what is pre emergent herbicide

{"title":"Acetochlor 95% TC\/Used pre-emergence\/pre-plant to control annual grasses","imgUrl":"https:\/\/img.chinax.com\/nimg\/76\/bf\/a9882d7b9330c96805cd94463dfd-200x200-1.jpg","attrs":{"Price":"depend on the market supply","Brand Name":"Qiaochang","Model Number":"AC-01","Minimum Order Quantity":"1000L","Delivery Time":"25-30days","Certification":"SGS","Place of Origin":"China","Payment Terms":"T\/T\/L\/C\/OA\/DP","Supply Ability":"60000T","Packaging Details":"1L\/200l or according to customer requirement","Acetochlor":"Homogeneous liquid "}}