You have probably read that wild horse herds grow at a rate of about 20% per year.
If you start with 100 horses, 20 foals will have to be born over the next year to bring the herd to 120, a 20% increase over the original 100. That’s if none of the adults die. A census would reveal that foals accounted for roughly 17% of the total.
If adults die at a rate of 5% per year, 25 foals would have to be born over the next year to bring the herd to 120. A roundup would yield 95 adults and 25 youngsters, with foals accounting for approximately 21% of the total.
When you look at gather stats and see foals representing 13% of the total, that herd may not be growing at a rate of 20% per year. Similarly, if you see foals accounting for 26% of the horses captured, that herd probably doesn’t conform to the 20% rate.
By how much can the death rate vary and not have an appreciable effect on birth rates?
Western Horse Watchers Association 
Would you please cite your sources for the information on wild horse herd growth rates. Thank you for your assistance.
The growth rate is often quoted in EAs for wild horse roundups. For example, search the Pancake Draft EA for ‘20%’, https://eplanning.blm.gov/public_projects/2003386/200393740/20029353/250035554/Pancake_Prelim_EA.pdf. You can also search the BLM news site for ’20 percent’ and look at the news releases involving wild horses, https://www.blm.gov/press-release.