The ACT government begins its annual cull of kangaroos on Monday, closing a number of reserves overnight to allow shooters to kill up to 3253 Eastern Grey Kangaroos.

A thousand more kangaroos will be culled than last year as shooters target new areas, such as West Majura and Aranda.

News of the cull came as the first results of a two-year trial into managing the kangaroo population via fertility drugs were published, showing a reduction in the number of kangaroos breeding.

The $500,000 trial looked at whether the GonaCon vaccine, which triggers an immune response that stops the normal function of the ovaries and testes, could be administered remotely via dart.

The trial also compared the effectiveness of using the dart versus hand-injecting the vaccine.

Previous research had indicated the vaccine was less effective when administered by dart, possibly because the immuno-contraceptive is more widely spread throughout the body.

However injecting by hand requires the animal to be captured and sedated, and can take four to five hours and a team of up to six people. Administering the vaccine by dart only requires two shooters and a driver, and can take as little as 10 minutes per kangaroo.

A total of 148 kangaroos were captured, fitted with collars and given the vaccine during the trial - 81 by hand and 51 by dart. Ten were given a placebo.

Thirty-three kangaroos died during the trial - two as a result of darting injuries and one from overheating after recovering from anaesthesia. Nineteen of those died after a car crash and one had to be euthanised after it was fished out of Lake Burley Griffin.

Of the surviving kangaroos given GonaCon by dart, only 20.8 per cent had young in the year following treatment. At the time the dart was delivered, 96.1 per cent had a joey.

Of those given the vaccine by hand, just 13.3 per cent had a joey in the year following treatment, and no kangaroos that had the hand-injected vaccine had a joey the year after that. When they were given the vaccine 92.3 per cent had young.

ACT government ecologist Claire Wimpenny said their focus now was on monitoring the animals to see how long the vaccine lasted.

Sub-adult kangaroos injected with the vaccine in 2008 in another trial were still infertile, she said.

"Those animals that did breed in the first year had quite large pouch young so we suspect they’d been able to breed again before the vaccine became active," she said.

"We believe the sweet spot is around 30 days, before those antibodies are reaching levels that are causing infertility."

Ms Wimpenny said fertility control was not a replacement for culling, as it did not reduce the population of kangaroos in a short time.

"We think the best way that fertility drug would work is you cull your population down to the desired level, you'd then come in with your fertility control and hopefully that would mean you won’t have to cull as much and you didn’t need to cull as frequently," she said.

ACT Parks and Conservation Service director Daniel Iglesias said they believed it would be cheaper to rely on the fertility drugs over firearms in future.

"Potentially in the future by bringing reserves to sustainable levels if we have the fertility trial we may never need to cull again using firearms in those reserves," Mr Iglesias said

"It’s a big 'if' and the science still needs to be proven but if we can bring reserves to sustainable levels we can look to fertility to keep them there and hence not have to use firearms."

Mr Iglesias said the cull "protects biodiversity and maintains kangaroo populations at appropriate levels".

He said ecologists analysed each reserve's kangaroo population, vegetation, and rainfall among other factors to determine what a sustainable number was.

"It’s been just over 10 years now since we’ve been culling and what we’ve been finding now is now in area where we consistently cull every year we’re having to cull very little animals or not having to cull at all because we’ve arrived at a sustainable number," Mr Iglesias said.

There were 2592 kangaroos killed in last year's cull, after authorities set a target of 2606. That was the highest since 2011.

The 2011 cull saw 1989 kangaroos shot and another 800 pouch young killed. Contractors are required to kill young by clubbing them in the head.

The cull has been controversial in the ACT. It was recently discovered kangaroo shooters had been using illegal silencers since 2009, but had been given permission by the ACT's justice directorate.

Parks and Conservation recently floated the idea of hunters no longer needing an annual license to take part in the cull, instead cementing it as a regular event.

The following sites will be closed during the cull:

Sites will re-open Friday July 27, or earlier if the program is completed sooner. Special opening hours will be in place for Mulligans Flat Woodlands Sanctuary to allow its night-time tours to continue.