12th September 2022

Heat Unit Map aids maize variety selection

For the first time farmers now have access to heat unit data for their post code, allowing them to more carefully select varieties with the appropriate maturity class for their location.

Tim Richmond, maize product manager with LG explains that the new online system available at www.lgseeds.co.uk/heat-map uses Ontario Heat Unit data collated by the Met Office and shows the 10 years average across the country broken down into 5km blocks.

“Ontario Heat Units (OHU’s) are the internationally recognised system to show if maize can be grown successfully in a particular location and this new tool allows a whole new level of accuracy, helping ensure varieties are selected that are suited to the farm.”

He explains that maize is a sub-tropical plant and needs a minimum amount of accumulated heat over the growing period to mature. The OHU system is the most widely accepted system for measuring accumulated heat.

“OHU’s are calculated for the maize growing season from mid-April to mid-October combining maximum daytime temperatures above 10°C and night time minimum temperatures above 4.4°C, at which maize actively grows. If there are too few OHU’s, then crops will struggle to mature which can lead to a number of problems, especially with increased environment concerns regarding maize stubbles.”

Mr Richmond says a delayed harvest will potentially lead to harvesting in more difficult conditions, increasing the risk of damage to soil structure. In addition, it will reduce the opportunity to establish a successor crop, leading to stubbles being over-wintered with a greater risk of soil run-off. From a herd point of view, a delayed harvest will mean maize silage is later going into the diet.

“Varieties differ in the number of OHU’s they require to mature (see table) and this is why it is important to choose varieties that will mature within the heat units typically achieved in your area.

“Using the online system, farmers will be able to more accurately assess the maturity class of the variety they should go for and it can help manage the risk when choosing varieties.”

The map shows the national results. The red areas are where there is usually plenty of OHU’s to grow maize successfully. The blue areas are those unsuitable for growing maize, while the marginal areas are shown in the bordering lighter colours. When a postcode is entered, the 10 year OHU average is displayed.

“It is important to appreciate there can be significant swings around the average. For example, 2012 was a bad year with lower accumulated OHU in all parts of the country. Conversely, 2016 was an outstanding year when maize could have been grown in majority of England, Wales and parts of Scotland.

“We believe the data can help choose possible varieties and recommend farmers to look for varieties which can be grown comfortably within the average OHU. It is better to err on the side of caution than to stretch the point. If the average OHU postcode score is close to 2750, it may be better to select a variety with a lower score like Ambition (2,720 OHU’s) that will mature earlier, rather than a higher scoring variety like Equity (2,780 OHU’s) that is slightly later maturing.

“Once a shortlist of varieties have been selected that will mature within the average OHU’s received, look to choose the variety which combines good levels of starch and high cell wall digestibility with high dry matter yields to deliver a high quality silage to drive production and intakes.

“It is important to remember that the map only records OHU’s. Factors such as soil type, altitude and topography will all impact on the actual OHU’s accumulated. For example, in a warmer part of the country and on a south facing site with good soils it may be possible to choose a variety with a later maturity class and higher OHU score, such as LG31.211 (3,010 OHU’s) than on a north facing site with heavy soils.

“Whichever variety is chosen it will be essential it is not drilled until soil temperatures have consistently reached 8-10°C over a five day period.

“Armed with this new information farmers can increase the prospects of high quality feed from a variety suited to the farm and location,” Mr Richmond concludes.

Variety class FAO and maturity class OHU required
Very early FAO 140-180 MC12-9 2500-2750
Early FAO 180-200 MC 9-7 2750-2900
Later FAO 200-230 MC 7-4 More than 2900
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