Among other things, the app can also work out how many tonnes of dry matter are contained in a silage stack or how much grass cover is needed in a particular area.
Mr Hathaway said all the grazing equations in the app were ones commonly used by farmers but instead of having to carry a calculator or use a pen and paper, he could now do it all on his smartphone. Because so many pieces of information go into working out the grazing, an app can be more straightforward than a calculator, he says, and eliminates error when working out the complicated equations.
Mr Hathaway got the idea when he was working out his own grazing and began wondering if there was an app which would do the job for him.
There wasn't anything that was simple and suited to New Zealand, so he turned to his sister.
"He said 'do you think you can make this?'," Miss Hathaway said, "and I said 'yip'."
They spent about three months developing the app, and Miss Hathaway made Apple and Android versions.
She said it was important to keep it simple so it would be easy to use on the farm, but still allowed farmers to compare grazing scenarios by changing variables such as paddock size without having to redo the calculation.
Once she had come up with a prototype, the pair tested it and gave it to friends and other farmers to try out.
"We had a lot of people when we were first developing it and showing it to people, saying it was such a good idea," Miss Hathaway said.
The app costs $2.59 because the pair wanted to keep it affordable, and they're hope as word spreads, sales will rise.
