Cac-ing - which stands for Compost Added at Casing is a currently widely used cultivation technique in the Netherlands. The aim of cac-ing is accelerated growth, better colonisation of the casing soil, evenly spread flushes and encouraging stronger mycelium in the casing soil.

Cac-ing is mixing mycelium in the casing soil at the same moment as casing. Irish growers McCanna and Flannagan first used this technique in the 1970s, so they could leave out ruffling when growing in the tray system. At that time ruffling in trays was technically impossible. Since that time, trials have been carried out in numerous countries using all sorts of mycelium hosts, mixing methods and times. In the 1980s this method was refined for the Dutch shelf system at the experimental station in Horst. This technique is now employed on virtually all Dutch mushroom-growing farms.

Why cac-ing?

The primary reason growers opt for cac-ing is the time gain. The mycelium no longer has to colonise the casing soil from the compost. As the mycelium is mixed through the casing soil from the very start, growth starts at once which can shorten a schedule by four days. Shorter schedules enable more cultivation cycles per year. Cac-ing also gives a higher yield.

Another reason is that growers are increasingly using heavier casing soil. This results in less mushrooms, but a higher individual piece weight. Heavy casing soils can however often have highly compacted structures. Adding mycelium lightens the casing soil structure, and creates more constant evaporation. This leads to easier growth later on and means the mushrooms can be sprayed with water. This will often help to prevent post-harvest discoloration and watery stipe.

Another reason for cac-ing is the development of stronger mycelium - if the watering schedule is adapted. If less but stronger mycelium develops, there will be fewer pinheads - but they will grow into better quality mushrooms.

Influence on quality and cultivation

In neighbouring countries, particularly in the UK and Ireland, so-called ‘cac-ing inoculum’ is used. This is a type of spawn with the added advantage of being pure and pathogen-free. Inoculum is also easy to weigh and mix in with the casing soil. When incubated compost is added as a cac-ing material, between 500 and 1000 gram per square metre is used. With commercially manufactured cac-ing spawn this is maximum 20 litres per square metre.

Using these maximum quantities colonisation can be accelerated by up to four days. Even though the period is shorter, the same amount of water should be given as normal so the sprinkling schedule should be adapted. If incubated compost is used it is important to allow the mycelium to colonise the casing soil in a dry state in the beginning, otherwise a greyish mycelium will develop which is unable to transport water. This fine, grey mycelium will cause too many small, light mushrooms and bacterial blotches in the second flush. The cac-ing inoculum used up until now cannot handle over watering in the early stages so altering the watering schedule is a necessity. However, if a combination of controlled spraying and the right casing soil is well-managed, the mycelium has the potential for strong growth resulting in very heavy mushrooms.

Another aspect that mustn'tt be overlooked is the fact that cac-ing introduces more compost to the surface of the casing soil. This compost tends to dehydrate very quickly, particularly when the pinheads are developing. This must be carefully monitored. The compost in the casing soil helps retain a good open structure, which guarantees constant evaporation. This leads to whiter mushrooms which are quite water resistant.

Using cac-ing

It's essential that the mycelium in the cac-ing material is the same strain as the mycelium in the compost. Strains are often mixed up in practice if incubated compost from a different growing room is used or commercially produced cac-ing inoculum.

Adding exactly the right amount of cac-ing material is vital to achieve the right increase rate. This is not possible with a ruffling machine. The actual blending is good, but the precise amount can't be estimated. The depth of ruffling is an influencing factor here, as well as the compost structure and the moisture content of the casing soil. Wet casing soil on short structured, dry compost will always result in more cac-material in the casing soil than intended.

Mixing followed by casing always gives the best quality. Immediately after mixing with cac-material, the casing soil must be brought up to maximum moisture content. Until now, this maximum moisture content is considerably lower for cac-ing spawn than for cac-ing with incubated compost. After moisture has been added the whole thing is left to rest for a day. The maximum moisture content must be retained - which means that as the end of the incubation period in the casing soil approaches, the amount of water per day drops. Finally, just before the recovery period, it is just the amount of evaporated water lost that needs to be added to the casing soil.

Another critical point is the development of the pinheads. A very constant RH level must be maintained at this stage - any drop in RH must not be too abrupt. A difference of more than 2% in 24 hours must be avoided at all costs. If the cac-ing is heavy, growers should take into account the possibility that mycelium on the surface of the casing soil continues incubating further than on un-cac-ed casing soil. Applying water or lowering the temperature can prevent this.