The most well-known member of the C-4 plant group is corn. Equally important members in this botanical country club is a grouping that I call hot climate summer annuals (HCSAs). Let’s briefly review HCSAs, which include sorghum, Sudangrass, their hybrids, and millets: the first three crops were developed in very dry sub-Saharan Africa (called the Sahel). Millets are believed to have originated in very dry regions of India and China. C-4 plants consist of carbon building blocks that are crafted with four-carbon modules. Without getting too technical, the biggest benefit of this trait is the increased ability of the plant to retain moisture. Moisture retention — as regulated by stomates (openings on leaf surfaces) — is much more efficient with C-4 plants, than is the case for non-C-4 plants. The C-4 class boasts such members as these just cited (don’t forget corn), plus sugarcane. And sadly, a really ornery weed called Palmer’s amaranth is a C-4. The total number of C-4 plants in nature runs in the hundreds.
There’s a lot of beautiful-looking corn just about anywhere I’ve driven in Upstate New York. A lot of the standing corn boasts blue-green foliage. Some of this corn will run high in nitrates (-NO3), particularly in the lower part of the stalk. This is caused by a combination of too much warmth, too little precipitation, and too much fertilizer nitrogen. That last factor can include high doses of animal manure. And the first two factors are what most of the Northeast was “blessed” with during the first three weeks of this past July.
High levels of -NO3 in standing corn may result in high -NO3 in the resulting corn silage feed, which in turn predisposes to higher than usual levels of nitrogen dioxide (NO2) gas in silos. According to the website, “Nitrogen dioxide (gas) is found in higher than normal concentrations within farm silos and is the predominant toxin in silo filler’s disease. The gas causes significant irritation of the mucosa lining the eyes, nose and airways. Nitrogen dioxide damages the lower airways in particular and the lung tissue. It breaks down into nitrous oxide and nitric oxide within the airways and lungs. Apart from the direct effects of nitrogen dioxide as an irritant, it also results in the formation of free radicals which can cause further cellular damage.”
In situations like these [prolonged hot dry spells and heavy nitrogen fertilizer (whatever the form)], most agronomists strongly advise that the first couple loads of chopped fresh silage be analyzed for nitrates. High forage test nitrate levels may indicate the wisdom of leaving tall stubble, or combining and/or picking the corn; very little nitrate makes it into the grain part of the plant. Until forage tests come back proving that the blue-green freshly chopped corn forage is not high in nitrates, I’m convinced that feeding such roughage straight to cattle is a bad idea. Forage test nitrate numbers will indicate to what extent such freshly chopped feed should be diluted by other Ingredients in a total mixed ration. Corn forage stored horizontally poses less NO2 gas threat than corn stored vertically.
Moving on to HCSAs, we see that sorghums, Sudangrass, and sorghum/sudan hybrids typically fared better than corn during the three-week dry spell so common in early July. University of Texas (UT) field trial research is very flattering to HCSAs. UT data shows that when moisture is very limited, these three HCSAs can produce twice as much whole plant dry matter, as compared to what whole plant corn silage yields for the same amount of precipitation. A caution here with these HCSAs is that after harvest, their early regrowth tends to run high in prussic acid [another toxic nitrogen product, with chemical formula HCN (hydrogen cyanide)]. After this regrowth surpasses 20 inches, any HCN threat is pretty well gone. Giving the devil its due, a plus for HCN is that its residue in the root mass of these three HCSAs pretty well knocks out any rootworm threat that might confront corn planted the next year in the rotation.
And millet, the last HCSA member, poses no HCN threat, which means that grazing cattle can be placed on its aftermath at heights less than 20 inches… though that’s still a good target to aim at in terms of stand vigor. Millet boasts two other pluses… compared to the other C-4s (including corn): millet isn’t as discouraged when pH dips slightly below 6.0. And millet keeps performing on even less field moisture than sorghum, Sudangrass, and their hybrids. Unfortunately, unlike these other three HCSAs, millet doesn’t come in many brown-mid-rib versions; BMR Pearl millet is the only one I’m familiar with. But most of my millet recommendations involve the Japanese type, one I’ve worked with one many years.
When ensiled, all five C-4s in our discussion should be at least 35 percent dry matter. Much less than that increases the likelihood of vertical silos juicing, as well as the fermentation process drifting in the direction of butyric acid. Not only does this volatile fatty acid smell bad, it increases the likelihood that dry dairy cows who ingest it might come down with ketosis once they freshen.
I can’t ignore the black sheep of the C-4 family. That critter is Palmer’s amaranth (Amaranthus palmeri), one of the most aggressive weeds known to attack modern agriculture. Because of the overuse of the weed-killer active ingredient glyphosate, A. palmeri has developed resistance to this herbicide, becoming tougher than any of its pigweed cousins. Modern weed control science has countered by developing corn and soybean varieties which are immune to being sprayed by dicamba herbicide. As a result we now have A. palmeri plants that are immune to both glyphosate and dicamba. If we can personify A. palmeri individuals, I can imagine them laughing amongst themselves, wondering if some of our learned weed scientists ever studied plant population genetics.