When using any herbicide, carefully read and follow the label directions for application procedures and animal withdrawal times. I am not really familiar with MSMA because it is more often used in cotton fields than in pasture applications. However, the label information I obtained states that the field should not be cut for forage use or grazed for 5 weeks following application of MSMA. Because there are many different formulations of MSMA, be sure that you follow the label directions for the specific formulation you intend to use. MSMA is an arsenic-containing herbicide, so proper use and withdrawal times are important. After the MSMA application, using the Grazon herbicide, as needed, should control weeds while allowing the horses to remain on the pasture.