Make changes quickly to prevent:

Animal Tips  
Suckler Cows and calves <12 months old Cows will milk off body reserves. This is acceptable if cow is in good condition and if grass growth is in the green within 2 weeks.
Cows in poor condition will lose weight and reduce milk production reducing calf growth
Dry Cows Cows will start to lose weight
Weaned Store Cattle Stock will start to lose weight
Finishing Cattle Cattle growth rates will be reduced and cattle may "store"
Lactating Dairy Cows Milk yield from forage will be reduced
Bulling Heifers Heifers growth rates will be reduced and cattle may "store"