Due to declining quantity of fossil fuels and growing awareness about climate change, search for viable alternatives of fossil fuels has received a lot of attention in last couple of decades. Biofuels, from crops such as maize, is one of the potential options to replace (even partially) the fossil fuels (such as petrol or diesel). A similar idea is the use of corn for ethanol generation, which is added to the gasoline in US. The problem with the use of these crops is the fact that they are also food and fodder crops. That is, people and cattle depend on them for food. If we promote and use more of these crops for biodiesel instead of food, we may have potential shortage of food. This shortage will ultimately affect the livestock's health, productivity and population. This will negatively affect the entire food chain, as the various levels are interconnected and any 'shock' (or sudden change) at one level is bound to affect other levels. For example, the birds who are in a symbiotic relationship with the livestock will have less food due to livestock population reduction. The manure obtained from the dung of such animals will also reduce, which will either reduce the quantity of manure applied to fields (thereby reducing the soil fertility) or increase the fertilizer input (thus adding xenobiotic chemicals to the environment). Hence such a practice is unsustainable in the long run.
Hope this helps.