When asked about how to reduce the environmental impact of agriculture, many people will think of organic food as an answer. It reduces global warming by not using natural gas to make fertilizers (plus other environmental advantages, such as maintaining soil quality and biodiversity). But tractors still emit CO2, cattle and rice still generate methane (and the lot still consumes water), so it is an improvement rather than a silver bullet.
The key health advantage of organic food is that it has no pesticides (the long-term health consequences of which are unclear), which is especially important with fruit and vegetables that are eaten raw with their skins (which are rich in vitamins and minerals). A recent study also found more antioxidants in organic food (but not everybody agrees that this matters so much, see e.g. Harvard's "Antioxidants: Beyond the Hype"). The environmental benefits exist (even though I cannot quantify them), the health benefits exist (especially for raw fruit and vegetables) but organic food is more expensive.
Since agriculture is mostly about food, another possible way of reducing the environmental impact of agriculture is simply eating less (not starving, simply not overeating). Obesity is on the rise and reducing food intake by, say, 10% will reduce the environmental impact by 10% (and the cost of one's food by 10% as well), on top of curbing obesity. Putting an end to the "supersize" phenomenon is better for the environment and for health, and cheaper.
One can also consume less meat, especially beef (milk and eggs are produced more efficiently and therefore have less impact). In the US, beef production uses corn and soybeans that could otherwise be used for humans; in South America, beef directly or indirectly causes deforestation. In rich countries people consume far enough proteins already and reducing meat intake will help to reduce the intake of cholesterol and saturated fats. Again, this is about switching from too much to enough, not stopping eating meat altogether. Since meat is expensive, consuming less also saves money. Like consuming less food, consuming less meat (i.e. consuming enough instead of too much, not turning vegetarian) is better for the environment and for health, and cheaper. Also, while milk is typically our main source of calcium (a mineral we genuinely need), "it's not clear, though, that we need as much calcium as is generally recommended [moreover] high intake can increase the risk of prostate cancer and possibly ovarian cancer", so we should probably consume less milk as well.
A fourth possibility (quite compatible with the other three) is to waste less. Any food that consumers waste had to be produced (with all the environmental consequences) and had to be bought (thus costing money). Wasting less is better for the environment and cheaper (but of course has no impact on health).
Of these four possibilities (not an exhaustive list),1 three have clear health benefits (wasting less does not) and three save money (organic food costs more). Organic food, often touted as healthier, mostly benefits people who are already fairly healthy and who feed themselves fairly well2 — someone obese or with diabetes or atherosclerosis (clogged arteries) will benefit only marginally by switching to eating too much (organic) food, eating cloyingly sweet (organic) food or (organic) food full of salt and saturated fatty acids. The priority should thus be eating a sufficient and balanced diet, of the right quantity and quality, without wasting. Once we get all this right, we can certainly switch to organic food.
If wheat is not harvested because harvesting would cost more than the wheat is worth then some wheat is lost (however, not harvesting during years of overproduction because it would be uneconomical only puts a limit on the overproduction — this has a bigger impact on statistics of waste than on hunger). Likewise, food is wasted if an apple is discarded because it is too small or ugly, or when stale bread is thrown away. The basic idea of wasting food is simple, quantification less so.
One often hears that 30% or 40% of food is wasted. I don't know if such numbers are reliable (the bigger the number the more media exposure the report will get, so there can be a clear bias). In any case, they miss an important point: we should waste less food quantitatively but also qualitatively. Back in the days, inedible leftovers would be fed to pigs or chickens; as much food is lost from human consumption this way, but it is not lost as much. Today, peels, leftovers, etc. can be composted; again, not as good as eating them but not as bad as throwing them away (especially if they end up in a landfill, where they produce methane and warm the planet). If all the food being thrown away were fed to animals or composted, then as much food would be wasted as today, but it would not be wasted nearly as much.
It is often mentioned that producing 1 kg of meat protein requires feeding 2–10 kg of plant protein to the animals. Meat is thus quite wasteful (beef more so than chicken). If meat consumption were divided by two for instance, instead of having 100 g of meat protein, one would have 50 g of meat protein plus 100–500 g of plant protein — quite a lot more food!
Yet this argument assumes that the feed is edible by humans. In fact, cows raised in pastures in mountains or in lowlands where no crop would grow do not take land away from food production for humans. Turning 10 kg of grass into 1 kg of meat does not waste 9 kg of food, rather it creates 1 kg of food. (Also see the last chapter in Vaclav Smil's "Should we eat meat?".) I would like to see people who disagree with this argument eat the grass themselves.
But purposefully wasteful production is not limited to meat: if a low-yielding (but perhaps better-tasting) cultivar is chosen instead of a higher-yielding one, is food not also wasted? A good example is wine: a plot of land where 1000 liters of wine could be produced in fact yields half as much because of the use of a grape variety that has more appeal to wine drinkers. This is not very different from selecting the best grapes to improve quality and discarding the inferior half (which clearly qualifies as wasting half the harvest).
And what if the yield is kept artificially low because techniques that could raise it (e.g. fertilizers or pesticides) are not used? In both cases less food gets eaten than could be (just like with meat). So, is organic agriculture on the wrong side of the fence, in eschewing productivity?
1 And then there is everything else. Unhealthy food is neither intrinsically better nor intrinsically worse for the planet. While corn syrup is a spearhead of (sweet) processed food, it is merely processed corn (and if processing is evil in itself so should be fruit juice or pasta). In Europe, the Common Agricultural Policy (CAP) subsidizes the production of oil and sugar rather than fruit and vegetables. These problems deserve to be tackled in their own right, even if they have no major environmental impact.
2 Also see B. Parmentier, "Manger tous et bien": "le pain complet et les pains fantaisie au seigle ou autres céréales sont bien meilleurs, la différence (en termes de bénéfices santé) entre le pain complet et le pain blanc étant sans commune mesure avec celle existant entre le pain « normal » et le pain « bio »." (p. 239).