Some musings on this topic from a rural omnivore Buddhist of a Himalayan tradition.
I will begin by saying that in general there is no natural farmland in nature. Every farm has been created by destroying a natural ecosystem and nearly every life form residing therein. Not only has this ecosystem been destroy, but in order to maintain a farm this process of selective life-form elimination must be maintained continuously (otherwise the "unwanted" plants and animals return and interfere with the production of crops). QUAM - Personally I do not believe that the life forms destroyed to create and maintain a farm are (in general) any less valuable than say a fish that has been caught in a pond to be consumed. This is a view I have come to since moving from an urban to a rural environment. Now that I live in the country I can see that growing crops does indeed involving the killing of countless sentient beings.
CREATING THE FARM: First there is the destruction of the aforementioned ecosystem and it's maintenance, then there is the cultivation of the ground (worms, rodents, birds and even baby deer can be killed in this process). The planting of crops also inadvertently BUT INEVITABLY destroys other life forms. Maintenance of crops (even just watering and inspecting the crops) inadvertently BUT INEVITABLY destroys life forms. The reason is, because out in the country you just about can't do anything without destroying some sort of life, be it walking you your car, painting a fence or grabbing a can of nails from the shed. Which is to say that nature is chocked full of life, it's everywhere and when we drastically alter the environment to create a farm we are killing innumerable sentient beings. To maintain a farm is to be involved in the destruction of innumerable life forms. To harvest a crop involves the inadvertent BUT INEVITABLE destruction of lifeforms. To transport the food to market (think bugs on the windshield and roadkill) involves the destruction of countless lifeforms.
The markets where this food is purchased, use various lethal methods to keep insects and rodents from entering the store and interfering with customers and the produce. So, from a certain perspective we can see that a meal prepared with vegetables has a demonstrable history (a karma) of death and destruction associated with its creation. This, from a Buddhist perspective, is a tragic aspect of our worldly existence: that there is NOT a kill-free diet and that we MUST kill other lifeforms to survive. It is my belief that we cannot consume our way out of this sad truth, despite our best intentions. An omnivore going to the grocery store and purchasing some organic free-range chicken has basically paid for a farmer to raise and kill that chicken. In the same way, a vegetarian or a vegan who goes to a grocery store has basically paid the farmer to destroy an ecosystem - killing the "unwanted" plants and animals therein and subsequently any that attempt to to return.
That said, and knowing that plants themselves can be observed to display awareness (think of flowers tracking the sun), from a strictly moral perspective, I personally can see no advantage to a vegan or vegetarian diet. Innumerable beings are killed no matter what we eat. Does this mean I think we should eat endangered species or cats? No, the first does not make logical sense and the second is not within my SUBJECTIVE (conditioned) comfort level. In fact ALL diets are an expression of comfort level. For example some vegans will argue that yes they kill life forms to live, but their diets are morally superior because they are less destructive to the planet. HOWEVER, vegans (often unknowingly) have their own personal comfort levels in regard to the consumption of food. For example, we can all agree that the cultivation of tea and sugar and coffee all have negative effects on life forms (for the reasons outlined above) and yet vegans will consume these items instead of just drinking water. "All the tea in China," is an expression we know well, I say, consider, "All the tea cultivated in China." So, some folks have a comfort level of consumption that involves the eating of fish and the drinking of milk, but not cats or whiskey respectively, while others can drink cultivated tea and crops planted (who knows where) that have destroyed (who knows what), but abstain from the eating of fish and the drinking of milk.
So, I do acknowledge the ASPIRATION of vegans to consume a diet determined by moral consideration and personally I find such an aspiration to be of the highest order. However, what I OBSERVE, is a subjective comfort-zone diet (tea, juice and coffee instead of water) that is steeped in death. Furthermore, using as much logic as I can muster, I can see very little substantive difference in terms of morality and and the saving of life between the diets of vegans and those of thoughtful diet-conscious omnivores. This does NOT mean I endorse factory farming of any kind.
My conclusion (from a Himalayan Buddhist perspective)? Life forms are life forms and the consumption of ANY life form that sustains us is (at some level) a tragic aspect of our worldly (samsaric) existence. Also, in general, when it comes to the moral filtering of what we eat, I view such considerations as subjective and/or aspirational in nature and NOT ULTIMATELY nor INNATELY possessed of moral superiority. Don't eat cat's, don't eat endangered species, buy organic and don't eat too much meat. Other than that, if you eat you kill and if you want to save the planet, don't have kids. That will work a whole lot better than avoiding butter or honey or whipped cream!

A FINAL NOTE: As an aspirational PRACTICE intended to promote an AWARENESS of the suffering of ALL sentient beings, abstaining from meat, etc. can be helpful. However, since such a practice does not constitute an actually reduction in the overall suffering of beings, its morality lies not in the act of abstinence itself, but rather in the aspirational regard for ALL life such a practice engenders. ) - Pax
A FINAL FINAL NOTE: For those of you interested in such things and cited here merely as a point of interest (and not to prove a point), the Buddha himself was not a vegan. In fact, it is known he didn't even require his followers to be strict vegetarians. There is a great deal written about this online. - Again, Pax