We studied the effect of heat stress on milk quality in Spanish organic dairy farms using
published milk productivity equations. We collected data from 23 weather stations and
14,424 milk test-days for milk yield and milk fat and protein content for the period
July 2011 to June 2013. As an indicator of heat stress, we used the maximum daily
temperature–humidity index (THI) from 2 days before the milk test date. We fitted the
data using hierarchical regression models stratified by farm, cow parity and monthly testday
milk records. The effect of THI was deemed low on biological costs through milk
yield. However, the known negative relationship between milk yield and milk quality
(protein and fat content) became even steeper when the THI increased, suggesting a
significant negative correlation between heat stress and milk quality. Therefore, although
the milk yield of cows in the organic farming systems analyzed appeared resilient to heat
stress conditions, milk quality, a major selling point for organic dairy products, was
negatively affected. The model presented here could be used to predict the potential
impacts of different climate change scenarios on dairy farming, and to delineate adaptation
strategies within organic systems.info:eu-repo/semantics/publishedVersio
