One of the most important things in our understanding
of how cotton grows is our understanding of the impacts of temperature. An important thing
we use to understand difference between regions and different seasons and climates is the
use of the day degree function. We use a measure of the heat load the crop
is exposed to on a particular day. So each day we calculate day degrees from maximum
and minimum temperatures to establish that heat load, and over time when we collect enough
data we work out how much of that heat load goes towards meeting the crop development
stage like first square or first flower. We accumulate the day degrees each day, we add
that up, and that’s how we come up with the day degree target.
Currently in the Australian industry, we use a day degree function that was developed by
Greg Constable in the 1970s. Greg went out and measured time to first square and time
to first flower, and he was also able to establish the minimum temperature where he wasn’t getting
any crop development -about 12 degrees. The challenges we have in using the current day
degree function that Greg developed is that industry’s expanding into new environments
that have more temperature extremes; we’re often seeing a lot warmer starts to our seasons
that weren’t experienced in the 70s and early 80s, and the fact that we’re growing new varieties.
In recent times, we’ve gone out to test the validity of the 12 degrees with our current
varieties, and the new growing areas that we have, and our re-assessment of that data,
we find that our base temperature’s more like 15 degrees Celsius, and importantly, we find
that because we’re growing these crops in these different regions, we’re having to account
for the fact that crops don’t continue to rapidly grow when we get to very high temperatures.
We have to include in our day degree function an upper threshold of 32 degrees Celsius.
Now that we’ve established our new day degree function, it’s given us more confidence to
predict crop development in a broader range of environments, and also in times where we
have extreme temperatures, either cold or hot. Going forward, the day degree function will be available for new tools to help you with
your decisions in monitoring crop development.