Every calf rearer has had that sinking feeling.
The calves looked fine on colostrum or transition milk. You changed them over to powder. Then two or three days later the tails are dirty, a few calves are hollow in the flank, one is not as aggressive on the teat, and suddenly the question becomes: Is it the powder, an infection, the shed, or something we’ve done? That is one of the most common calf-rearing headaches in New Zealand, and DairyNZ is clear that scours are not one single disease. Scours can be nutritional, often linked to sudden feed changes or poor digestion, or infectious, caused by bugs such as viruses, bacteria or protozoa. Either way, the danger is dehydration and salt loss, not just loose manure.
A very typical upper-North-Island scenario goes like this: a rearer in Waikato or South Auckland buys a mob of calves that have had a reasonable start, then moves them onto a new milk replacer too quickly, or changes both milk type and routine at the same time. Maybe the calves are also shifted pens, maybe the weather turns cold, maybe the mix strength is not quite consistent between staff. None of those things on their own always cause a train wreck, but stack them together and calves often tell you fast that they are not coping. DairyNZ’s current guidance makes that point clearly: when changing feeds, the calf digestive system needs time to adapt, the risk of nutritional scours is higher when feed is changed suddenly, and other stressors should be avoided at the same time.
That is a big shift from the old thinking that a calf should just “harden up” and get used to it. Better rearers now understand that calves do not read the label on the bag — they respond to consistency. DairyNZ recommends stepping between calf milk replacers over several days, using a rough progression such as 75:25, then 50:50, then 25:75, rather than flicking from one product to another overnight. They also note that older calves generally adapt more easily than very young calves, and that non-curding powders are better suited to older calves.
The other common mistake is what happens after the scour starts. Some rearers panic and remove milk altogether. That feels logical, but it can cost calves condition fast. DairyNZ advises that if a calf has mild to severe diarrhoea, an electrolyte solution should be considered while continuing milk feeding, not instead of it. NZ vet guidance says much the same: electrolytes are the main treatment for dehydration, but milk is still needed to provide energy and help the calf hold condition. Clean water and clean bedding also matter more than many people think.
So what does a smart response look like on farm?
First, separate in your mind the question of cause from the question of support. You may not know in the first hour whether you are looking at nutritional scours, crypto, rota, coccidia later on, or a hygiene blow-up. But you do know the calf is losing fluid and salts. That means warmth, dry bedding, clean water, and electrolytes are urgent. If calves are dull, sunken-eyed, slow to rise, cold, or the problem is spreading quickly, that is the point to involve your vet.
Second, look hard at the basics of your mixing and feeding routine. Are you actually following the bag directions? Are staff rounding scoops, guessing temperatures, or mixing stronger because they think it helps? Are calves getting fed at the same temperature and the same concentration every time? Sudden inconsistency is one of the quiet causes of “mystery scours.” DairyNZ specifically warns that mixing rates vary between products and labels should be followed carefully.
Third, check what else changed that week. In plenty of real-life cases, the problem is not the powder alone. It is the powder change plus cold rain, plus a pen shift, plus disbudding, plus calves of mixed ages, plus wet bedding. Nutritional scours often show up where the system has lost rhythm. Infectious scours then take advantage of calves that are already under pressure. DairyNZ and NZ veterinary guidance both emphasise hygiene, bedding, clean troughs and avoiding overcrowding.
The rearers who come through these patches best are usually not the ones chasing miracle cures. They are the ones who get very boring, very fast: same mix rate, same temperature, gradual transitions, dry pens, clean gear, and no unnecessary stress piled on young calves. That is not flashy advice, but it is the sort that saves calves.
Practical takeaway
If calves scour after a feed change ask four questions first:Did we change too quickly?
Did we change anything else at the same time?
Are we still feeding milk alongside electrolytes?
Are the calves dry, warm and clean?
Most of the time, the fix starts there. And the farms that rear the best calves year after year are usually the farms that make transitions feel almost boring.
