An analysis of data from past El Niño years suggests farmers in some regions can expect a drop in pasture growth this summer, Dairy NZ says.
The current El Niño weather pattern is expected to be one of the most severe since 1950.
Dairy NZ has reviewed pasture growth data from the three strongest El Niño years - 1972/73, 1982/83 and 1997/98 - which this summer is predicted to mirror.
Senior developer for productivity, Kim Mashlan, said if similar years were anything to go by, then many farms could grow two to three tonnes less feed per hectare than normal.
There were steps farmers could take to lessen the impact, Ms Mashlan said.
"This information isn't meant to be a crystal ball but highlights the importance of preparing for the worst and hoping for the best, which many farmers are already doing.
"The key is to have completed a written summer management plan that identifies triggers for a change in farm management.
"For example, if grazing residuals go below 4cm, what will happen? An action might be to feed 4kg grass silage per cow per day."
Ms Mashlan recommended body conditioning, keeping an eye on young stock, regularly reviewing the weather forecasts and adjusting plans based on predicated pasture growth rates."