I'll guaranty you that it was not the Magazine publisher how financed the ad, typically the person who is advertising the product will pay for ads.
Now as to if Bayer is making a mistake here, I would say no. When you think about it the modern farmer is a very lucrative target market. They make their own buying decisions, most of their expenses are in consumables (Pesticides being a major one) and capital (vs labour as in most other industries) and like all business owners are always willing to buy a product capable of increasing yield.
Bayer also gets a very high yield per ad unit, after all how many people do you know subscribe to farming magazines without owning a farm? This isn't your average magazine where most units will sit on shelfs until being sent back to the publisher.
