Authors: Nick Andrews, Small Farms Program, Oregon State University & Len Coop, Integrated Plant Protection Center, Oregon State University
Publish Date: Fall 2011
There are other factors that can affect the development rate of organisms such as moisture, day length and competition. However, simple degree day (DD) models can accurately predict development rates within a few days, especially if some assumptions can be made about other factors affecting development rates, such as the presence of adequate moisture as provided in an irrigated field.
Nearly all seed catalogs report the time to maturity of different vegetable varieties in number of days to maturity. If all vegetables were bred in our region, that could be fairly accurate despite considerable variation in weather from year to year. However, a month in Florida or California provides a lot more DDs than a month in Oregon or Washington. Reported days to maturity can give a relative idea of which varieties take longer to mature, but they normally don’t help schedule plantings or harvests with much accuracy.
Many farmers get a rough feel for this after years of experience with a variety, and some farmers and agricultural companies have developed their own DD models for the main varieties they grow. They can use this information to schedule plantings to provide the volume of harvest they want at different times of the year. When inclement weather interrupts the planting schedule, they can use this information to select different varieties that will get their harvest schedule back on track. As harvest time approaches, they can also use this information to communicate with buyers when their crop is ripening a bit earlier or later than expected. If climate change impacts local weather in the Pacific Northwest, this sort of information will help farmers adapt to years with poor growing conditions and take advantage of years with better growing conditions.
Crops have upper and lower development thresholds outside of which they don’t develop physiologically. One of the simplest ways of counting DDs for one day when the crop’s lower temperature threshold is known is: [(Tmax + Tmin) / 2] – T lower. For example, assume the crop’s lower threshold is 50°F and upper threshold has not been determined or is very high and therefore not used. On a day with a Tmax of 90°F and a Tmin of 40°F, the DDs for that day = [(90 + 40)/2] – 50 = 15. This method of calculation is known as the “Simple average DD method.” Since many crops such as corn are only responsive within a range of temperatures defined by the thresholds, substitutions are made; if the daily Tmax or Tmin are above or below the thresholds, they are reset to the threshold.
OSU’s Integrated Plant Protection Center (IPPC) has a collection of more than 90 DD and hourly-driven plant disease risk and chilling unit models, all pre-configured for individual insects, plants, and plant diseases, plus a generic DD calculator, that are freely available online at http://uspest.org/wea/. The models on the website link to a large network of more than 15,000 weather stations across the United States. In Fig. 3 we display an example output from running the Jubilee sweet corn model (reference above) at the website, using May 1, as the planting date over the years 2009-2011, and the AgriMet weather station ARAO located in Aurora, Oregon. This particular model has been run over 2,600 times at the website and has been used for scheduling planting and harvest dates for processed sweet corn.
The sweet corn DD model example in Fig. 3 shows how DDs may be used to improve the scheduling of harvest dates in years with differing temperature regimes. It illustrates how you can begin to develop models for your own needs by careful record keeping of crop development for selected events and then later running a DD calculator that will help you determine the average DD requirements for each event. If funded, a new proposed project (see below) will provide training and instruction on how to do this step-by-step. This online system is widely used in the tree fruit and nut industries to manage pests by estimating phenological events (e. g. egg laying) that can be used to reduce risk and time treatments. From the website (Fig. 4) go to Quick Start then select the crops you are interested in. When you enter your zip code the program automatically selects the closest weather station and produces a graphical display of all the available models that are relevant to your crops. Currently, this system has about 11 models of interest to vegetable growers including the late blight, early blight, cabbage maggot and Jubilee sweet corn development models. If you know the maximum and minimum thresholds and DD accumulation required for the varieties you grow, you can use the full featured DD calculator at this site: http://uspest.org/cgi-bin/ddmodel.pl. In addition to the models hosted by the IPPC, there are some other sources of DD models. For example, Crookham Company (www.crookham.com/) has DD models for their sweet corn varieties and UC Davis has an extensive collection of DD models (http://www.ipm.ucdavis.edu/MODELS/index.html). We are working with several vegetable growers on a new project proposal to increase the number of development models available to vegetable growers and develop a new interface for vegetable crop scheduling and management. If funded, Jim Myers (OSU Vegetable Breeder) would develop DD models for the varieties he works on, Dan Sullivan (OSU Soil Scientist) would work on a N-mineralization model, Ed Peachey (OSU Weed Scientist) would work on weed development models and Nick Andrews (OSU Small Farms Extension) would work on cover crop development models. Len Coop (IPPC) would lead development of the website and modeling system. The project team would work with growers to help them develop DD models for their own vegetable varieties.
We would also develop a new webpage that would help vegetable growers use these models to plan successive plantings, manage crops and schedule harvests. We are already working with a group of about 10 local producers on this proposal, but are looking for other interested growers. If you are interested in this project please send your contact details to Nick Andrews (firstname.lastname@example.org). Your interest may help us secure funding, and if the project is funded, we’ll get in touch to let you know how you can participate.