graze after 7-10 days, but I usually try to wait at least 14 days to make sure everything is dead (I'm a belt and suspenders kind of guy and try to play it as safe as I can).
Nitrate poisoning from sorghum is an entirely different concern and some people seem to confuse nitrate and prussic acid poisoning so do your homework if you are thinking about grazing or baling any of the sorghum family. Nitrate poisoning usually results from higher applications of nitrogen and/or drought conditions, so I'm not that worried about this field since I didn't apply any nitrogen and we had decent growing conditions this summer.
Prussic acid and nitrate poisoning isn't anything to be taken lightly, so if anyone is reading this and thinking about doing something similar, make double sure that you learn all there is to learn about the subject before you turn your cattle out into a field of grain sorghum, sorghum-sudangrass, or any other similar grass.
Whenever I put cattle out on grain sorghum stubble, I usually just let them have access to the entire field, but I'm going to divide this field of sorghum-sudangrass into at least four divisions for a number of reasons. I'm interested in feeding cattle with this field of sorghum-sudangrass, but I'm also interested in improving the soil in this field. Letting the cattle only have access to part of the field should help ration the forage out a little bit longer and should also stop the cattle from eating the higher quality parts in the first few days and leaving the worst for the rest of the winter. Concentrating the cattle on only part of the field should also result in a more even distribution across the field of manure, urine, and any other cow goodness (saliva, milk, hair) they might also be contributing.
It's tough to eyeball a field and accurately determine exactly how big it is, and it's almost impossible to easily figure out where I need to put my electric fence to fence in a 5 acre area in a field of 8-foot tall sorghum-sudangrass by just looking at it. So I used the website: http://www.acme.com/planimeter/ to try different fence placements on an aerial photo.
After trying about fifty different fence placements, I finally decided to divide it up into four sections (I might subdivide those sections even smaller depending on how the cattle actually graze and trample everything). Before I can build an electric fence through a field with grass taller than my head, I'm going to need to mow some strips across the field. I've read accounts of people just driving an ATV or a tractor across a field to knock down the grass before they build a fence, but I think I'd rather mow it now instead of coming back after I've built my fence across the field and trying to mow it when the grass decides it doesn't want to lay on the ground anymore.
|Diving into the field and hoping I'm sort of going in the right direction|
|Looking back at my mowed strip (don't worry, it's supposed to be curved)|
After mowing those strips, it already "feels" like growing sorghum-sudangrass and grazing it over the winter is going to make a difference in the soil health in this field.
I've been thinking that if I had grazed it back in late-September when it was ready to cut for hay, then let it regrow until it winter-killed, I think it might have worked even better (twice the amount of livestock impact, possibly more total growth, etc.). Another idea is that I should have added something like oats or turnips to the sorghum-sudangrass, (oats will grow if planted in late summer) so that I'd still have something growing after the freeze killed the sorghum-sudangrass.
Now I need to build my electric fences and see how the cows eat sorghum-sudangrass "standing hay".