On the farm about two years ago, his Labrador-cross Oscar licked some leftover milk replacer in a calf feeder.
About 30 minutes later, after noticing Oscar panting and dragging his legs, he rushed him to a veterinarian.
The vet gave Oscar medication to make him throw up and discovered the milk replacer in his vomit. Cotter was unaware some milk replacer was toxic to dogs.
"There's no treatment for it - they have to give them an intravenous drip and you hope for the best."
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After Oscar recovered and returned home, Cotter inspected the bag of replacer and discovered a warning to keep it away from dogs on the bottom of the bag "in very, very, very small writing - smaller than the instructions".
To stop Oscar from eating any more of the replacer he installed a range of strict measures, such as locking it away.
"You don't want to take any risk - he's a member of the family."
Despite his vigilance, he noticed Oscar panting, with limp limbs, last month. He was rushed to a vet and made to vomit, which showed the nearly 40kg dog had ingested a tiny amount of the replacer. As the vet worked on him, Oscar's heart rate dropped and he died.
"It was just enough to shut his heart down - the poor bugger."
His vet told him as Oscar's heart was damaged from previously eating milk replacer, existing scar tissue on his heart would have ripped from the second attack.
Oscar was 6 years old when he died and had been part of the family since he was rescued from the pound. "He's been part of my life since he was a puppy."
NZ Veterinary Association head of veterinary services for companion animals Sally Cory said some calf milk replacers contained ionophores, compounds added to reduce the risk of coccidiosis infection in calves.
Ionophores could be toxic to species such as alpacas, cats, dogs, cats, goats, horses, llamas and sheep. Dogs were particularly susceptible, and clinical signs could develop within 12 hours of ingestion. Neurological signs usually developed first with generalised weakness and wobbliness.
Dogs would develop weakness in all four limbs, which could progress to full paralysis and lead to breathing problems. Drooling and a loss of tongue control were common. Often a dog's pupils were different sizes and it became especially sensitive to noise and sudden movements, and its body temperature would rise.
Milk replacer should be mixed at the correct ratio of water to powder for the age and weight of the animal, as indicated by the instructions. Open bags should be stored in a container which could be completely sealed and out of reach of all animals.
Farmers should never leave empty bags or storage containers lying around as the remnants of the product could be toxic, Cory said.
All equipment used to prepare and store milk replacer should be immediately cleaned to ensure dogs were not able to access milk remnants.
If a dog accidentally ingested milk powder, it should be taken to a vet, where it might be made to vomit to help retrieve some of its stomach contents and reduce the absorption of the potential toxins.
Activated charcoal might then be given to help bind any further toxins within the gastrointestinal tract and reduce the risks of absorption.
"If clinical signs develop then unfortunately there's no specific antidote."
Treatment would be supportive and often intensive. Animals might be in hospital for extended periods and could be left with ongoing heart problems if they survived.
