Having discussed this category of plant life recently, I apologize for mentioning the subject again. However, I think the currently unfolding growing season merits a review of some of the members of this botanical grouping. Toward that end, I’ll paraphrase from Wikipedia: C-4 plants use the carbon fixation pathway to increase their photosynthetic efficiency by reducing the amount of moisture being respired out of their “body” tissues. Such losses are most prolific in situations of low atmospheric carbon dioxide concentration, high light, high temperature, drought and salinity. There are over 8,000 known C-4 species, which are classed as flowering plants. Among these are important crops such as maize (corn), sorghum, sudangrass, millets and sugarcane, but also weeds and invasive plants. Although only 3% of flowering plant species use C-4 carbon fixation, they account for 23% of global primary production. Defined simply, primary production is synthesis of organic (carbon-based) compounds from atmospheric or water-dissolved carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its energy source. It’s quite impressive to me that so comparatively few plant species create a disproportionately large amount of Earth’s total plant biomass.
A brief mention of C-4 weeds is in order. The most troubling is Palmer’s amaranth (a member of the pigweed family) which has proficiently developed immunity to almost any manmade ag chemical sprayed its way. Then there’s Johnson grass, a cousin of sorghum and sudangrass. Last is barnyard grass, a cousin of the millets. Not surprisingly, most of the other C-4 weed specimens live in tropical and subtropical climates, where they can practice their efficient carbon retention and moisture retention “skills,” thus aggravating crop growers in those regions of the planet.
These moisture and carbon retention skills may be more important this year than during other growing seasons due to some climate misbehavior in the Pacific. What’s going on far west of us are cooler than normal sea surface temperatures. Without getting too technical, climatologists refer to such happenings as La Niña events. With these happenings, strong winds push warm water toward Asia and upwelling increases of the west coast of the Americas. This means that cold, nutrient-rich water rises to the surface in the Pacific, which pushes the jet stream northward. As a result, southern and western states in the U.S. tend to experience drought, while the Pacific Northwest and Canada see heavy rainfall and flash flooding. What’s actually taking place right now is a blockage of moist air originally destined for the U.S. West Coast. This deflected precipitation thus never becomes rain to bless already parched states of California, Arizona, Oregon, New Mexico, Texas, Colorado, Utah and even Kansas. How much farther east La Niña’s scorching misbehavior actually strays remains to be seen.
Erring on the side of caution, I recommend that crop growers evaluate the moisture storage capabilities of the land they work. In my part of Central New York, during May, there have already been two major brushland/forest fires, one an hour north of Utica, another in Otsego County’s Worcester Township. Both required over a dozen mutual aid fire departments to contain. All too many places, even in the Northeast, are very dry. This fact should make growers consider summer annuals that don’t need a lot of water. It should also make them acknowledge the reality that the ability of a soil to retain moisture is directly dependent on the sponging properties of its organic matter. Classic USDA research shows that for every 1% loss of soil organic matter (OM), the water reservoir benefit has been reduced by 16,000 gallons/acre or about three quarts per square foot. So it’s particularly important to have abundant soil OM to hang onto what little precipitation our farmland receives.
Our all-too-common soy/corn non-rotation – lacking winter forages – slowly but surely dissipates soil OM. These crops both lack the fibrous root system so critical for forming soil OM. Although corn is a C-4, soybeans aren’t. And corn doesn’t rank as well as other C-4s in the moisture retention arena. Texas University research agronomists have proven that when rainfall is deficient, a given quantity of precipitation supports twice as much forage dry matter growth from sorghum (and sudangrass and their hybrids) as from whole plant corn forage. C-4 group members use their stoma to limit water loss and retain acquired carbon much more efficiently than C-3s (the other much larger plant anatomy classification). Of the C-4s, here’s how these species rank in terms of physiological moisture conservation: millets, sorghums, sudangrasses, sugarcane and corn, in that order. My own opinion regarding corn placing last in this trait is that its non-fibrous root system may be undermining its otherwise efficient water management skills.
Purdue agronomists are also flattering millets, saying that this crop can be grown in a wide range of environmental conditions, being better adapted than most crops to hot, dry regions. Because of their short growing season (generally figured to be 65 days) they fit well into semi-arid, higher altitude regions. They’re some of the earliest corps to be cultivated in prehistoric times, being a staple food in Eastern Asia as well Europe and parts of Africa. From agronomists at North Dakota State University, situated in an area blessed with only 17 inches of annual rainfall, we learn that millets are even more efficient at converting water into forage dry matter. Prompted by the idea that millets need even less water than sorghum, I recommend planting millets on sods with less than 4% OM. Plant sorghums or sudangrasses (or their hybrids) on soils in the 4% – 6% OM range. With soil OM exceeding 5%, the moisture reserve situation should be adequate to support short season silage corn hybrids. Should a grower wish to return to corn after a year of sorghum, they will observe that immature sorghum’s prussic acid trait does a great job of discouraging corn root worms.