The estimates of yield reduction and the data on herbicide efficacy that WebHADSS provides depend on the types of weeds present, and their population. Weeds differ greatly in how much they reduce yield, depending on the weed, and the crop they are competing with, so correct identification of weeds, and accurate estimation of their population is important to obtaining good recommendations. You can experiment with running individual weed species to see how species differ in their ability to reduce yield. You may also notice that when populations are high, adding additional weeds may not have much effect. This is due to the weeds beginning to compete with each other, in addition to competing with the crop. For extremely high populations, WebHADSS may actually recommend not to treat at all in some cases. This is because the weeds remaining after treatment may do almost as much damage as the original population. In this case, we recommend that you select a treatment to apply from the list and then reevaluate the situation after treatment. Another herbicide application may further reduce the weed population and economic loss.
The recommendations made by WebHADSS are based on weeds that emerge at about the same time as the crop. Weeds that emerge several weeks after the crop may not compete as much, and WebHADSS will overestimate the losses associated with these very late-emerging weeds.
The weed population estimates you enter in WebHADSS are based on a fixed ground area. This area can be of any shape. For US versions of WebHADSS that use a 100 square foot sample area, if you count weeds between two rows spaced 30 inches apart, you should count 40 row feet. In drilled crops, you can use any shape that is convenient for estimating population.
WebHADSS assumes that the number of weeds of each species entered will be controlled with a herbicide and not by cultivation. If the field in question will be cultivated to control weeds between the rows, then only those weeds in the row that you expect to remain after cultivation should be counted.
The number of samples to take in a field is not an easy decision to make. We know that the more samples taken, the better the estimate of the weed population is likely to be. However, there is a point of diminished returns due to the time requirement for sampling. Generally, 10-12 samples per field should suffice.
WebHADSS provides useful information in low weed population situations where the need for herbicide treatment is in doubt. It is also useful for making recommendations on fields with high populations. When weed populations are very high, it is obviously impossible to get accurate counts of all weeds. For these fields, an estimate of number of weeds will suffice because as weed numbers increase, the effect per weed decreases. In these situations, it is more important to estimate the relative populations of different weeds accurately so the best possible treatment can be recommended. |