One licence has already been issued by Alberta Environment to capture up to 200 feral horses in the central Alberta area around Sundre.
That angers opponents who worry that many of the horses are destined for slaughterhouses or will die during the roundup.
"Why are we killing our horses? We live in Alberta. We pride ourselves on horse culture," said Anita Virginello, one of about 50 protesters in Calgary on Thursday.
"We're home to the Calgary Stampede, numerous rodeos and ranches."
She suggested it is hypocritical for Alberta to promote its cowboy culture when the province is "the horse slaughter capital of Canada."
"There's duplicity in that and it has to change."
The government says the feral horse population is continuing to balloon and numbers need to be balanced.
"It's actually a capture ... not a cull. What we're trying to do is balance the users of the grassland in that area," said Carrie Sancartier, a spokeswoman for Alberta Environment and Sustainable Development.
"The feral horses eat the grass, but (so do) wildlife such as deer and elk, and this grass is quite sensitive to overgrazing, so we have put in place a capture season to remove a small portion of the feral horses."
Sancartier said the number of horses in the Sundre area increased to 980 last year from 778 the year before. The department is confident about the count, which is done at the same time every year by helicopter, she said.
The horses are descendants of domestic animals used in logging and mining operations in the early 1900s.
Joe Anglin, environment critic for the Opposition Wildrose party, said there isn't enough scientific evidence to support a cull.
"We don't have answers to any questions and now they're going to move forward and cull the herd," said Anglin, who represents the Sundre area in the legislature.
"If there's about 1,000 horses ... what's the appropriate size of the herd for the habitat and what we have? How do you make a decision if you don't know if the habitat and the range can sustain the existing size?"
The Alberta government last issued a capture order in 2011 and 216 horses were removed. Sancartier understands that some people are upset.
"It is very emotional and we certainly understand that, but we're also trying to manage the resource for all users, the feral horses as well as wildlife and livestock."
Anglin said most Albertans are probably opposed to the plan.
"A lot of Albertans identify with the horse culture. It's something that's sort of germane to our past of independence and strength.
"It fits into the Alberta psyche, so there's a lot of emotion attached to the issue."
The roundup is being allowed until March 1.
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