On a super basic level, poverty and malnutrition among famers and fishers has reduced worldwide during the last 50 years of so while these agro conglomerates have been in action. Of course this doesn't mean the agro conglomerates aren't doing harm - they could have been slowing the impovement during this time.
The gross figures (from Raj Patel, Eric Holt-Gimenez, George Monbiot, and others) are as follows: the percentage suffering from under nutrition has declined; however, the number suffering from under nutrition and especially malnutrition has grown alongside a large growth of calories per capita. There is more money to be made selling soy and corn to beef and poultry companies than to poor people. Grain is dumped into Asian fisheries while fishing families, especially women and children suffer from malnutrition; menstruating women and girls are especially vulnerable to anemia from a diet lacking iron. To earn needed cash fishers sell the best parts and suffer from malnutrition. I can get more precise references if you like.
Leaving aside the issue of growing under nutrition and malnutrition, we should look at other effects of corporate farming. Undisturbed soil has a natural fertility which is destroyed by ploughing, herbicides, and pesticides. (The first three chapters of Monbiot’s Regenisis are very good on this.) Corporate farming substitutes petroleum-derived fertilizers to compensate for the loss of natural fertility. Besides polluting waterways and creating dead zones in oceans, these fertilizers contribute more to climate change than all the fuel used in transportation.
Traditional peasant farming produces more calories per unit of land than does corporate agriculture. The latter’s advantage is that it produces more calories per worker than traditional farming, hence more profit for capitalists, incentivizing land grabs and pushing peasants onto increasingly marginal land.
Peasant farming is both labor and knowledge-intensive. The farmer must be a practical scientist studying by trial and error how to get maximum production without chemicals; she must be a student of the soil, of various crops and their effects on soil fertility and control of pests. Monbiot is particularly good in explaining how this works.
There's a lot of good, old, semi-formal content on the GiveWell blog: https://blog.givewell.org/ If you do some searches, you may be able to find the subject touched on.
I'm not sure if they have done any formal review of the subject however.
I couldn’t find anything at Give Well searching the names of the most prominent food radicals.
I am trying to find literature where Effective Altruists engage the arguments of the food radicals (Eric Holt-Gimenez, George Monbiot, Raj Patel, Wilma Dunaway, Philip McMichael, IPES Food). Can anyone help me? I am particularly interested in replies to the radicals’ arguments that poverty, malnutrition, and hunger among farmers and fishers are caused by grain-dumping and the actions of seed, chemical, and agro conglomerates.
My best guess is that EA didn't really engage much with this, because they never engaged much with starvation relief as an intervention.
The closest thing I can remember is holding up Norman Borlaug as an example of a very impactful career (implicitly in favor of bioengineering food).