Mark Goodwin
Cyanamid Crop Protection
Winnipeg

There are three main factors that determine how well a weed control chemical is going to work. All three factors can affect results on their own. To complicate things, the three factors interact with one another as well.

THE THREE FACTORS

1) Applicator error (such as timing problems, rate errors or tank mixing errors).

This is generally declining in importance as a common factor in performance problems. Most farmers recognize that herbicides cost enough money to warrant great care in proper timing and careful application. Most herbicides are applied on time unless environmental conditions keep farmers off the field as weeds advance in staging.

Rate errors have actually declined significantly over the years, with the most common deviation from rate occurring as farmers take the calculated risk of rate cutting below label rate

2) Inherent strength of the product on spectrum.

All herbicides have weeds that they are strong on, and weeds they are less strong on. Take the Manitoba Agriculture ratings for several canola herbicides in the chart below. The chart ranks products based on consistency over a range of conditions. You'll notice that all products have a range of degree of consistency across weed spectrums. You'd expect then that adverse weather, or other stresses would affect lower rated weeds sooner than higher rated weeds. You'll also notice that the ratings vary widely from product to product.Chart l. Weed control consistency ratings

Manitoba Agriculture ratings

E = excellent

G = Good

F = Fair

P = Poor or suppression

3) Weather factors (the big one!)

Weed control technology is every bit as complex and technologically intense as computer chips. But computer chips are carefully handled from the gowned, gloved hands of the factory floor to the humidity/temperature controlled conditions that exist in your computer. In contrast, the elegant architecture of the herbicide molecule is thrown out in a micro-thin blanket into the wild and woolly world of our prairie weather.

Each product has to make it's peace with that weather. And each product does so in a different way. Chart 2 shows how some different wild oat product react differently to different weather. Generally, herbicides work best when relative humidity has been moderate to high, and when temperatures have been moderate (15 to 28 cel/60 to 80 fahrenheit). Wild swings from day to night are not good-especially if frost temperatures are hit.

Generally speaking, the Golden Rule of weed control is that weather that favours plant growth also favours herbicide uptake, translocation and activity.

Effects of Weather on Some Wild Oat Products

PRODUCT	EFFECTS OF WEATHER
ACHIEVE	Unacceptable crop injury (crop yellowing, stunting) can occur if Achieve or Achieve tan mixes are applied when temperatures of 50o C or below occur within 48 hours of application control of grasses could be reduced when they are stressed due to drought, heat, lack of fertility, flooding or prolonged cool temperatures.
ASSERT	Do not apply Assert 24 hours before or after a frost It works best at warm temperatures. Performs relatively consistently under dry conditions. If cold, wet soil conditions persist in the days after application, retillering of wild oats may occur. Do not apply to drought stressed sunflowers.
AVENGE	Crop injury is worse on cold or hot, humid days than when temperatures are moderate. Best weed control is attained when temperatures are 20 to 300 C, particularly when these temperatures follow application. Do not spray when crop or weeds are wet with heavy dew or rain. Do not apply when crop is under stress from drought or excessive moisture.
HOEGRASS	Application under hot (above 280C) or droughty conditions will result in reduced weed control. Hoegrass 284 provides optimum results under cool, moist conditions. Under certain conditions, yellow blotches may appear on barley leaves. Under certain environmental conditions, especially hot, humid weather, leaf scorch on cereals and leaf cupping and bum on broadleaved crops may result from HoeGrass 284 application
MATAVEN	Wild oat control may be less than acceptable if cold or dry conditions persist for 1 week preceding application. Best control under high relative humidity.
TRIUMPH PLUS	Do not apply to crop that is stressed by severe heat, frost, low fertility, drought, water saturated soil, disease or insect damage as crop injury and poor weed control may result

SOURCE: MDA 1995 WEED GUIDE

4) Resistance problems

This is still thought of as a worrisome "over there" problem But with only 15 percent of western producers (higher in Manitoba) rotating multi active ingredients, it's just a matter of time before this problem becomes a routine source of herbicide failure.

THE INTERACTIONS

Nothing in nature is simple. Seldom does one thing clearly end and another thing start. This is true of the factors that affect herbicide performance. Consider the traffic light model below. The relevant rule is called the "Five Light Rule". This rule says that a herbicide still work well if at least four out of five of the "lights" are green (favourable).

For instance, if a farmer cuts a rate, he or she turns the "rate/surfactant light" red (unfavourable.) He still gets good performance though if all other factors are still green -in other words, if his crop is competitive, the weather is favourable, the weed species is on e that the product is inherently strong on etc.