He was speaking at a Southern Organic Group field day on Bruce and Merril Catto's Waipahi property in front of 115 people on November 23.
Organiser Allan Richardson said the theme of the event was ''Growing great crops and pastures with less chemical inputs''.
''The 115 people there were from across the spectrum, from conventional to organic and people who want to be organic,'' Mr Richardson said.
Mr Merfield covered weed biology and control, growing winter crops and grain crops using organic and biological systems, how to reduce chemical use to fight weeds on conventional properties, how to keep pastures weed-free, and chemical-resistant weeds.
''We had good feedback and we may hold another event with Charles,'' Mr Richardson said.
''There is a lot of interest, particularly from dairy farmers looking at converting to organics, which is quite promising for our sector.
''Some local machinery agents brought out equipment to have a look at.''
There were also demonstrations of implements including adjustable scufflers and a modified roto-crumbler.
''The roto-crumbler creates a false weed bed,'' Mr Richardson said.
''It cultivates only down to about 4cm and takes out the weeds.
''Beyond 5cm, seeds do not germinate and you get a very clean bed for crops and pasture, rather than using spray.''
He said Dr Merfield also talked about weed chemical resistance.
''There is a real concern about roadsides, which are sprayed year after year, and if we are going to get resistance, we will get it from there.''
Dr Merfield said more than 125 species of weed had been identified as herbicide-resistant in the United States. In New Zealand, there were already seven or eight weeds that were resistant to some herbicides and issues were appearing on some vineyards and farms in parts of the country.
Perennial and Italian ryegrass that were resistant to glyphosate had appeared in some vineyards in Marlborough.
Dr Merfield said another key problem was spray manufacturers were not able to find or develop new chemicals to combat the issue.
He predicted it would rapidly become a significant issue in vineyards.
''Five years is the timeframe to find an effective solution.''
He said the European Union was considering banning glyphosate (Roundup) and the flow-on effect would impact New Zealand growers. If the EU banned it, it would not accept products from growers that had used it.
He attended one conference and heard a chemical company representative saying there were no new chemicals for sprays discovered or on the horizon. The speaker said he had a message for farmers: ''The cavalry is not coming over the hill.''
However, Dr Merfield said there were other options for weed control, including using cultivation and ploughing techniques, as well as biological controls, and some regional councils had ''dabbled'' in using flames and steam for roadsides.
-By Yvonne O'Hara