When it comes to pre-emergent, all blends are not created equal. There are three important factors to consider when selecting a pre-emergent to protect your yard from annual grasses; coverage, chemical, and concentration.
Coverage is very important to the efficacy of granular pre-emergent. In order for the chemical to be taken into the plant it must come in contact with the weed's root system once it germinates. If the spacing between particles is too great, the weed seedling may grow too large before it absorbs the chemical and root growth is shut down. This is how the chemical remains selective in your lawn. The mature grasses in your lawn are not impacted by the chemical at the surface because their root system is mature and fully developed below the barrier. If the weed seedlings get mature enough before they contact the herbicide they may tolerate the herbicide, which results in breaks in control. I recommend applying at least 4lbs per 1000ft2 to ensure adequate coverage (remember always read and follow label directions). To give you an idea a 50lb bag should cover no more than 12,500 ft2, any higher coverage per bag and the risk of a control break increases.
Chemicals commonly used as pre-emergent in home lawns usually contain trade names like Dimension (dithiopyr), Barricade (prodiamine) and Halts (pendamethalin). The standard for years has been pendamethalin. These products typically last for three months and will degrade fairly quickly in homeowner type products. This is where the saying, "Apply your crabgrass preventer when the redbud trees bloom" comes from. If you applied the chemical too soon the chemical would break down before the peak of crabgrass season resulting in multiple breaks in control. If you applied it too late the seedlings would be too mature to be affected by the chemical. This is why you will often see a recommendation of two applications on the label 60 days apart. When barricade and dimension came onto the market they were more expensive than the pendamethlin types. however, at a full rate they could be applied earlier because they don't degrade as fast in cooler weather and are less water soluble than pendamethalin. Barricade and Dimension can last for 5 months at a full rate. This is why you need to be careful when you purchase the product.
Concentration, when dimension and barricade entered the homeowner market they were generally released at half strength to compete with pendamethalin's lower price and shorter window of control. It resulted in a cheaper more comparable product but arguably not a more advanced one! In the commercial industry these are called split application products. They contain roughly .10% dimension or .20% barricade. Compared to a full rate of about .20% dimension or .40% barricade. These split applications are available all over town and cost less, but to be completely effective must be applied twice, once in March and again in 60 days. Some of these products may claim "season long control" but that depends on how long the growing season is where you live. What works fine in Wisconsin may not be inadequate in Kansas. Most commercial companies who use a split applications will come back with the second half of barricade or dimension in May. Commercial companies will usually combine the second app with a broadleaf treatment. They do this to insure adequate coverage. Premier Farm and Home sells both the full strength version and the split applications but we recommend the full version for a convenient one time application.
Hopefully this will help you sort through the multitude fertilizer/pre-emergent combo's on the market. Remember at Premier Farm and Home we sort through all that for you! We stock the full strength line of pre-emergent to make sure you get the best control possible for the money.