That's certainly true of other animals, and historically true for humans as well. See the population boom in Europe after introducing super foods from the Americas (potatoes, corn) which enabled calories grown per acre to double. However, in developed nations food is only one part of people's expenses. Food would be cheaper if there's more land available, but it won't get hugely cheaper, because it still has to sell for more than what it costs to grow it. Also much of the land used for livestock grazing is unsuitable for farming - too arid or poor soil. I don't think at the end of the day it would affect people's decisions about how many children to have in any material way. The other considerations around having children would dominate the decision making. Even unrealistically large changes to the cost of food would not likely factor into the top 3. The developing world is another story all together, but those countries are changing and will eventually more closely resemble the developed countries. Any affect on those countries would be short-lived (but still encompass multiple generations, which could have a huge effect on world population.)