CBH has revised this year's expected harvest total down to 12.5 million tonnes.
CBH general manager of operations David Capper is attributing the further 500,000 tonne drop from the last harvest report to heavy losses as a result of wind, hail and fire across the State.
He said harvest was now 80 per cent complete in WA as better harvesting conditions in all regions allowed growers to move quickly before rain over the weekend.
"Quality continues to be a frustration for growers and we appreciate their support and co-operation as we try to get the segregation mix right and help growers to maximise the value of the remaining crop," said Mr Capper.
Receivals surpassed the 10mt mark last week, with Albany having received 1,890,620t, Kwinana 4,741,653t, Esperance 1,841,998t and Geraldton 2,162,748t as of Monday.
Agrarian agricultural consultant Chris Pinkney, Geraldton, said only the minority of grain was left in paddocks in the north of the zone, while the south would be going for another couple of weeks.
He said weather issues and moisture was delaying the harvest in the south and the season had proved a "mixed bag" across the zone.
"It's proven to be a nice harvest for a lot of the guys north on solid country that had good summer rainfall but on the lighter country where frost can and does occur, it really hit home this year," he said.
"We probably haven't seen frost to that extent since 2004 so that curtailed a few 3-3.5 tonne per hectare crops down to 2-2.5t/ha."
CBH Geraldton zone manager Duncan Gray said bin sites were being closed as growers finished delivering.
Frosted grain and dry conditions has caused Mr Gray to downgrade his estimated yield for the region, saying the wheat was the let down.
"My initial yield estimate was about 2.45mt which we won't get - I don't think," he said.
"We will probably get close to 2.3mt, maybe a little bit more.
"From a barley perspective nearly all of it was feed, but we've gone over our estimate of barley and we've well and truly gone over our estimate of canola.
"We've gone 25pc above my estimate of genetically modified canola and we're about right for lupins.
"Where we will miss out on 200,000t will be on the wheat estimate and that's just due to small grain from the heat and lack of rain and frosted grain due to that cold spell in September.''
Mr Gray said screenings were an issue and a lot of grain cleaning and mixing was happening on farm.
Albany Zone manager Greg Thornton said screenings were encouraging a high proportion of growers to hold grain on farm for cleaning.
"We're mainly receiving wheat and a significant percentage of it has been downgraded due to high screenings," he said.
"Growers are holding a lot of grain back and cleaning it before delivery and the cleaning is making a difference.
"That is bringing it down as low as 5pc screenings and even below 3.5pc allowing growers to increase their flexibility with optimising protein."
He said he expected deliveries to slow as cleaning continued.
Mr Thornton was also concerned about rain across the Albany zone, and he expected deliveries to drop off as growers waited for paddocks to dry out.
"Potentially, depending on how much rain they get, it will do a bit more damage in regards to quality and we will look at the falling numbers aspect," he said.
In Esperance, growers are almost finished delivering an above average harvest despite the widespread affects of the November bushfires.
CBH Esperance Zone manager Mick Daw said they were expecting a 3mt harvest this season 1.8mt of which had been delivered as of December 3.
"There's potentially been hundreds of thousands of tonnes predominantly barley and canola lost to the wind, but we've still already met over half our target," Mr Daw said.
He expects to receive a further 600,000t-700,000t of grain, which would total about 2.5mt.
"This will equate to around $750 million worth of grain in the Esperance Zone and many businesses in the community will benefit from this revenue," he said.
Scaddan farmer Nigel Norwood lost his entire crop in the November bushfires, but decided to act quickly in the aftermath by sowing barley over 1070ha and wheat over 650ha.
Mr Norwood said he hoped sowing a summer crop would reduce soil erosion over the coming months.
Statewide Grain Cleaning owner Michael Swain, who is based in Albany, but cleans grain across WA, said work was being done in all areas.
"We're always busy but we're doing a lot this year and there's a lot I can't get to," he said.
"I only just finished cleaning last season's grain two weeks prior to the new season starting so we're cleaning all year round."
He said the harsh, hot finish and frost had created screenings issues that could be dealt with through cleaning, but growers should weigh up their costs and whether making a higher grade would be profitable.
"It is very hard to pick areas affected by frost that need cleaning because of high screenings, there's no rhyme or reason to it so once we start cleaning we're cleaning the entire paddock," he said.
"In one paddock, it'll be from a range of 13-14pc screenings to 4pc screenings which don't even need cleaning.
"Because it's hard to see the worst hit areas we've cleaned it anyway and because we don't charge a lot for what we do, the ability for the farmer to optimise all of his lower grades has almost cut out our bill entirely."
Mr Swain said some growers were able to gain about $40 a tonne from effective cleaning.
