The German chemical makers Bayer and Evonik Industries will together invest $200 million in two chemical units in Mobile, Ala. Both facilities will produce precursors used to make Bayer’s Liberty brand glufosinate herbicide. In addition, Bayer will spend $50 million to expand production of glufosinate in Muskegon, Mich. The projects are part of Bayer’s plan to double its global production of the weed killer, used mainly on canola, corn, cotton, and soybeans. The rise of weeds resistant to rival herbicide glyphosate, known as Roundup, has spurred demand for glufosinate as well as for seeds engineered to be tolerant to both herbicides. The company expects the additional production to start in 2017.