Contrary to common belief, vegetables are in fact murder. While eschewing the flesh of sentient beings may give you that warm and cuddly feeling that comes from not being directly culpable for the destruction of creatures capable of suffering, the agricultural processes used to produce non-animal foodstuffs will still be likely to kill animals. Let's now go from seed to market and go through all the ways killing will be involved.
The first part of agriculture involves planting the seed. The seed needs to be injected into the soil and then be watered before it can germinate. In order to break up the soil so that it can be sown, sharp metal tools need to be used. This brings us to the first point where sentient beings can be harmed through agriculture. The tools used to break up the earth are sharp, and are usually applied with force so that the entire field can be tilled quickly. Any animals living in the soil to be tilled, such as earthworms, ants, or even moles or rodents can be harmed or even killed by the act of tilling, especially if it's done with mechanical tillers. While the chances of any given animal being killed per seed are small, if you multiply it by the total area tilled, chances are, quite a few animals will be killed by tilling for your oatshake.
After the seed is implanted, the crop needs to be grown and maintained. The installation of an irrigation system also involves digging, which can also result in the death of animals, but the amount of deaths from irrigation are lower, as it's a one-time process and affects less of the land. The greater amount of killing, however, in growing the crop, comes from pesticides. Pesticides are by definition intended to kill pests, so the application of pesticides will result in the death of pest animals. Some farms choose not to apply pesticides, avoiding this potential for killing, but they may also use GM crops, which will still induce animal death. They can also opt to use biological controls, which will also result in the killing of animals.
Lastly, we get to the harvesting stage. Human-harvested crops can limit much of the killing necessary in harvesting, but with machines, the same issues apply as in tilling. The heavy machine needs to roll through the fields, and it needs to use a cutting tool to harvest edible portions for the market. This can result in the deaths of animals that live in the fields. Harvesting fatalities vary per crop; fruit trees, for instance, use machines to shake fruit from trees, reducing significantly the amount of killing involved, while grain threshers mow through entire fields.
After the food is harvested, the field still needs to be made ready for new crops. Not all crops are annuals, but the ones that are need to be cleared from the field. This can sometimes be done by hand, or by machines, which will still increase the amount of animal deaths, or it can be done by burning the field, which will drive off or burn to death all the animals living on the fields, but not below it. At the completion of the task, the field is left fallow until the next crop, upon which we return to the tilling process.
I do expect to shock a few vegetarians and vegans, who thought that their all-natural organic wheat drink was good for the environment and completely innocent, but that is my intent, and I think that people deserve a greater consciousness of the implications of their survival. I read somewhere once that the Buddhists believe that all life, even monastic or vegetarian life, must involve the suffering of other living things, and I think that perspective is correct. While I don't intend to equate veganism with an all-meat diet, I do state that between a carnivorous diet, a flexitarian, a vegetarian, and a vegan one, it is merely a difference of degree. Raising animals for meat does involve more killing than just directly eating the feed, but it's not an absolute dichotomy. Knowing this, people who abstain from animal products ought to be more compassionate towards people who don't.