Many dairy farmers were facing issues of bloated calves this season, she said.
"We have had awesome grass growth this spring."
Mrs Hanning and her husband Maurice milk about 650 cows on Bristol Grove Dairies in Grove Bush, northeast of Invercargill.
A couple of seasons ago, calves up to 10 weeks old were bloating and dying.
The newborn calves had been given as many feeds of high-quality colostrum as possible and were given access to fresh water, meal and hay in a calf shed.
Milk was always available to calves.
"They can drink as much as they want whenever they want so they don’t guts themselves and drink little and often."
The calves, at the age of 4 weeks old had their milk supply limited to up to six litres per day, depending on their breed.
By limiting the milk intake, it encouraged the calves to eat more hay and meal to stimulate and increase the volume of its rumen to prepare them to move on to pasture.
A couple of days after the calves were released from the shed on to lush pasture, it was noticed a calf was bloating.
The calf died the next day, another the following day and a third was bloated and lying on its side.
Mrs Hanning had a background in animal science and hoisted a dead, bloated calf to hang upside down on a front end loader and performed a post-mortem.
She discovered symptoms of an ulcer in the abomasum, a compartment in its digestive system.
Further investigation revealed the abomasum had a hole in it and "red, angry-looking" tissue around it.
A greenish slimy material was floating in the gut cavity.
"We changed their diet too quickly. We went from having lots of roughage and carbohydrates to having lush, post baleage, regrowth grass."
As the calves had no access to hay, they were eating the "juicy" grass, which would have contained less than 10% of dry matter.
The calves had not been chewing the grass enough, she said.
A lack of chewing lowered the sodium bicarbonate levels in the gut, changing the pH and causing an ulcer in the abomasum.
"They end up having a gastric catastrophe."
In response, they restricted the access to the pasture and offered hay.
The move resulted in an immediate change in the calves, which were no longer bloating and dying.
Calves were delicate and did not handle rapid change well.
"Whatever you do, do it slowly and carefully."
She urged farmers to "be curious" and investigate if an animal had fallen ill and died.
"You’d be surprised what you can find with a pair of gloves and a craft knife."
She encouraged people to take photos of the investigation process.
"If you come across something weird, it’s documented and you can get someone who’s a bit more skilled to have a look and help you."
A solution was often a lot less complicated than first thought.
"In this case, hay. That was it, simple."
