> ## Documentation Index
> Fetch the complete documentation index at: https://docs.agrihub360.com/llms.txt
> Use this file to discover all available pages before exploring further.
# BYDV Day-degrees
> Degree-days from emergence towards the 170 °C day threshold.
## What It Tells You
The **BYDV day-degree count** is how close an autumn-sown cereal is to the second aphid generation. That generation spreads barley yellow dwarf virus plant to plant. Aphids develop with temperature. The count sums degree-days above 3 °C from crop emergence, or from the last aphicide, towards the 170 °C day threshold in the AHDB tool. A field at 40 of 170 has a reason not to spray yet. A field at 150 is the point to start walking the crop. The window closes at stem extension, when a new infection costs little. The count is computed by the [BYDV Risk](/models/bydv) model. It is a thermal proxy for aphid activity. It is not an observation of aphids.
## How It Works
A daily job accumulates degree-days from a gridded weather archive and forecast. Emergence is inferred from the crop's own degree-days since sowing. The state moves from accumulating to approaching at 75 per cent of the threshold. One notification is sent when 170 is reached. A new aphicide in the spray record moves the start date and the count begins again.
