It can sometimes be tricky to decide whether there is a need to go in for a application of Glyphosate or Liberty on your canola crop. There are a number of things that must be considered before pulling the trigger on another application.
The first is simply is there a need for it? Lots of growers use canola as a way to clean up their field so they automatically go in a second time no matter what. This isn’t always the approach to take. You must go out into your field and check to see if there are any weeds actively growing. If there are more than a few here or there then you can assume that going in again is a possibility, but consider the second question.
How big is the canola crop? This question must be answered for a couple of reasons. The first is that if you are spraying a chemical such as Liberty which is a contact herbicide, coverage is key to killing the target plants. If your crop canopy is large enough that getting enough contact for the herbicide to be effective is a very low chance then automatically scrap going back in. Even with glyphosate you must be able to get adequate coverage of the target weeds. The second reason you must consider the size of the crop is because canola is a very competitive crop and if it is much larger than the target weeds then there is a very good chance it will out compete it causing no yield loss to you and even the potential for the crop to choke the weeds out.
The third thing to be considered is how big are the weeds? As stated before if the crop can out compete the weed then it probably isn’t worth getting going back in. But some growers may be concerned about the potential of the weed feeding the weed seed bank in their field. For example if it is already June and you are simply concerned about a large number of 1 leaf wild buckwheat in your field and don’t want to have to worry about them going to seed then you probably aren’t in a position to worry. The likeliness of those seedling wild buckwheat making it to the point where they produce viable seed probably isn’t a big enough concern to force you back into the field for another herbicide application.
Even if you have a thin plant stand and think you can get good weed coverage, spraying a herbicide to late can have a negative impact on your crop. I have personally witnessed Liberty sprayed on a thin Invigor stand at flowering set back the canola crop significantly. With that said I have also witnessed glyphosate sprayed on RoundUp Ready canola at flowering and not noticed a significant loss in plant health. When making your first application Invigor canola is registered to take a shot of Liberty at the cotyledon stage. This is typically ok with glyphosate resistant varieties as well, however, it is generally considered to be a little safer to wait until the 1 leaf stage in glyphosate resistant crops. I have always heard that the farther down the line a Roundup Ready crop is in terms of its breeding lineage, the better it is at standing early or heavy doses of glyphosate.
Taking into account all of these considerations should allow you to decide whether a second herbicide application is warranted.