Dr. Calvin Trostle, Extension Agronomy, TAMU Dept. of Soil & Crop Sciences, Lubbock, (806) 746-6101, ctrostle@ag.tamu.edu
August 13, 2020
Producers across Texas know that field crop stubble on fields offers potential benefits. There is erosion control, the reduction of raindrop impact (protection of the soil surface), potential protection to a subsequent crop that is fragile in the seedling stage (e.g., cotton), etc. USDA-NRCS often uses a rating scale for fields with stubble to estimate the erodibility. Other programs through FSA may require at least 30% of the soil surface be covered with residues.
But also, farmers note that in some instances substantial crop residues may curtail yields in a subsequent crop. Perhaps the most common concern is that high amounts of crop residues, especially if they are incorporated near the planting of the next crop, lead to a tie up of nitrogen thus reducing growth of the next crop, at least in key early growth stages where components of yield potential may be determined. College students in agronomy are often taught about the need to add supplemental N for a crop if there is a large amount of incorporated wheat stubble stimulating a great increase in soil microbial activity. For residues, like wheat, where the ratio of carbon to nitrogen is above 20:1 then microbes are scavenging nitrogen so they can grow. And if they are competing against a crop root the microbes win. The crop must wait for recycled N or have supplemental N applications.
In general, farmers likely view crop residues as a means to reduce evaporation of existing soil moisture. This is generally true due to reduced wind speed at the soil surface, shading of the soil, etc. But there are instances where this might be detrimental. One example is published in Hockley/Cochran/Lamb AgriLife Extension IPM agent Kerry Siders’ August 13, 2020 newsletter (https://hockley.agrilife.org/newsletters/ipm/). In this example, wheat stubble appears to have potentially wicked moisture out of the soil thus reducing cotton yields.
But in this example, the possible solution to the wheat stubble issue (or other residue) does not mean ‘soil incorporation,’ rather leaving the residues on the soil surface. This would apply to most crop residue decisions. Some farmers might like to think of the residue as a nutrient source, and they are correct. There is a short-term (incorporation) vs. long-term aspect (leave stubble standing—no stalk cutting, no tillage) to this question. The intermediate term comparison is tillage or cutting operations that leave the stubble on the surface. This hastens breakdown, and in high residue situations this may be necessary at a minimum. But for growers across the Texas, the windier your production environment then increasingly is the value of standing stubble.
Your Planter Capability
What you as a farmer do with crop residues may depend on your planter and how it is equipped to plant into higher residues. (I will write later this year about my opinion on what a farmer’s most important piece of equipment is and why.) A planter with the needed trash whippers, coulters, etc. enables you to plant into at least moderate residue environments. This gives you the farmer an option. You may be able to eliminate an expensive—and potentially erosive—tillage operation.
Most planters are equipped with some sort of disk to move stubble or soil for desired seed placement. Moving soil is more likely a priority in dryland farming in drier regions of Texas as you reach for sufficient soil moisture to germinate your seed. The disk opener itself can serve as a cutting coulter in many cases, and that is all that is needed (Fig. 1).
Fig. 1. Standard planter unit with no specialized equipment planting into thin sorghum/sudan stubble, Lubbock Co., TX. In this instance the disk openers are doing a good job of cutting the stubble as the furrow is opened for cotton planting. Heavier stubble might require use of the trash whipper disks to clear a narrow band for planting. (This figure was published originally in an October 2018 Row Crops Newsletter on cover crops.)
Higher biomass present on fields may present additional planting issues. Corn production produces so much residues in some instances that direct planting for a subsequent crop is difficult if not impossible. (But burning that residue or shipping it off as forage has a downside as well.) Biomass that is five tons per acre or more would need substantial soil contact—surface contact, not necessarily incorporation) to degrade for ready planting the next year.
Some dryland cover crop work in West Texas has produced several tons per acre of sorghum/sudan stubble. A late fall observation might be “How could I possible plant into that next year?” But after lots of wind and the gradual contact of residues with the soil, farmers at our demonstration sites agree they wouldn’t have too much trouble planting into the stubble (Fig. 2). The key is having a coulter to cut the residues, so they don’t hairpin around the planter unit.
