Dr. Ellen Mallory Ph.D., University of Maine
Topdressing, an in-season application of nitrogen, is a strategy some organic winter wheat growers use to increase grain yield and enhance protein. Because there is little research-based information available on topdressing, it can be difficult to decide when it is needed, appropriate timing of application, and what materials should be used.
Photo credit: Ellen Mallory, University of Maine.
Grain protein is a key quality measure for bread wheat, affecting gluten strength and loaf volume. Wheat grain must have a protein concentration of 12% or greater to be considered suitable for bread flour. Grain that does not meet the acceptable level either receives a discounted price or must be sold into alternative markets. High protein wheat is often rewarded with premium payments. More information about grain protein and other bread wheat quality measures can be found in the publication Understanding Wheat Quality: What Bakers and Millers Need, and What Farmers Can Do.
Nitrogen (N) is a primary building block of protein, so it follows that N availability is one of the critical factors influencing the protein content of a crop. The timing of N availability, as well as the total amount, is important. Nitrogen taken up by the plant during its vegetative period can increase both yield and protein, whereas N available after stem elongation primarily increases grain protein concentration. Production practices that increase yields without supplying enough additional N can reduce grain protein concentrations due to protein dilution, i.e.,the same amount of N is contained in a greater quantity of grain. For these reasons, assuring adequate available N for grain yield and protein is a top challenge for winter wheat production. Winter wheat yields tend to be higher than spring wheat yields, yet N supply can be lower due to loss over the winter months of N applied before seeding. In conventional winter wheat production, a standard recommendation is to include a spring topdress application of N to increase grain protein content and baking quality. For more information, see Nitrogen Management for Hard Wheat Protein Enhancement.
Organic winter wheat producers face additional N fertility challenges. The most economical and practical approach to supplying N is to incorporate amendments prior to seeding. However, amendment sources with low C:N ratios (e.g. green manures and liquid dairy manure) may release substantial N in the fall and promote vegetative growth and tiller production; yet excess soil mineral N is susceptible to leaching over the winter. Sources with higher C:N ratios (e.g. solid dairy manure) may have better synchrony with fall crop uptake, but may not mineralize quickly enough in the spring to supply adequate N for the crop to attain acceptable grain protein levels. Many researchers have observed lower grain protein and bread loaf volumes for organic compared with conventional wheat, which they attributed to inadequate N supply (Annett et al., 2007; Casagrande et al., 2009; Fredriksson et al., 1997; Gooding et al., 1993).
While some organic producers use topdressing, there is limited research-based information currently available as to the best N sources and timing of application in organic systems. A study in France found that topdress applications of guano or feather meal, applied to winter wheat at various times from early tillering to heading, always produced higher gross margin from increases in grain yield, grain protein or both as compared with a no nitrogen reference treatment when there were no other limiting factors (e.g. weeds, disease, water); and that later topdress applications produced greater increases in protein than earlier ones for both materials (David et al., 2005). A similar study in the United Kingdom observed increased grain yield and grain protein with early spring applications of either broiler litter, cattle slurry, or pig slurry to winter wheat but found a high degree of variation in their effectiveness from year to year, as well as among the manures (Nicholson et al., 1999). High rates of topdress N were applied and evaluated as a sole source of N in both cases, which may not be practical for many organic farmers. See below for links to current research on using topdressing as a supplement to preplant N applications to boost grain protein levels of organic winter wheat.
Manure is not generally an acceptable N source for topdressing bread wheat because there may not be enough time between application and harvest to satisfy the 90-day pre-harvest interval specified in Part 205.203 of the United States Department of Agriculture [USDA] National Organic Program [NOP] regulations for crops whose edible portion does not have direct contact with the soil surface or soil particles. Other N sources for topdressing include properly composted or heat-treated/processed manures, plant and animal meals and emulsions (e.g. soybean meal, feather meal, blood meal, fish emulsion), and sodium nitrate.
Any topdressing materials must meet input standards for organic certification (see Can I Use This Input on My Organic Farm?). Sodium nitrate, also known as Chilean nitrate, is currently allowed under NOP standards but has been under scrutiny and may be restricted in the future. See the eXtension article Organic Soil Fertility for more information on different N sources for organic production.
Topdress sources can be applied as dry materials or as liquid foliar sprays, the latter being well-suited for irrigated systems depending on the liquid formulation. While soil-applied N is absorbed via plant roots, foliar-applied N may be absorbed directly through the leaf cuticle and/or indirectly via plant roots, as some of the N solution reaches the soil&emdash;either initially or with subsequent rainfall or irrigation. Foliar N application rates are limited by how much the leaves can physically absorb at any one time, and by the potential for leaf-burn from high N concentrations.
Topdress N rates depend on the yield potential of the crop. The higher the potential yield, the greater the additional N needed to increase protein. Researchers in the Pacific Northwest estimate that to achieve 14% grain protein, hard red winter wheat requires 0.4 pounds of N per bushel of grain above the amount of N needed to attain optimal yields (Brown et al., 2005). This amounts to 20 lbs N/acre for a 50-bushel per acre crop and 30 lbs N/acre for 75 bushels per acre. It is difficult to predict crop yield potential and protein increase resulting from added N since both depend on late-season growing conditions. Field and production history, including N credits for legumes used in soil-building crop rotations, should be used to help gauge if topdressing is needed and how much topdress N to apply. In conventional systems, in-season diagnostic tests at two key wheat developmental stages have been developed to guide topdress N decisions. Tiller density at spring green-up (Feekes 2) is used to determine if topdress N is needed at that time to stimulate more tillering and optimize yields. Tissue N concentration at jointing (Feekes 4-5) is used to determine if the plants have sufficient N for good protein levels or if topdress N is needed. For more information see Alley et al., 1999; Brown et al., 2005; Weisz and Knox, 2009. These tools have yet to be adapted to organic systems.
The type of topdress material applied also needs to be factored into the application rate. Materials that mineralize slowly may need to be applied at higher rates, but care should be taken to synchronize N release with crop uptake as much as possible to avoid excess N mineralization after crop harvest.
Topdress N can be applied as soon as soil conditions allow traffic on the field in early spring. However, numerous studies under conventional production have shown that later applications increase protein more than earlier ones. Similar results were found in an organic field trial of different topdress timing and N sources conducted in Maine and Vermont in 2010 and 2011. Averaged over both sites and years, topdress N applied at the late tillering, flag leaf, and boot stages increased crude protein by 0, 0.4, and 0.8 percentage points, respectively, for dehydrated chicken manure and 0.4, 0.9, and 1.3 percentage points, respectively, for sodium nitrate (Mallory and Darby, in press). In drier areas, late-season topdress N applications may not be fully utilized by the crop without adequate soil moisture. There is also concern that N applied very late in the season (at flowering and later) may increase grain protein levels but does not always improve baking quality. A number of studies in conventionally grown wheat have found no improvement in dough properties or bread loaf volume with application of a foliar urea solution at flowering despite increases in grain protein concentrations (Gooding and Davies, 1992). There is evidence that N taken up by the plant this late in the season does not get fully incorporated into functional grain proteins (Finney et al., 1957), and changes the protein composition in ways that negatively affect dough properties (Timms et al., 1981).
The decision of whether or not to topdress should include consideration of the added costs and potential returns. Topdress costs include the cost of the N product and application, and any damage that may occur to the crop from field traffic. Potential returns from topdressing depend on changes in yield, increase in protein, and whether the higher protein level moves the wheat crop from the feed-grade market to the food-grade market, or qualifies it for a protein premium when sold.
The following links will take you to abstracts and recorded presentations.
This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic's articles on organic certification.