How to manage pre-harvest sprouting in cereal crops?

Before harvest, grains can sometimes sprout in the ear. This phenomenon called pre-harvest sprouting can affect grain weight and specific weight, as well as human food uses including the baking quality of the flour.

Conditions favoring pre-harvest sprouting

Three factors increase the risk of pre-harvest sprouting in cereal straw crops:

Varietal resistance to pre-harvest sprouting.
Climate during the grain maturation phase, approximately in July. Warm weather favors the breaking of seed dormancy and thus early germination.
Post-maturation climate: humid and cool weather favors germination.

Observation and measurement of pre-harvest sprouting

Sprouting can be easy to observe if the coleoptile is developed on the ear, but visual examination does not always allow detection of sprouted grains.

Measurement of the Hagberg falling number confirms the presence of grain sprouting. This index indirectly measures amylase activity (enzymes that degrade starch), which can become excessive in the presence of sprouted or germinating grains. Below a falling number of 180 seconds, baking quality is adversely affected. The degradation of the Hagberg falling number thus indicates the onset of amylase activity, which may not immediately manifest as visible grain sprouting.

Pre-harvest sprouting is measured by counting the number of visually sprouted grains, expressed as a percentage. For commercial transactions, the threshold is 2% sprouted grains.

Avoiding risk situations

For a seed to sprout, several conditions must be met: temperature, humidity, and internal physiology, namely dormancy. A grain with more than 37% moisture content will always sprout at low temperatures, below 10°C.

At higher temperatures, a grain can only sprout if its dormancy has been broken.

Dormancy breaking phase: dormancy can be broken by excessive temperatures accumulating from the milk-dough stage. The amount of heat required is highly varietal; sensitive varieties need less accumulated heat to break dormancy.

Germination expression phase: once dormancy is broken, the grain will sprout under any temperature conditions as long as the critical moisture content threshold is exceeded. Grain imbibition by rain or very humid weather plays an important role.

This imbibition allows oxygen to reach the embryo, a process favored by low temperatures. Water retention on the grain surface is obviously enhanced in the presence of lodging.

Harmfulness of pre-harvest sprouting

In animal feed: sprouted cereals retain their full nutritional value for feeding pigs and poultry. However, it is necessary to ensure that harvesting, drying, and storage are properly done to stop the sprouting process and prevent fungal development.

In human food: baking quality can be severely impaired. Measurement of the Hagberg falling number evaluates the level of sprouting.

In parallel, specific weight (SW) deteriorates due to rain during grain drying. Thus, sprouted grains will also have a reduced SW (-0.5 point per 10 mm of rain). In cases of heavy pre-harvest sprouting (visible sprouting on the ear), the thousand grain weight (TGW) can be affected, leading to yield loss.

Preventive and corrective solutions

Varietal sensitivity: depending on genotype, dormancy duration varies from 7 days to several months. There is a sensitivity scale for varieties to pre-harvest sprouting established by GEVES. Consult the ARVALIS documentation.

Harvest as soon as possible to minimize risk.

Species sensitivity

Durum wheat

Durum wheat is a species that easily sprouts pre-harvest.

Except in extreme cases, sprouting has little effect on semolina and pasta production. However, the impact on color due to browning (Maillard reaction during pasta drying) can be notable in modern high-temperature processes but negligible in low-temperature processes.

Barley and oats

Barley and oats are less sensitive to pre-harvest sprouting than wheat because their seed coats are more tightly fused to the grain walls. This physical characteristic limits water entry into the grain. It also reduces oxygen entry to the embryo, necessary for germination (oxygen is transported by water). Moreover, barley seed coats contain more quinones, molecules that act as oxygen traps when water penetrates.

Barley sprouting affects its quality because sprouted (or pre-germinated) barley, even if properly stored, loses its ability to germinate during malting.

Triticale

Due to its genetic characteristics, triticale is the cereal most sensitive to pre-harvest sprouting. Indeed, triticale dormancy is shorter than wheat and dormancy breaking is faster.

However, early sprouting does not alter triticale's nutritional value. Conservation can be done by wet anaerobic methods. Grains can be stored whole inerted or crushed and ensiled.

What to do if harvest was not possible before pre-harvest sprouting?

Switch to plan B! If the harvest can no longer be used for human food, alternatives exist.

Utilize sprouted grains for ruminants

Certain precautions must be taken during harvesting and storage. Let's take triticale as an example.

How to harvest?