Fig. 2. November high sorghum/sudan biomass from a summer cover crop with intent plant a spring crop into untouched biomass (Lamesa, TX). This looks difficult for later planting, but after a winter and spring of wind and some precipitation, direct planting conditions appear manageable without tillage if a planter is equipped properly. In this case area farmers said they would have no problem planting if they used a coulter to cut the biomass in front of the planter unit.
Plowing in Crop Residues for Organic Matter—A Trostle Agricultural Pet Peeve
In my region of the Texas South Plains most farming is cotton. Cotton doesn’t produce much residue, but more farmers now recognize that even allowing the cotton stalks to stand to near planting the next cotton crop offers some slight advantages on erosion protection. But more commonly in the low residue system of continuous cotton, occasionally the cotton fails, and farmers may plant grain sorghum as a catch crop. When that crop is harvested, the response of some farmers is to plow the sorghum assuming it will increase organic matter. Wait a minute!
As one moves from humid to semi-arid regions of Texas, whether far south Texas back from the Gulf Coast, or through the Rolling Plains into the High Plains, soil organic matter levels decline. It is common in the Lubbock region to find soil test reports with 0.3% carbon (C) which would be about 0.5% organic matter (OM). This is unbelievably low. But if sorghum residues are plowed in that should increase OM?
For example, a farmer ekes out a 2,000 lb./A catch crop sorghum yield on dryland near Lubbock after a failed cotton crop. That would be about 3,000 lbs. of dry matter from the stubble, leaves, and threshed head. That is somewhat marginal in protecting the soil from strong winds, but if left standing it can help. Is this a tradeoff versus incorporating the stubble? Yes, it is. And, importantly, because the typical soil in West Texas tends to be sandy it does not afford good stability to soil organic matter. Tillage disturbs the little organic matter you have. Yes, some of above-ground stubble if incorporated does become stable long-term organic matter. But the tillage operation disturbs some of the organic matter you already had.
So, incorporating that stubble, at least in this drier situation, is likely and merely only an exchange of organic matter. It is probably not an increase in soil OM. And in the process you have lost the value of the residue to protect the soil from erosion.
But there is a silver lining here: the potential slow increase in soil organic matter in this scenario comes from the roots. Left undisturbed this contributes to soil organic matter without the potential losses of what you already have.
If you are in a wetter region of Texas and you have 2% or more organic matter, then consider that a blessing. It means you also have more rain.
An Additional Downside to Reduced Stubble Value—Harvesting for Forage
Stubble harvest for forage may be an attractive source of additional income. Corn farmers in the Texas Panhandle often bale stalks to reduce biomass on the field so they can sometimes plant fall small grains. But apart from forage harvest reducing erosion protection, there is a hidden downside that too many growers don’t consider. Crop residues that leave the field are removing nutrients from your cropping system. Is it a lot? Or a little?
Most farmers that occasionally sell stubble, in contrast to a regular forage grower who is likely to account for nutrient removal in forages, usually don’t count the replacement cost of the nutrients. For many stubbles of corn, grain sorghum, perhaps wheat, protein content can be in the range of 5 to 6%. For simplicity, in general 6.25% crude protein is about 1% N. So, for each ton of residue removed, you are taking off about 20 lbs. of N. At today’s N cost that typically has a value of about $10-13 per ton. I estimate the value of all remaining nutrients (P, K, S, micronutrients) is about 1/4 to 1/3 the value of nitrogen. So, a ton of stubble may remove about $13-18 of nutrient value from your cropping system.
Is the price you are receiving for the stubble—which is not high—accounting for this? When I ask farmers about it, most are not accounting for nutrient removal. If you are getting $30-40/ton for the stubble, then as much as half of that income would be needed to replenish nutrients. The stubble sales may seem like free income, but it may not be a good deal.