WHY DIDN’T THE WEEDS DIE?

Last month I wrote about the critical weed-free period in corn. This month I’ll continue the theme by discussing why you may still be seeing weeds growing in your fields weeks after the corn was sprayed.

While herbicides are very effective at controlling weeds when everything goes well, the reality is that many dairy operations end up with more weed material in their corn bunk than they would like to admit. In my experience, this only affects animal performance in rare cases. However, the effect of weed escapes on yield and future weed pressure is often substantial. While cleanup sprays can help remedy these problems when they occur, the ideal solution is to try to control the weeds well the first time. There are a number of reasons why weed escapes happen – all of which are avoidable.

Spraying in the wrong conditions:

No, we can’t control the weather, but we can try to work with what we have. Even if you use a custom applicator, you can request that they avoid certain conditions when they spray. If they don’t listen, at least you can point to the application conditions if the weed control is poor. As a general rule, emerged weeds need to be actively growing in order to take in herbicides and succumb to them. Warm and humid weather, in particular, results in the best activity for many modes of action. Most pre-emergence herbicides need at least half an inch of rain to be activated. Any weeds that emerge prior to that point may become escapes.

Using too little or the wrong herbicide:

As a general rule most farmers tend to be frugal. There’s always a temptation to try to get by with less product or a cheaper product. This doesn’t usually result in the cleanest fields. While I’m not suggesting that you always need to apply the maximum allowable rate for the products you are using, substantial rate reductions need to be considered carefully depending on the weed pressure expected. Even at the maximum labeled rate, every herbicide will be weak on certain weed species. If you have that weed in your field, and you don’t have at least one other effective mode of action, you are going to get some weed escapes. Identifying your weeds makes all the difference!

Mixing and application errors:

All herbicides must be mixed and applied in accordance with the label. Skipping adjuvants, a nitrogen source, or anything else that enhances herbicide uptake by the plant will result in lower effectiveness. Additionally, mixing multiple herbicides together can weaken the effectiveness of one or more of the products, even though the additional mode of action may still be useful. Glyphosate, for instance, is a great option for killing quackgrass. However, a higher rate would be required to kill the same amount of quackgrass if you are mixing it with dicamba.

In this case, the dicamba stresses the plant as it takes in the glyphosate and less of it gets moved to the roots and rhizomes. Another thing that reduces the effectiveness of glyphosate is poor quality spray water. If not properly treated, solutes in the carrier water can get stuck to the glyphosate in the spray and inactivate some of it before you even spray it on the weeds.

If you see weed escapes in your silage corn, now is the time to take action. Identify the weeds and look at their density and distribution. That will tell you if the sprayer just missed a spot, or if there was something else going on. Look up the weather on the day the field was sprayed and try to determine what might have gone wrong. Learn from your escapes and you’ll be making fewer mistakes!

— Allen Wilder

Wilder@whminer.com