heat units vs growing days

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Carlos Alberto Arnillas

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Jul 19, 2021, 8:06:33 PMJul 19
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Dear Luca
I am trying to implement APEX in a Canadian area. Talking with farmers, they indicated that many crops here are commercialized using number of days since germination instead of heat units, and that photoperiod may be more important as a trigger of some biological responses. What would be the best way to parametrize those crops in EPIC/APEX?
Once more time, thanks for your help.

Carlos Alberto

Luca

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Jul 20, 2021, 4:49:04 PMJul 20
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Dear Carlos,
If you have the number of days since germination, the average planting period, and the weather data, you can estimate the heat units required to reach maturity and use that value (plus 100) as potential heat units (PHU) in the management file.
About the photoperiod, the crop growth model in APEX does not simulate processes like flowering, where the photoperiod plays an important role. It can simulate a winter dormancy regulated by the day length, and it can be adjusted using parameter 6. What other processes were you considering as affected by photoperiod?

Best,
Luca

Carlos Alberto Arnillas

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Jul 20, 2021, 7:42:55 PMJul 20
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Thanks Luca. I will try that approach for heat units.
About photoperiod, a farmer mentioned that he would cut hay before flowering, so I was curious about how to represent it.

Luca

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Jul 22, 2021, 9:50:54 AMJul 22
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Carlos,
Unfortunately, the model does not provide information on the flowering period and does not simulate the crop phonology in a detailed way. What I would do is to get information on the number of heat units required to reach the flowering stage. When you calculate the total heat units as explained in the previous message, you can calculate the ratio between heat units at flowering and heat units at maturity. With this number in mind, run the model for many years and look at the DGN output file. You should find an output called HUI (heat unit index). It is calculated as the ratio between the accumulated heat units and the heat units at maturity. I would assume that when the HUI reaches the value of the ratio you have calculated before, the plant is at the flowering stage. You can do this for several years to have a rage of dates when the plants reach the flowering stage.

Let me know if the explanation is not clear.
Best,
Luca
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