Conserving end-of-season N with effective seeding dates for cover crops based on fall growing degree days

Impacts of Results/Outcomes

Results from field studies and further activities are discussed in detail below.

Seeding Dates for Cover Crops:

Most farmers and extension and UDSA-NRCS staff have known the appropriate date for seeding cover crops based on achieving 30% cover prior to winter for control of soil erosion. In the Deerfield area of the Connecticut River Valley in Massachusetts the seeding date has been taken to be September 15. However, this seeding date for cover crops had been determined only for controlling water and wind erosion. Thus, seeding dates were based on plant canopy cover, not on root development for their nutrient accumulation. Growth of cover crops with varying seeding dates is illustrated in Figure 9.

Our research from 2004 to 2009 has indicated the seeding date for maximum biomass growth and N accumulation was two to three weeks earlier than the date for adequate soil erosion control (Figs 10-12). Based on these results most cover crops planted by farmers are not effective in accumulating end-of-season nitrogen because they are planted after the desired date. If nitrogen, for example, is not accumulated by a cover crop it will be lost during the fall as indicated by the lower soil N values for the no cover treatments during the fall (Fig. 13), and very low soil N values for rye and oat cover crops regardless of seeding date (early September or late October) after winter in early spring. Our earlier studies reported that cover crops planted in early September had 50% more nitrogen accumulation than cover crops planted in late September. Results from our studies in this project confirmed these earlier reports for both rye and oat cover crops and that rye retained more of the accumulated N through the winter than oat.

Early establishment dates for cover crops were confirmed in UMass and farmer studies as necessary for effectiveness for end-of-season N accumulation by cover crops and this translated into higher PSNT levels with earlier seeding (Fig. 14). Later seeding of cover crops even in heavily manured soils indicated N fertilizer was needed due to PSNT levels below the 25 ppm sufficiency level of nitrate-N for corn silage.

Accumulated GDD was calculated from Sept. 1st to Dec. 30th. The base temperature of zero degrees C was found to give the best fit for normalizing data based on GDD accumulation from different seeding dates in several experiments and years. It was determined that 1030 units (Base temp. 0 degrees C) were needed for maximizing rye cover crop growth. There appeared to be no benefit to greater GDD accumulation in terms of N accumulated. Data showing the relationships for biomass increase and GDD accumulation or loss, biomass and N accumulation with change in seeding date are given in Table 1. A delay in the planting date reduced N accumulation exponentially. Even a small delay in planting cover crop in fall reduced its efficiency for N accumulation dramatically. A small drop in GDD can lead to exponentially larger reduction in biomass and N accumulation.

Using the data from the 14 weather stations, we were able to estimate GDD and therefore could estimate biomass and N accumulation for each date of planting in any locations in the State. To increase the accuracy of our model and for better estimates a non-liner model calculated using the weather station data the GDD was based on temperature and the elevation and precipitation (R squared 96%). Thus, the estimate of GDD for each location was determined with greater accuracy.

GDD = f(temperature, elevation, precipitation), and Biomass = f(GDD), N uptake = f(Biomass)

In this way we can estimate potential N uptake for each location in the State, using the elevation and precipitation data and we can do that for any planting date from August 1st to October 31 or later. A digital map integrating temperature, elevation and precipitation was developed for the critical planting date zones based on GDD accumulation (Fig 15). Biomass and N accumulation for weekly interval planting dates. These maps are the basis of a decision making tool for the dairy farmers and to assist them in deciding the best practical cover crop seeding date. This tool is being finalized as a web-based decision aid.

Strategies for Earlier Seeded Cover Crops:

Previously cover crops have been inter-seeded in standing corn using helicopters. This is now probably not a viable option because of cost and absence of cost-share programs for such aerial seeding methods. In all of the inter-seeded cover crop studies rye seeded in June in the first year, with good growth in July, plant material was almost absent after corn harvest in late August to early September (Fig. 16). Based on this data and early July inter-seeding with similar results, this method was abandoned in later years. Farmers were encouraged to use other options of combine seeding of cover crops with the harvest operations or with the incorporation of manure.

Seven years of field studies at the UMass Research Farm have shown that yield of early hybrids was similar to late hybrids. The average yield weighted for the number of hybrids each year was 28.6 t/ac for early hybrids and 28.7 t/ac for the late hybrid group. In all maturity groups there are high and lower yielding hybrids of varying quality (Fig. 17). Hybrids were not always consistently high yielding at differing sites. However, planting early hybrids may increase nutritional quality without lowering silage yield.

Early seeding of corn and choice of an early maturing hybrid helps to ensure an earlier harvest date and greater chance of seeding a cover crop near to the optimum date for maximizing N accumulation. Several studies were established to demonstrate crop response to date of planting in relation to date of harvest (Fig. 18) and to maturity group of hybrid (Figs. 19 and 20). Any delay in planting tends to reduce total yield as well as grain contribution to yield and hence overall quality. A delay in planting reduced yield more than the differences in yield between early and late maturing hybrids (Figs. 19 and 20) with yield of early maturity hybrids being similar to late at varying planting dates for silage and ear yield.

