The mission

To develop a grazing heat load index that enables forecasts or alerts of heat stress risk for dairy cattle grazing pasture in New Zealand. Commonly used indices only include ambient temperature and humidity. In New Zealand, predictions of heat stress in grazing dairy cows requires additional weather variables, including solar radiation and wind speed. The aim is to help identify and predict heat stress more accurately and have ways to reduce its impact.

Project progress: developing

Identifying heat stress in cows

The physiology and behaviour of cows changes when they experience heat stress. Increased breathing rate, seeking shade and grouping are all signs that cows are trying to keep cool. When looking at a cow’s individual response to heat stress there is variability in susceptibility - that is, some cows cope better than others.

Predicting heat stress

Heat stress risk is expected to increase in future as daily temperatures continue to rise and extreme hot weather events become more frequent. The ability to predict heat stress in dairy cows could help on-farm management decisions, such as timing of milking or choice of paddock, changing milking frequency, breeding choices, tree planting, and installation of portable shade structures.

Measuring heat stress

Until now, there have been a variety of ways to assess heat stress, including animal measures and weather conditions. Animal measures include breathing rate, panting and drooling, and weather conditions typically focussed on air temperature and humidity.

The Temperature Humidity Index (THI) is a common way to measure cattle heat load. It looks at both the temperature and humidity in the air. However, it doesn't include factors like wind or sunlight. But when cows are outside, sunlight can be a big part of how hot it feels to them.

Progress to date

DairyNZ, in partnership with AgResearch and Fonterra, recognised the need for a new New Zealand thermal heat load index suitable for grazing cattle. As a result, the grazing heat load index (GHLI) was developed, and considers air temperature, solar radiation and wind speed.

Collecting data

To begin with, most of the research took place in the Waikato and was then extended to collecting data from across New Zealand farms which had existing on-farm sensor technology. The goal was to refine and increase the accuracy of the index data collection from more locations and conditions, and compile data into one benchmark database.

A national grazing heat load index

The next step was to see how this data could be scaled and to consider different regional weather conditions. The national data collection verified that New Zealand cows experience different summer conditions and the hotter in gets the more risk there is of heat stress. This data was used to update the grazing heat load index, taking it from a Waikato index to a national index, regionally applicable index.

What's next

Heat stress will continue to become more important as the climate changes. The research project will keep refining geographical location data to understand which regions are more prone to heat stress.

The scope of the research is being widened to investigate the use of existing on-farm technology to identify indicators of heat stress before and after it occurs. The research is currently examining sensory data collected from collars and rumen bolus to understand a cows’ individual heat stress response. Technology will facilitate gathering data from larger herds without needing to manually observe cow respiration in the paddock.

How this will help farmers

Using the grazing heat load index, customised for New Zealand farming, means it is possible to predict which weather conditions create heat stress.

The results of this research will be used in the development of tools and technologies that can monitor and inform when cows are likely to experience heat stress and will support proactive management to keep your cows cool.