Here is another example that I did just right now calculating how many that could be fed in Ethiopia if everyone ate a vegan diet and there wasn’t any animal agriculture.
Arable land (hectars): 13606000.0
“Only 400-480 pounds of meat can be produced by one acre of land [Of an animal that has the right to live, and shouldn’t be killed for someone’s taste], compared to 20,000 pounds of plant foods that could be grown on the same slot.” 
More specific of the current agriculture methods in Ethiopia:
“Ethiopia's potato area had grown to 160,000 hectares, with average yields around eight tons per hectare.” 
The average adult person would need about 1 ton of potatoes per year. Some of this would be replaced by vegetables and fruits in order to get all nutrition needed. Of course one can also grow other starches other than potatoes, this is just an example.
Plant foods that can be grown on the arable land:
13606000.0 * 8000 kgs of potatoes per year
People that could be fed = 13606000.0 hectars * 8000 kilograms per hectar / 1000 kilograms per person = 108 848 000 adults
Everyone could be fed on a vegan diet. Today because of that they also have domesticate, breed and kill animals for food, they don’t grow enough foods to sustain all the population.
Starvation in Ethiopia:“The U.N. World Food Program estimates that 2.7 million Ethiopians will need emergency food aid because of late rains — nearly double the number who needed help last year. An additional 5 million of Ethiopia's 80 million people receive aid each year because they never have enough food, whether harvests are good or not.”
“Samuel Akale, a nutritionist with the government's disaster prevention agency, said the hunger will get worse. "The number of severely malnourished will increase, and then they'll die."”
“Ethiopia is a country with a history of hunger. It's food problems drew international attention in 1984 when a famine compounded by communist policies killed some 1 million people. Pictures of stick-thin children like Bizunesh were broadcast onto television sets around the world.”
Quotes from: http://www.nbcnews.com/id/24731042/ns/world_news-africa/t/once-again-children-are-starving-ethiopia/
Do you know how much of Ethiopias arable land that is used for animal agriculture?
Ethiopia's total land area of 1,221,480 square kilometers, the
government estimated in the late 1980s that 15 percent was under
cultivation and 51 percent was pasture. It was also estimated that over
60 percent of the cultivated area was cropland. Forestland, most of it
in the southwestern part of the country, accounted for 4 percent of the
total land area, according to the government. These figures varied from
those provided by the World Bank, which estimated that cropland,
pasture, and forestland accounted for 13%, 41%, and 25%, respectively,
of the total land area in 1987."
Mulatu. "Agriculture" (and subsections). A Country Study: Ethiopia
(Thomas P. Ofcansky and LaVerle Berry, eds.) Library of Congress Federal
Research Division (1991). This article incorporates text from this
source, which is in the public domain..
Quote from: http://en.wikipedia.org/wiki/Agriculture_in_Ethiopia
I am advocating a vegan diet. All production of animal foods involves or indirectly causes unnecessary harm and death: http://abolitionistvegansociety.org/tavs-initiatives/tavs-articles/how-to-go-vegan/should-i-go-vegan-by-adopting-a-vegetarian-diet-first/
What about droughts???
2.With a plant-only agriculture they will be able to feed all the population and also save some of the harvest, which can be used when there are droughts, etc.
In any instance, a plant-only-agriculture is much more efficient compared to an animal agriculture - both in terms of plant protein required, water required (especially important in Ethiopia!!!), etc.:
According to Cornell University Professors David Pimental and Marcia Pimental, it takes 13 kilograms (2.2 pounds) of grain and 30 kilograms of forage to produce one kilogram of beef; 21 kilograms of grain and 30 kilograms of forage to produce kilogram of lamb; 5.9 kilograms of grain to produce a kilogram of pork; 3.8 kilograms of grain to produce a kilogram of turkey; 2.3 kilograms of grain to produce a kilogram of chicken, and 11 kilograms of grain to produce one kilogram of eggs. Livestock in the United States consume 7 times as much grain as is consumed by the entire U.S. human population and that the grains fed to livestock could feed 840 million humans who had a plant-based diet. One kilogram of animal protein requires about 100 times more water than does 1 kilogram of grain protein. It takes between 20 and 100 gallons of water to produce most vegetables or fruit. The Pimental study says it takes more than 2000,000 liters of water to produce one kilogram of pasture-raised beef. The estimates of water required for a pound of beef go from 2,000 to 12,000 gallons; it takes about 110 gallons of water to produce a pound of wheat. Most estimates vary between 1000 to 2000 gallons of water to produce a gallon of milk.
The Food and Agriculture Organization of the United Nations states that animal agriculture contributes more greenhouse gases, which are linked directly to global warming, to the atmosphere than does burning fossil fuel for transportation. According to Worldwatch Institute, animal agriculture produces an estimated 51% of the worldwide total of greenhouse gas emissions from human activity. Moreover, a significant amount of fossil energy is required to yield an animal-based product. The average fossil energy input for all animal protein sources is 25 kcal of fossil energy input to 1 kcal of animal protein produced, which is more than 11 times greater than for grain