GOSHEN, Ind. — Waterhemp and Palmer amaranth are two of the most loathed weeds locally. Both are members of the pigweed (amaranth) family. Both can produce half a million seeds or more on a single female plant. Both species are able to quickly adapt when farmers use the same herbicides over and over again to combat them.
Both weeds germinate over a long period of time and can be seen popping up through the canopy late in the season. Despite all the attention given to Palmer, waterhemp is the predominate species in our local fields.
One of the ways both of these weeds get introduced into fields is through manure. When feedstuffs contain the seeds, particularly hay, the seeds are distributed when the manure is spread.
Hay from other regions of the country where the weeds are more prevalent is often the seed source. A dairy farm I visited in June of 2014 had purchased hay from southern states, and within a few years, all the fields where manure was spread had Palmer. It could have just as easily been waterhemp.
In a recent article, Extension specialists in Nebraska and Minnesota wrote about managing manure that has the seeds in it (https://bit.ly/32358KM). The first point they made was the seeds will survive the trip through the digestive system because the seed coat on pigweeds is tough.
Ensiling the crop did kill a high percentage of the seeds. Eight weeks after ensiling, up to 87% of the seeds were killed, and what’s more, the remaining seeds were more likely to die when they passed through the animal digestive track.
Composting manure property can significantly reduce the seeds too. The key word is “properly.” With truly composted manure, you cannot tell what the “parent material” was. Too many composted manures are simply just aged manure. If you can see residue of bird feathers, or corn kernels, or other feedstuffs, it is not thoroughly composted.
Several studies have found 90-98% of weed seeds are killed when the manure compost is at various temperatures between 140⁰F and 160⁰F for three or four days. A key point to maintaining the temperatures that high is to keep the moisture content between 35 and 50%, which helps to evenly distribute the heat. When the temperature begins to drop, turn the pile and add water if needed. Another study found that it took between 21 and 50 days of composting with proper management to eliminate amaranth seed.
Liquid manure is more problematic. It cannot be piled for composting, and heating it would be expensive. The best option is probably applying near the barn where the field can be frequently scouted for weeds.
It’s important to keep in mind that a 98% kill of weed seeds sounds great, but if a single plant can produce 500,000+ seeds, it does not take many years before the weed seed population builds up in the soil. A survey of fresh dairy manure in New York found an average of 75,000 viable seeds per ton of manure. A 2% survival rate of 75,000 seeds would leave 1,500 viable seeds per ton remaining. Applied at 8 tons per acre, that would increase the weed seedbank by 12,000 seeds per acre. This “numbers game” is especially troublesome in the case of amaranth and waterhemp, a prolific seed-producing weed species.
While most dairymen don’t consider themselves well versed in crops, this is one case where you certainly want to learn which herbicides to use, when to use them, and which ones are no longer acceptable for the pigweed family. A trusted, up to date crops consultant can be extremely valuable when it comes to control advice.
— Jeff Burbrink, Extension Educator, Purdue Extension Elkhart County
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