"Springtime is when the new growth is providing good nutritional value for the cows to eat.
"In summer the nutritional value goes down as the grass goes into survival mode if there is not enough moisture.''
Hartigan said Northland's milk production was "very much stable" with about 5 million litres a day at this time of year.
"It's more than we can manage at our two processing plants at Kauri and Maungaturoto so the excess Northland milk is being taken out of the region to be processed at other Fonterra plants,'' he said.
A strength of the Fonterra network was the ability to move milk around to other processing plants wherever it was needed. "That's the beauty of Fonterra. With that flexibility we are able to keep pretty fluid,'' he said.
The Maungaturoto plant can process up to 2 million litres a day and Kauri can process up to 3 million litres a day.
The two plants have a different product mix, and this affects how much milk they can process at a time according to the types of orders they are aiming to fulfil.
The Kauri plant produces mainly high-revenue products such as milk powders, butter and cream.
Maungaturoto is used for producing mainly casein, whey, whole milk powder and buttermilk.
Drinking milk sold in Northland supermarkets is generally produced in Takanini, south Auckland.
Hartigan said more than 95 per cent of the products processed at the two Northland plants are made for export.
"We tailor-make our production according to the orders.
"We might do an 18-hour run or 12 hours depending on the type of materials we are producing.''
Hartigan said the butter produced at Kauri was not in the small block sizes sold in supermarkets.
"We produce 20kg blocks, which are sent elsewhere to be refined down to pottle size,'' he said.
About 300 operations and support staff keep the Kauri plant operating, and about 115 staff look after the Maungaturoto factory.
"Currently, we have the entire milk tanker fleet at capacity, with no time for breaks until the shoulder season when there is less milk to collect.''
Hartigan said he was incredibly proud of the way staff were handling the challenges of the busiest time of year on top of the restrictions of the Covid-19 pandemic.
"I want to reassure farmers and the public that we are doing everything to keep our plants and staff safe. The region might be at level 2, but here we have always stayed at level 4 in terms of restrictions.
"We all wear masks, even in the office areas, and we keep in separation zones.
"People are getting tired and it can be frustrating wearing a mask all the time, but we take our jobs as essential workers very seriously,'' he said.
Hartigan said the payoff has been the gratifying end-of-year results, where Fonterra had performed well despite the challenging and dynamic conditions in the export markets.
"It's one of the best performances on record and it was great to read,'' he said.
He said people around the world appeared to be reflecting on their health and looking for high-quality, nutritious products that were produced in a sustainable way.
"Fonterra is doing a good job on the sustainability journey,'' he said.
Fonterra has raised its 2021-22 farmgate milk price to $8.40/kg of milk solids for the year. If realised, this would inject $13 billion into the economy and equal the highest price ever paid, the co-op said.
