June 1997
Easing Tough Crop Management Decisions
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Prairie Grains is the
official publication of
the Minnesota
Association of
Wheat Growers,
North Dakota Grain
Growers Association,
South Dakota Wheat,
Inc., and the
Minnesota Barley
Growers Association.
by Jochum Wiersma, U of M Small Grains Specialist
As a farmer, you are faced with a lot of decisions during the growing season: What crops to plant, when to spray herbicides, when to treat insects or fungal diseases. Questions such as, "Is this the best time to spray?" or "Do I have enough pest and disease pressure to warrant an economic return by spraying?" are gut-checks to crop management.
Several tools can help make these hard decisions easier, and can save you both time and money. The first tool that might come to mind is a crop consultant. Many of you use professional scouts to aid in the decision process. If you lack the time or expertise yourself, you hire someone that can do it for you, right? These consultants have a vested interest to make you money; they want to keep you as a paying customer.
But what tools are there to make your life easier without hiring the work done? Several tools have been developed through research that may help. One of the most commonly used tools is the degree days model (DD-models).
The concept of DD is an attempt to model the growth and development of a species, whether it is an insect, a weed or a crop, by using a single variable that is easy to measure: Temperature. The idea is that for each species, growth and development can only occur within a specific range of temperatures.
Through monitoring and experimentation, scientists have been able to determine the minimum temperature below which no growth occurs. This temperature is called the minimum threshold. The rate of growth increases with higher temperatures up to a maximum developmental threshold.
In its most basic form, degree days are calculated by taking the average daily temperatures minus the minimum threshold and summing the degree day value over a period of days.
For example:
The date to start the accumulation of DD differs for each model but the most common starting time is when the temperature first exceeds the minimum threshold.
Because of its relative simplicity, DD-models have their limitations too.
1) The accuracy of a DD estimate is dependent on the temperatures used to calculate the degree days. Temperatures from distant weather stations obviously will differ from your field or farm.
2) The precision of the DD model in predicting growth and development stages might be limited if other factors, like lack of moisture, have limiting effects within the range of temperature of the degree day model.
How to use DD models
Despite its simplicity, degree day models have a definitive use in planning scouting activity. By using degree day models, we have a chance to eliminate unnecessary scouting activities. By the same token, the models also avoid missing pest problems, or critical growth stages of the crop. This results in better and more efficient use of your time and may help prevent economic losses.
Where to find the DD-information
DD-information is currently available via the NDAWN weather network. NDSU's NDAWN system has 49 weather reporting stations spread over all of North Dakota, northwest Minnesota and eastern Montana. It's the same system that is used to inform you about the level of risk for Cercospora in sugarbeets or late blight in potatoes. The growing degree days information can be accessed via the Internet at http://www.ext.nodak.edu/homepages/regeberg/ndawn/.
You will find summary maps for the whole area that is covered with the NDAWN system and a list of the 49 weather reporting stations. Within each weather station, the daily weather observations, crops water usage, and degree day information are reported. To calculate the number of growing degree days for wheat, simply subtract the total accumulated growing degree days of the date you planted the field, from the total accumulated of the current date. For the insect degree days, you just take the reported accumulated total as insects are not affected by the date a crop was planted. Instead of insect species, the insect degree day information is reported for a number of minimum threshold temperatures. For OWBM the base temperature is 40° F.
Another tool that has been proven very useful are decision-aids for applications of fungicides. Examples are the Cercospora Management Model for sugarbeets, and the Foliar Fungicide Decision Aid for spring wheat. Both decision aids are examples of what I would call epidemic prediction and prevention models.
Through research we know which conditions favor the development of a fungal disease. By asking a number of questions and doing some scouting work in the field we are able to predict with a degree of certainty the disease level and thus whatthe yield losses will be in one to two weeks from now. Couple this with prices for the crop and the fungicide and you have yourself a decision aid. A decision aid for foliar fungicide in spring wheat is on this page.
Also, your local county extension office is another resource to draw upon for production questions this growing season. n