The change in the weather and the massive increase in grass growth rates are the likely causes. Mineral content of grass is least when grass growth is highest. Allied to this, many farmers would have reduced meal feeding levels after the burst in growth. Other forms of supplementation should be considered.
They all work well, even low-cost methods such as water dosing and paddock-dusting. It’s still early to dose calves for worms. The timing of the first dose should be based on exposure and this can be measured in faecal egg counts. Take dung samples from the group and get them tested. Your vet will help interpret the results. The other option is to wait for signs, such as loose dungs and poor thrive in some animals. The advantage of waiting is that the herd will have built up more immunity, which will stand to them for life.
The signs of grass tetany, or hypomagnesaemia