On the same day visitors will visit Morrison Farming near Marton, a 1500ha family business with 1600 stock units and a 50:50 sheep and cattle mix.
The other field trip will be to the OB Group of dairy farms in the Santoft sand country. These farms use a mixture of a new variety of tall fescue and irrigation to provide pasture for milking herds on drought-prone land.
The conference also has a range of speakers. One is Dr Jacqueline Rowarth, the new Environmental Protection Authority (EPA) chief scientist.
Mr Johnston says she is a straight talker. Her talk is titled Post truth, alternative facts and the role of the EPA.
Local speakers include Horizons Regional Council chairman Bruce Gordon, Atihau Whanganui Incorporation CEO Andrew Beijeman and Coastal Spring Lamb founder Richard Redmayne.
One of the focuses of the discussion will be the increased use of brought-in feeds on the sustainability of dairy farms in the Waikato.
The New Zealand Grasslands Association has 900 members and started in Palmerston North in 1931.
Conference organising committee chairman Noel Johnston lives in Raumati and owns two dairy farms, one near Foxton and the other near Waverley and run by his son Duncan. His daughter-in-law Clare Johnston is the conference secretary.
The association's conferences alternate between the North and South islands, and are always in spring. Last year's was in Timaru, next year's is in Twizel. The last time Whanganui was the host was 1989.
A committee of 19 has been organising the conference since late last year.
Four awards will be given at the conference dinner - two to excellent local farmers, a regional one for technology use and the Ray Brougham Trophy for someone who has made a national contribution.
The association produces a regular newsletter, and has many papers on pasture and animals on its website.
Mr Johnston has been attending its conferences for the past 20 years, and said it was always good to meet people farming in other parts of the country.
++ To register for the conference, go to www.grassland.org.nz.