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What exactly is heat stress, how does it affect dairy cows and why is it a growing threat to UK production?

All ruminants struggle with heat stress but Dairy cows are particularly susceptible.  Comfort, consistent health and optimal intakes are all essential to ensure dairy cows remain the balanced productive machines the industry expects.  Due to poor sweating capacity cows manage their thermal comfort zone through a combination of intake adjustment, drooling and panting or increased respiratory rate. 

This means with the onset of heat stress they will reduce intakes, often particularly the forage portion of their diet, as well as reduce rumination, cudding and therefore buffering capacity of saliva.  Raised drooling and respiration rate result in increased CO2 lossfurther limiting buffering potential of saliva, and this combined with dietary concentrate imbalance quickly places cows in ruminal acidosis territory.  Diseases related to acidosis are multiple and complexed but essentially result in reduced milk quality, quantity and often forced early herd exit.

Further to these issues, raised core body temperature damages cell function and leads to ‘oxidative stress’.  This has significant implications with respect to immune function, namely Mastitis and SCC, as well as reproductive health.

With the challenges the industry faces can you afford to let cows ‘manage through’ this seasonal issue, which is now presenting itself as having a yearlong risk period?

Heat stress is the result of rising environmental temperature in the face of high ambient humidity.  We use temperature humidity index (THI) to estimate stress bands as shown in (fig.1).

Figure 1

It is generally accepted that >72 THI units suggests early indication of heat stress and >80 is where significant affects can be appreciated.  The work behind this THI chart was conducted in 1990 across Arizona cattle holdings.  It should be kept in mind that their climate is a very different one with lower ambient humidity.  Here in the UK rarely fall below 60% and often hit the 90’s, particularly overnight.  From studying modern environment probes and heat stress warning systems we see that most alerts come from temperature spikes on relatively normal days with respect to humidity. The impact of global warming having caused us to have higher summer temperature spikes and a longer hot season with sunny spells starting in very early spring and extending right through autumn.

Temperature rises throw far higher risk at the cows, especially on a muggy day..or night; and so much important feeding time for the modern dairy cow occurs by evening and night.  With that in mind it should be highlighted that well stocked poorly ventilated cubicle sheds can easily carry a differential of upto +10oC to the outside temperature.  This is of huge significance, especially when we think who or what type of dairy cow is generally housed 24/7; high yeilders!  Just like poor feed bunk management, poor environmental management will result in reduced intakes, disease issues and poor unit efficiency.

Whats the solution?  Reduce the risk; accepting the evolving climatic challenge and adopting adaptations to the environment or system.  No one solution is perfect and solutions have to be selected to suit cows, businesses and farms.

Firstly what is the situation on your farm, in your sheds?  Fully integrated climate systems are available at great expense but so are cheap basic sensors.  Gather data on your set up and consult with advisers as to where to go with the information.

Over exposure of cows to sun all day at grass and reduced available energy density of ration is far from ideal and a step too far for some businesses.  Building an insulated roof line on legs, northern aspect roof lights only and perfect airflow is unavailable to many.  Retro-fitting huricane fans, destratifying fans and calving up old sheds to alow appropriate air intakes and outlets is often the best option.

Which fan is the best?  Large volume, low speed fans set up in series to ventilate a shed, maintaining consistent air exchange tend to be the most effective.  But this type of ‘tunnel ventilation’ is not going to work if there is no chance of air intake due to old fashioned yorkshire boarding or solid sheeting.  Walk sheds and yards with an advisor or your vet and calculate required intakes and outlets, which fan system is best suited to your shed, and where to place them.

If you are lucky enough to enjoy a green field development make sure you go and visit recent projects and see how cutting edge design ideas develop year on year.  The insulated rooves and considered roof light panels are often worthwhile investments where possible.  Three years in and we are seeing no summer yield or conception rate issues on such a unit and they have not yet installed fans.

If you would like further advice on preventative measures to reduce the effects of heat stress on your farm, please get in touch with the Molecare team who will be happy to help on [email protected]


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