Paul Mercer told Jackie Harrigan about the process of understanding and addressing metabolic problems in the Manawatu herd he manages.

Solving the metabolic problems around calving cows helped Manawatu farm manager Paul Mercer halve the incidence of milk fever and ketosis in his herd and reduce the empty rate for the following season by a third.
Paul manages the 410-cow herd on Garvaghy Farm at Rangiotu for Robert Ervine and Colleen Sheldon. Now in his third season, Paul remembers back to that first season with a shake of his head.
“We had a lot of metabolic problems – 20% of the cows had clinical milk fever followed by ketosis and retained membranes and it had a big impact on production, but then more problems affected mating performance.”
“We dusted the grass and the hay but the cows were still going down – it was really exhausting.”
Paul says he had a recipe for getting the cows up again and that “we got pretty good at it”.
“I usually put one bag of calcium in the ribs first and then a bag in the neck vein – you have to make sure there is no air in the bag.”
“But a couple of down cows take an hour out of your day, especially if you have to hip lift them.”
The six-week in-calf rate dropped to 62% that season and the empty rate sat at 18%.
“We had multiple issues that season – after the metabolic problems at calving then 18 cows went down at mating, the metabolic issues just carried on, as well as a flood on the riverbank land, and mating problems led to poor results.” “I remember thinking ‘this is no good – we need to do something about this’,” Paul says.
Half of the farm lies alongside the Manawatu river across a stopbank from the rest of the land and is prone to flooding at any time, but the regional council’s early warning system gives Paul time to move the cows off the area.
Paul is the type of person who likes to measure, monitor, and understand what is going on with his cows, his grass, and his soil. Currently embarking on a Bachelor of AgriCommerce degree at Massey by distance learning, he sees his future in farm consultancy, helping other farmers to solve issues through identifying an issue, researching a solution, and then monitoring its implementation.
After doing some research Paul and Robert decided the first step was to blood test the cows to see what was going on with them.
“If one cow goes down it’s good to blood test a group because the problem will pop up in others as well,” Paul says.
He started taking bloods from older cows that were at a similar post-calving stage, saying they were cheap to test and the problem was not as likely to show up in the two- to three-year-olds who usually handle the mobilisation of calcium from their bones after calving better than the older cows.
“With our relatively high-producing herd at 450kg MS/cow and on a diet of mainly grass and maize silage, the cows are on a metabolic knife edge.”
In a less intensive, lower-producing herd, if the cows are averaging 300-400kg MS/year, Paul says they don’t have the same high demand for minerals and can get by with enough from the feed.
Attending a metabolics field day was part of his research, and someone asked him if his down cows were crawling.
“They told me that’s a phosphorus problem, and I realised that was the funny moving gait they had.”
Sure enough, the blood tests came back as low in phosphorus (P) and cows were supplemented, but the poor results impacted mating later in the season.
“If the cows miss getting pregnant in the first cycle then you can miss out on 42kg MS early in the season at $6/kg MS or whatever the payout is.”


Soil tests were being carried out each year but Paul says they hadn’t looked hard enough at the results of the soil test after the flood.
“The nature of the farm, with particularly the Parewanui silt and to a lesser degree the Rangitikei silt loam, means that grass can grow very fast – up to 100kg DM/ha/day in the spring – and it’s almost too fast for itself. The soil doesn’t hang on to the minerals and the grass doesn’t pick up enough minerals.”
Now soil tests are carried out in early October with three paddocks on each block sampled to get a trend in fertility of the soils.
“With mineral issues it all starts with the soil and then the plants,” Paul says.
“Blood tests are crucial, pasture tests are crucial, and soil tests are crucial – prevention is the key.”
A flood can have a huge and very quick effect on fertility with P levels dropping, and so fertiliser needs to be applied.
“The silt soils can have the minerals washed out of them in a prolonged flood event and a dumping of fresh silt can be quite deficient in nutrients, depending on where it has come from.”


To gather data with more immediate results Paul took up the suggestion of a visiting rep who recommended pasture testing.
The farm now carries out monthly herbage testing from August and may take a sample any day for an extra test if they are seeing metabolic issues in the cows.
The R2 heifers start calving on 15 July and the older cows on 20 July, Paul said.
“We start the pasture sampling in early August to see what the P level is doing along with energy levels and other minerals.
“You just pick a handful in 5-6 different places pre-grazing and pop into a plastic bag then drop into the lab.”
This result outlines the mineral and energy content of the pasture and the dietary cation anion difference (DCAD), which indicates the risk of metabolic disease.


Fearing that dusting the pastures was not giving them enough mineral coverage – and little to none in a Manawatu wind or rain event when the dust was blown or washed off the pasture – Paul and Robert decided they wanted to feed a transition diet added into the maize silage they feed to their cows.
In consultation with Fraser Abernathy from The Dairy Vet Ltd, a transition plan was devised to solve the metabolic issues around calving and early lactation and to improve cow intakes and energy levels through to balance date. A nutrition plan was established with the springers being fed a low DCAD diet, which stimulates the cows own calcium reserves and reduces milk fever and the problems of retained membranes. A key focus was to reduce the incidence of ketosis (excess mobilisation of body fat) over the transition period by adjusting the cows’ diets and ensuring pasture quality was maintained. This resulted in the cows maximising post-calving pasture intakes.
Paul starts feeding the DCAD mix 21 days before calving through until the cows calve. With advice from Fraser and the herbage testing results, a custom blend of minerals is mixed in with maize in the silage wagon and fed on the feedpad.
The aim is to cut out some of the spring grass, which can be low in minerals, supplementing 4kg grass per cow with 5kg maize silage, 3kg palm kernel, 2kg hay, and the mineral mix.
Blood tests on a group of cows during the DCAD diet, and again 10 days after calving, track how the cows have some through the transition. The tests have same-day results for Mg, Ca, P, and energy (MJ ME).
The colostrum diet consists of a mix of 200g lime flour and 80g Mg blended into 2.5kg palm kernel per cow per day fed in the palm kernel trolley before going onto their grass ration.
“I calculate it daily, mix it up by hand, and feed it – it’s pretty rough and ready but it works,” Paul says.
“It’s also a good time to see if any cows are unwell. If one is standing off to the side and not going to the trolley I put a bag on her ribs and fix the problem really quickly. If she doesn’t perk up it could be retained membranes, so we take her temperature and look into it and then ring the vet.”
Then three weeks out from mating more blood tests look at levels of bicarbonate, Ca, Mg, P, and B-OHB, which is related to the ketosis threat.
“The best way to keep track is to keep testing – if my soil and pasture and bloods are good I know I can sleep at night.”
The pre-mating diet consists of 2.5kg palm kernel and 18kg DM pasture per day, depending on how the bloods look at that time.
The mineral supplementation at transition and pre-mating has resulted in a 50% reduction in milk fever cases and a halving of ketosis along with a lift to 69% for six-week in calf rate and lowering of the empty rate to 12%.
“It’s hard to put all that improvement down to the feed because we have also started vaccinating for infectious bovine rhinotracheitis (IBR) and using Flashmate heat detectors, with a really good farm team who are well trained and really observant.”
“A good team makes a huge difference to your mating performance,” Paul added.
“But I really think the diet made a difference – we got 28 cows in calf 21 days earlier, which is worth a thousand kg of milk solids at $6/kg, and it also gave us way more culling and breeding options.”