A Taranaki dairy herd recovered from salmonella overnight after a pelletised mineral supplement was withdrawn from their diet last spring.
A national study has since found pelletised magnesium supplement presents a high risk of salmonella in dairy cattle.
Salmonella bacteria are present in low numbers in the gut of many cows.
Affected animals spread the bacteria in large numbers in their faeces and milk.
Often only a few animals are infected by the original source and the disease spreads because infected animals contaminate water or feed. Salmonella bacteria can live and multiply for months in wet, swampy ground.
Symptoms include a sudden milk drop, high temperature, depression and a very watery scour. Some animals die.
Brian and Marian McDonald were among 16 Taranaki farmers who suffered large losses during a salmonella outbreak last spring.
The week before Christmas, the couple learned of a strong link to the disease from minerals in the supplementary feed they were giving their cows.
They had just spent $45,000 on a new in-shed system that supplied their cows with palm kernel meal, biscuit meal and minerals. After they removed the pelletised mineral, the health of their 280-cow herd changed dramatically.
"They stopped scouring within 24 hours," McDonald said. "The weather was rough when I stopped the mineral - not the conditions I would associate with cows firming up."
They had already sought professional advice about removing the minerals and were told they were unlikely to be the cause. "If I'd removed the mineral straight away, the cost would have been minimal, but that was the last thing to do, on the advice I was getting."
McDonald said the outbreak cost the couple at least $130,000 in lost production, dead animals, veterinary bills, the purchase of calves and a milk capacity adjustment payment to Fonterra.
"The cows were scouring the whole spring. I was constantly taking their temperature.
"I thought it was the time of year. It was reasonably wet and there was a lot of moisture in the grass. 'The cows were really loose and were going off their milk, but they'd come back."
Initially, he wasn't too concerned, and thought they would come right.
"When it really took hold, the numbers with salmonella went from 10 one day to 50 in three days and 200 within a week.
"Looking back now, I wonder how I handled it. I didn't know the cause, how to fix it, or if it was going to go away. Getting up in the morning was scary because I never knew what I was going to see."
McDonald said the smooth season this year highlighted to him how sick his cows were last year. Some of his cows were infected with salmonella twice and a few contracted it three times.
Worried about the effect of the disease on the herd's in-calf rate, he thought an empty rate of 10 per cent, after being 6 per cent in 2010-11 was not too bad, even though it almost doubled.
Only a third of the herd got in- calf in the first six weeks of artificial breeding last season, so this season's calving was spread out.
However, he received a dispensation to induce more than 4 per cent of his cows and calving was finished by the first week of October.
The outbreak appeared to have had little other effect on the herd this year.
"AB [artificial breeding] has been really good. I had a 92 per cent submission rate in the first four weeks and used no CIDRs [hormone treatment], so I'm more than happy."
At present, his cows were producing just under 2 kilograms of milksolids a day.
His production last season was 7 per cent down on the 2010-11 season, but he estimated it was 10 to 12 per cent below what could have been achieved in such good conditions.
This season's spread-out calving had lowered his early production, but it was now 8 per cent ahead of last season. Production this November was 70 per cent ahead of November last year when the herd was hard hit by salmonella.
In one five-day period last November, he supplied no milk to Fonterra and at times he was milking only 80 cows.
He said he was "extremely edgy" at the start of this spring.
"If the cows were slightly scouring, I wondered if it was was starting again."
He now gives his cows magnesium in a powder form.
Any farmer facing such a situation should not try to sort it out alone, he said.
Talking to others affected by the outbreak made a huge difference at the time.
It was also important to get away from the farm, he said. Watching his granddaughter have a swimming lesson was therapeutic.
He was still amazed at the support the couple received.
"Farmers were offering me cows - even for this season if I didn't get enough in calf - and they brought food to the house so we didn't have to worry about meals.
"The support was incredible. It certainly helped us get through it."
Mrs McDonald said the unknowns about the outbreak last spring were emotionally exhausting. Her husband's many interests off the farm helped him cope.
Magnesium pellets linked to salmonella outbreak
Feeding a pelletised magnesium supplement to dairy cattle poses a high risk for a salmonella infection, a nationwide study has found.
The study of 110 dairy herds shows the use of continuous troughs, pelletised magnesium supplements and palm kernel meal can increase the chances of a salmonella outbreak similar to the one that affected 16 Taranaki dairy herds last spring.
Massey University epidemiologist Mark Stevenson attributes part of the rise in the number of salmonella outbreaks over the past three or four years to the intensification of dairy farming.
He said the association between the use of pelletised magnesium supplement and increased salmonella use was discovered in a small study of Taranaki herds he undertook during last year's outbreak. The link had not been reported in international scientific literature before.
In the national study, farmers were asked about herd demographics, the amount and type of feeds offered including mineral supplements, the storage and mixing of ingredients and effluent management.
Stevenson said it appeared both the amount and the formulation of magnesium used on farms had a role in triggering the disease.
He is advising farmers to seek advice from vets or nutritionists on the dosage and formulation of magnesium.
Cows need extra magnesium in spring to prevent grass staggers, because the level in pasture falls during rapid growth.
He said that given the strong link between pelletised magnesium and salmonella risk, Fertco's voluntary withdrawal of its product, Mineral Boost, from the market in January for redevelopment was an important intervention.
Monitoring the incidence of salmonella using data from veterinary diagnostic laboratories, would show whether the removal of the product was successful.
Already, preliminary data showed the number of cases so far this spring was well below the same time last year.
Because continuous troughs provided several cows access to feed at once, a daily variation of supplement intake per cow was likely.
Fluctuations in intake could affect the balance of bacteria in the rumen, allowing salmonella to multiply and trigger the disease, he said.
But he is still advising dairy farmers to ensure they store and handle all supplementary feeds, in a way that reduces the likelihood of contamination from birds and rodents.
He said others who worked with cattle feed, such as feed importers, transporters and compounders, should also adopt proper storage and handling methods.
Significant inroads into understanding last spring's salmonella outbreak had been made in a short time because vets, Massey University, Fonterra, the Ministry for Primary Industries, the New Zealand Veterinary Association and the industry had adopted a co- ordinated and collaborative approach to it, he said.
- © Fairfax NZ News
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