Pre-Emergence Harrowing – Michael Miller
Pre-emergence harrowing works well to control early germinating weeds that have emerged before the crop has emerged. Pre-emergence harrowing is sampling running a set of tine harrows through the field before the crop has emerged through the soil to remove some of the early germinated and emerged weed seeds. This technique can be used for basically any crop that you want. One study showed that pre-emergence harrowing caused a 26% weed density reduced and a 22% weed biomass reduction (Brandsaeter, 2011).
Pre-emergence harrowing works best when you set your harrow properly based on the type of soil that you have and the amount of weeds there are growing in the field. If your have a hard packed clay soil it will take a more aggressive harrow pressure that than a sandy soil that is more loosely packed. Using harrows with bent tines usually works better for pulling up weeds and you need to ensure your harrows aren’t digging into the ground to deep and affecting the emerging crop.
The biggest problem to avoid with pre-emergence harrowing is being able to scout your field and ensure your crop is not emerged because if it is just beginning to emerge out of the soil it is very sensitive and cannot take much stress. If you harrow to late then you will cause more damage to the crop than the benefits from harrowing. Also making sure you do not how to much pressure on the tines so they are digging in and hurting the emerging seedlings is also important problem that needs to be avoided.
Harrowing your fields is relatively cheap, harrows themselves are not expensive implements to purchase and don’t take much to maintain. Fuel is the biggest cost associated with pre-emergence harrowing.
I believe that pre-emergence harrowing is something that can be fairly important to weed control but there is some controversy behind it as well. Some studies have shown that harrowing can also promote germination of more weed seeds that have not already germinated once you go to harrow your fields (Brandsaeter, 2011). But overall it can remove a lot of competing weeds.
Brandsaeter, L.O. K. Mangerud, J. Rasmussen. Interactions between pre- and post-emergence weed harrowing in spring cereals. December 2011. http://orgprints.org/21137/4/21137.pdf