Yes, there is a competiotin with food grains as far as renewables is considered. But the concentration should be more on non edible crops and grass variety to minimize the competition. The local government intervension is necessary to regulate the price and control without escalation. Technologies are available to convert non edible feedstocks to useful products. Every country has major fraction of waste land and they should think for effective utilization. One more solution to minimize the competition is to acquire the land by the organizations and dedicate land for crop growth.

The problem is the world population; if the world population would be low the use of grains for renewable energy make sense but with the current population it is a shame to use food for energy production; emerging technologies that use other less pregiate biomasses (not food) for energy production should set an equilibrium point

I like Luca Patauner's answer. The ethics depends on current conditions and circumstances. For example, two hundred years ago, the use of biomass as source of energy will be considered to be practical and sound. Today, human population is high and real estate farming population is on borderline enough to feed global population (not to mention very bad and erosive farming practices). But there might be exception like the corn example you mentioned. But even if biomass is thousand times more plentiful, I do not think that using biomass for energy purpose will be practical and beneficial at all in the future. As I mentioned before on this forum, it is not quantity of CO2 that we should be concerned (we should anyway) but the RATE of production (in exponential form) of CO2. Furthermore, its also a double edged sword since at the same time oxygen level in atmosphere is dropping at the same rate (in exponential form) as carbon dioxide is produced. Today, news made announcement that for the first time in 2 million years, levels of CO2 gas has reached 400 parts per million. They should also mention that for the first time in 2 million years, level of O2 gas has dropped by 400 parts per million. It does not give much room to breathe reading this news especially when this process took only two hundred years what nature created in 2 million years. Therefore, renewable energy such as biomass that require combustion with oxygen and release of carbon dioxide is not beneficial at all at the current (exponential) RATE of energy demand and consumption. We are far better off using biomass for food and civil engineering purposes ONLY and so far.