Monitoring grain temperature in store

Temperature is the earliest honest signal a grain heap gives you. Long before wet grain smells musty, before a crust forms on the surface, before insects show in a trap, the trouble spot warms up — often weeks before anything you could see or smell on a walk through the shed. Which means a farm that probes its heaps on a fixed grid, weekly, and writes the numbers down will catch problems while they're still a fan job. A farm that walks the shed and sniffs will find the same problems when they're a grain-moving job, or worse.

The kit can be a £50 spear thermometer or a fixed multi-point system. The habit matters more than the hardware.

The targets

The standard UK approach, once grain is dried to spec:

Stage	Target	Why
Into store (off the drier/field)	Start cooling immediately	Warm grain respires and insects breed fast
By early autumn	Below 15°C	Insect breeding effectively stops
Over winter	Below 10°C	Moulds and mites slow right down; grain is close to inert

Cooling is done with ambient air — running store fans on cool nights and cold snaps to pull the heap temperature down in stages behind the falling outside air. It costs pennies per tonne in electricity, and below 15°C it does the same job as an insecticide, without the paperwork. The relationship between temperature and moisture is covered in our grain moisture guide; the short version is that warmth and dampness together are where everything goes wrong, and temperature is the half you can keep adjusting after harvest.

Oilseed rape is a special case — it heats faster than cereals and wants closer watching, especially in the first month. There's a separate guide to storing OSR.

Probe on a grid, not on a whim

The common failure isn't not probing — it's probing the same three convenient spots near the door every time. Trouble doesn't start near the door. It starts where the wet load went in, over the duct that isn't blowing properly, under the roof joint that drips.

So: set a grid. On a typical flat store that means readings every 5–6 metres in both directions, plus any known risk spots — over the intake, against walls, anywhere a wetter load was tipped. Probe to more than one depth if the spear allows; surface readings miss heating that starts a metre down.

Then do it weekly, at the same points, and write the numbers down. Not a mental note. Numbers on a sheet or in a phone, dated, per location. A single reading tells you almost nothing — 12°C is fine if the heap was 12°C last week and alarming if it was 8°C. The signal is the trend, and a trend needs numbers.

Weekly is the baseline for settled, dry, cooling grain. Go twice-weekly in the first month after filling, during warm autumn spells, and for anything that went in borderline on moisture.

What a rise actually means

Ignore the absolute number for a moment and watch the differences. A useful way to read them:

• The whole heap drifting with the seasons — normal. Grain follows ambient slowly; fans speed the fall.
• One spot 2–3°C above its neighbours and climbing — this is the early warning working. Something in that column is respiring: a wet patch, a concentration of fines and admixture, the start of mould or insect activity.
• A spot 10°C+ above ambient — you've found it late. Heating is established and spreading, and grain around the hotspot is taking up the moisture the warm air carries.

The response, in order: run the fans hard and check that air is actually moving through that part of the floor (a blocked or badly sealed duct under the hotspot is a common culprit). Probe around the spot to map its size. Pull a sample — smell it, meter it, look for insects; a proper sample tells you which problem you have. If the temperature keeps climbing despite airflow, move the grain: dig out the hotspot, run it through the drier or shift it forward for sale. Moving a hotspot early costs a few hours of loader work. A tonne or two lost to a cooked, compacted core — plus the mycotoxin risk in what's around it, which buyers test for and reject on — costs far more.

Spears versus fixed systems

A hand spear — a probe thermometer on a 1.5–3m lance — costs roughly £50–£300 and does the job on most farms, provided somebody actually walks the grid weekly. Its weakness is entirely human: it measures nothing on the weeks nobody has time.

Fixed monitoring — cables or probes hung in the heap or mounted in the store, reading continuously, some versions with logging and alerts to a phone — runs from a few hundred pounds for a basic setup to several thousand for a large store with automatic fan control. What the money buys is consistency: the same points, every day, no gaps in the record, and fans that run themselves on the right nights.

As a rough rule, the more tonnes and the longer they're staying, the stronger the case for fixed kit. A 300-tonne shed sold by November is well served by a spear and discipline. A 2,000-tonne store carrying milling wheat to June is betting a lot of premium on nobody being too busy to probe.

Either way, the record is the point. Readings per location per week, kept somewhere they can be compared — that's what turns a thermometer into an early-warning system, and it's also what shows a buyer or assurance assessor that the store was managed, not just filled.

Frequently Asked Questions

What temperature should stored grain be in the UK?

Aim to have the heap below 15°C by early autumn and below 10°C over winter, using ambient-air fans on cool nights. Below 15°C grain store insects effectively stop breeding, and below 10°C moulds and mites slow to a crawl. Cold, dry grain stores safely for months with only routine checks.

How often should you check grain temperature in store?

Weekly, on a fixed grid of points across the heap, with readings written down so trends show. Increase to twice-weekly in the first month after filling, during warm spells, and for grain that went in close to its moisture limit. Oilseed rape wants twice-weekly checks for the first month as standard.

What does it mean if grain temperature is rising?

A localised rise of 2–3°C above the surrounding grain means something in that spot is respiring — a wet patch, concentrated fines, or early mould or insect activity. Run the fans, check airflow is reaching that area, and pull a sample. If it keeps climbing despite ventilation, dig out and dry or move the affected grain before the heating spreads.

Is a grain temperature spear good enough, or do I need a fixed system?

A hand spear at £50–£300 is perfectly adequate for smaller tonnages sold by mid-winter, if the weekly grid discipline holds. Fixed multi-point systems earn their keep on bigger stores and longer holds — they read every day, miss nothing during busy weeks, and the better ones control the fans automatically.