As another heatwave sweeps across the country, farmers will have to take rising grass dry matter (DM) into consideration when deciding grazing plans.

According to PastureBase Ireland data, Moorepark in Co. Cork hit 22% grass DM on July 6.

Many other farms across the country, especially those in the south-east, have seen grass growth slow drastically as soil moisture deficits exceed 50mm.

These farms are seeing growth rates drop below 50kgs DM/ha amid this hot spell of weather.

Where growth drops below 45kgs DM/ha, the farm is considered to have a restricted growth level, which is the initial or least severe level of drought.

During these drought conditions, grass dry matter increases significantly, so it is important for farmers to understand how this impacts grazing conditions.

Dry matter digestibility

Dry matter digestibility (DMD) is a better indicator of how much grass a cow must consume compared to dry matter (DM), as cows cannot digest 100% of the DM in grass.

Where the grass plant is headed out or in the stemmy phase, the DM will be high but the DMD will be lower, creating a balancing effect.

The UFL value, used to define energy value from a crop, can be used to understand DMD.

Fresh grass from the second to fourth rotations typically see a UFL value of around 1.01/kg DM. This drops to below 0.75/kg DM if grass is headed out.

Most farmers will have also seen UFL decrease to approximately 0.95 in the fifth rotation, dropping further with each rotation from there on after.

If the DMD is still high despite a rapidly rising DM content, which is commonly seen in fresher grass still in the tillering phase at this time of the year - then the farmer must re-evaluate the grazing area provided each day.

Grass allocations

When dealing with fluctuating dry matter percentages and digestibility levels, it is important to ensure the herd is being allocated the correct grass.

If too little grass is allocated, it will be felt in the milk cheque as production slips, or if too much grass is allocated, farmers may run through the rotation too quick with poor graze outs affecting the quality of grass in the following rotation.

At this time of the year, a lot of farmers find a cut and weigh system useful for maximum efficiency around grass allocations.

The following calculation can be a good example to follow;

0.210kg (weight of grass cut) x 18% DM x 40,000 (convert size to ha)= 1512kg DM/ha

Compare this to the same field with 20% DM = 0.210kg x 20% DM x 40,000= 1,680kg DM/ha.

That 2% increase in DM makes a substantial difference when it comes to allocations.

It is worth bearing in mind that an 80 cow herd consuming 18kg DM/cow/day would consume 1,440kg DM/day.

If the farmer was entering a field with the same grass as in the first example, they would need to allocate approximately 1ha for grazing.

Using the second example, the farmer will have to reduce his allocation. With 20% DM, they need to allocate 0.85 ha/day for grazing.

Water risk

A higher dry-matter level will result in the cow taking in less water through grass, so the water intake must be balanced with availability of trough space.

A farmer must also consider flow rate, as cows consume up to 50% of their water requirement within three hours of milking. The recommended flow rate is 15L/minute.

A herd's water requirement can be estimated by working out the production volume as a cow consumes 4-4.5L water/kg of milk produced, according to Tirlán data.

Cows must also have sufficient space to drink, with a recommended drinking space of 6m/8 cows.

If a farm does not meet this requirement, then competition between cows for water can induce heat stress in the warm weather, especially in older cows.

A farmer must plan his grazing to ensure cows have access to water.

If cows have to walk long distances for water, it can increase risk of dehydration and stress, especially in older and lame cows.