• Fig. 14. N accumulation and PSNT measured next spring
• Table 1. Growing degree day accumulation, potential biomass production, nitrogen accumulation, and economic value of nitrogen accumulated and loss related each time of planting in Massachusetts
• Fig. 19. Silage yield of corn hybrids on three planting dates.
• Fig. 20. Earcorn yield of hybrids on three planting dates.
• Fig. 10. N accumulation by rye seeded August to October
• Fig. 11. N accumulation in one farm study
• Fig. 15. Digital map integrating temperature, elevation and precipitation was developed for the critical planting date zones based on GDD accumulation.
• Fig. 16. Inter-seeded cover crops in one of several studies.
• Fig. 9. Cover crops seeded on dates shown and photographed the end of December
• Fig. 13. N loss with no Cover Crop
• Fig. 17. Mean yields of corn hybrids at the UMass Farm in 2008. Dash lines reference means of hybrid yields for silage and ears. #’s refer to days of relative maturity. Example of five trials in 2007 and 2008.
• Fig. 12. N accumulation in 2008
• Fig. 18. Planting and harvest dates for an early and a late maturing corn hybrid.

Economic Analysis

From first day of August to the end of the Julian year, average weekly GDD was calculated (20 weeks) for 14 weather stations in and around the state of Massachusetts. For each week, using ArcMap and Interpolation techniques, a raster map was created. For maximum N recovery winter rye requires about 1050 GDDs (based on data from all cover crop experiments) which we call Critical GDD (CGDD). In mapping we looked for regions with similar CGDD (we assumed that a GDD range between 1000 and 1100 can be considered as CGDD). Regions with critical amount of GDD (CGDD) were extracted from each raster map to create the growing degree day map for Massachusetts. (Fig 15). Ideally, cover crops need to be planted earlier in the western part of Massachusetts compared to the east. This is because the west is higher in altitude and hence cooler than the ocean moderated eastern climate. It also means the growing season is shorter with later planting of corn in the western hills, meaning later corn harvest and less opportunity for planting cover crops on schedule for maximum N accumulation.

The effect of a delay in planting cover crops on GDD accumulation is shown in Table 1. The trend is linear for up to 4 weeks of delay in planting. More than 12% of GDD will be lost by each week of delay in planting. Four weeks delay will cause about 50% loss in GDD. The effect of a delay in planting cover crops on biomass loss had an exponential trend. Four weeks of delay will cause about 80% loss in biomass. A one week delay can reduce the biomass production by 29%.

Nitrogen accumulation is more important economically than biomass production. When the cover crop is planted on time, it can accumulate more than 100 lb N/ac. Nitrogen loss also has an exponential trend. One week delay in planting cover crops can reduce N uptake by 25% (about 27 lb N/ac loss). It again demonstrates the dramatic effect of planting date on cover crop efficiency. Four weeks delay in planting cover crops reduces N uptake by 74% (about 78 lb N/ac loss).

This delay in planting cover crop translates directly into economic loss. Assuming the price of each pound of N in fertilizer is about $0.52 (based on 2010 urea price) it is possible to save about $55 per acre by planting cover crops on time. This number comes from multiplication the price of each pound of N with the amount of N recovery. Economic loss due to delay in planting cover crop has also an exponential trend. More than 25% of the economic loss happens with just one week delay in planting cover crop.

Farmer Adoption

The results of the multi-year and multi-location comprehensive cover crop research project and associated research into the corn-cover crop system was presented to farmers who attended several annual field days, farmer research group meetings, and barn twilight meetings. A list of the events is presented in the above section of this report. Attendants were impressed with the potential nitrogen recovery (~ 120 lbs N/acre) by early planting of winter rye cover crop. However the short growing season is the major constraint to plant effective winter rye cover crop in Massachusetts. A combination of earlier planting of silage corn (late April-early May), when is applicable, along with using shorter season corn hybrids (90-95 days RM) provides a greater opportunity of planting winter rye by the first week of September. At UMass 10-years of corn hybrid trials have shown that on average, shorter season corn hybrids yield similar to full-season corn hybrids. Many farmers now have decided to select earlier maturity corn hybrids, at least for a few fields, which enables them to harvest corn silage earlier, and plant winter rye cover crop earlier for higher nutrient recovery. We are aware of 3 farmers that were not planting cover crops before for one reason or another and now have started planting cover crops. Six other dairy farmers have been able to plant cover crops earlier by using shorter-season corn hybrids and/or earlier planting of their silage corn. At least 5 farmers (2 of them never planted cover crops before) have decided to plant corn earlier and/or use earlier maturity corn hybrids for the 2010 growing season. Some of these farmers have noticed that their silage corn yields have been reducing possibly because of lower organic matter in their fields.

The short growing season was complicated by two consecutively wet growing seasons (2008 and 2009) which delayed planting on many farms with delayed crop maturity resulting in a delayed harvest and late planting of cover crops. To further extend the education related to this project we have selected 3 farms in Pioneer Valley, Central Massachusetts, and Southeast Massachusetts as pilot farms to demonstrate the potential N recovery by winter rye cover crop planted at various time and the effect on yield of subsequent corn, planted in the same fields. These three pilot farms will be used as demonstration sites for our educational workshops next year.