New York investor Ivan S. Boesky and a group of companies he controls lowered their combined stake in Cooper Laboratories to 3.7% of the common shares outstanding from 8.5% . . . Phoenix-based Southwest Forest Industries said it has been informed that Jefferson Smurfit Group Ltd. has suspended its efforts to obtain Southwest's stockholder list . . . Dow Chemical said it will close its styrene plant in Midland, Mich., and supply its customers from plants in Texas. Dow said the closure was temporary but could not say when it might reopen the plant . . . The Agriculture Department reported that February's milk production totaled 10.6 billion pounds, down 3% from a year earlier.