How long after using crabgrass preventer can I use a fertilizer?Also, will using a broadleaf weedkiller on new weeds have any effect on the shield made by the crabgrass preventer. Thanks.
There is no problem applying crabgrass preventer with lawn food. In fact they are often sold as a blend to be applied at one time.
Your crabgrass preventer should be applied within the next couple of weeks.
More on timing you crabgrass preventer application HERE https://blog.timesunion.com/gardening/finally-its-time-for-crabgrass-preventer-step-1-to-many-of-you/7124/
Even though it is warm out this week, it is too early for a weed killer application. Nighttime temperatures need to be above 50° regularly for the weed killer to work. That is more a mid-May or later application depending on the weather. More on timing your weed killer application HERE https://blog.timesunion.com/gardening/winning-the-war-on-weeds/6652/