It is preferable to harvest the plots as grain. At this maturity stage, triticales can no longer be ensiled. The whole plant dry matter content is too high to allow successful forage conservation: impossible silo compaction preventing lactic fermentation necessary for acidification...

At grain harvest, two situations arise:

Sprouts are less than 1 cm: grain moisture at harvest must be below 20% to ensure dry conservation in grain silo (drying and ventilation).
Sprouts are more than 1 cm: grain moisture at harvest must be at least 30%. This way, the grain retains some soluble sugars necessary for lactic fermentation essential for wet conservation. If grains are not moist enough, they can be moistened during storage.

In all cases, combine harvester settings must be adjusted: maximize product ventilation, open lower separation grids, and reduce forward speed.

How to store?

Drying or inerting: what strategies for these cereals intended for animals?

If the sprout is less than 1 cm

Grain storage is still possible if moisture is below 20%. Grain moisture at harvest must be below 20% to ensure dry conservation in grain silo. For moisture below 16%, simple ventilation suffices.

Before silo filling, it is particularly important to clean the grain well to separate good grains from plant debris and wetter sprouts that could affect storage quality and cause heating.

Storage is then done under the same conditions as non-sprouted grain, respecting all usual good practices:

Cool as quickly as possible, in stages, respecting at the first stage a maximum temperature difference of 10°C between grain temperature and ventilation air temperature to avoid condensation on silo walls.
Do not wait until the silo is full to start ventilation. Ventilation can begin as soon as the ducts are covered.
Monitor grain temperature scrupulously and record it for better tracking.

If ventilation is properly done, the risk of mold development is controlled, and the sprouting process is stopped.

If it is not possible to harvest below 16% moisture, drying before silo filling is necessary. Drying must be done quickly (avoid drying ventilation which does not inhibit sprouting fast enough) with air at 80-90°C followed by effective cooling.

If the sprout is more than 1 cm

Storage of crushed sprouted grains in tube silo. Photo credit: FIDOCL Livestock Consulting

In the case of wet grains (DM<80%) and/or sprouts longer than 1 cm, drying is strongly discouraged. Conservation must be done by wet anaerobic methods, preferably crushed or flattened. This is called inerting. The cereal is harvested, crushed, and placed in a hermetic silo tower, corridor, or tube. The sprouts respire for a few days, consuming oxygen and releasing carbon dioxide. Inerting is a conservation technique under carbon dioxide atmosphere. This technique is widely used to harvest and store wet corn grain (with or without crushing) for ruminant and monogastric livestock.

In tube silos, as in "classic" ensiling, flattened grains are compacted, covered with plastic, and sealed (hermetically) immediately after filling. If grains are not moist enough at harvest (<30% moisture), water can be added to promote compaction and conservation. High-capacity crushers are equipped with nozzles to facilitate water addition.

Beware of mycotoxins

The risk of excessive mycotoxin presence is low if the cereal was dried or inerted quickly after harvest and if the product looks good. If the delay between harvest and inerting was long or if the product's appearance is doubtful (presence or smell of mold, silo heating, etc.), do not feed it (especially to young animals and females in lactation or reproduction), or have mycotoxin testing done by a laboratory beforehand.

How to utilize?

Early pre-harvest sprouting does not alter triticale's nutritional value. Starch is converted to sugar without energy loss. Protein content is unchanged and grains have better digestibility.

Here are some references from a very wet harvest in Brittany (1992) on wheat, after quality analyses on 4 lots of sprouted wheat: 40 to 60% sprouted grains, 23% moisture at harvest. Results are expressed compared to a non-sprouted wheat lot.

Hagberg falling number (s): 62 vs. 338
Proteins: identical to control lot
Starch: -3%
Sugar: +1 to 2%
Metabolizable energy: identical to control lot

For more on the subject, click here.

Making tube or corridor silos

The ideal is to use the plastic "tube" conservation technique if you have a contractor or Cuma equipped. If making a small corridor silo (you can use square bales), as with "classic" ensiling, flattened grains are compacted, covered with plastic, and sealed (hermetically) immediately after filling. A minimum advance of 10 to 20 cm is needed (1 m³ of crushed cereal weighs about 800 kg). Crushing must be neither too coarse nor too fine. If grains are not moist enough at harvest (<30% moisture), water can be added to promote compaction and conservation. The wetter the grain, the more the silo must be compacted and hermetic. However, beware that the product can clump and reopening is more difficult.

Utilize sprouted grains by methanization

If storage for animal feed is not possible, methanization remains a possible valorization route.

To find the nearest methanization unit, consult this map.

Sources

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