This is a huge deal, one of the biggest in any industry. Bayer or whatever it will be called, could potentially control almost a quarter of the worlds' food and pesticide supplies. Farm incomes are dropping and now grain prices are very low. So having that much market control could hurt farmers by making them spend more on seeds and fertilizer/pesticide.
Yes, Bayer's large market share will hurt farmers and everyone else in the U.S. Monopolies are not meant to exist and the huge merger will create less competition. This will be bad for the economy in general. The government should not have allowed the merge to take place and will hopefully break it up.
There are a couple of reasons to be concerned about an agricultural landscape dominated by just a handful of giant companies. If firms can corner key markets in seeds and chemicals, they might be able to raise prices of their products on farmers, which in turn could make food more expensive.
Clearly, Bayer's takeover of Monsanto will hurt farmers. Anytime competition is reduced, Especially in the form of mergers or acquisitions, the consumer is the first to feel the pain in the form of higher prices and more expenses. Of course, those associated costs are always passed on to the end consumer.