Is heat stress affecting yourherd reproductive success?

Words by: Lisa Whitfield Production Animal Veterinarian

Heat stress has been mentioned a lot in the last few years and is a buzzword at discussion groups leading up to summer. It has been clearly demonstrated that heat stress has a significant impact on cow welfare and production.

The reproductive impact of heat stress is not insignificant either, and consideration should be made to heat stress days which occur over the mating period. Cows experience heat stress where there are ambient temperatures exceeding 25C, or at lower temperatures when the humidity index is above 70%.

Heat stress impacts not only the fertility of the cow and her ability to become pregnant in the first place, but it also reduces her ability to maintain pregnancy.

The behaviour of a cow is altered when she is experiencing heat stress – she is less active, spends less time grazing and spends less time lying down. Negative energy balance from reduced feed intake, changes in reproductive hormone levels and damaged follicles are all substantial problems when it comes to establishing pregnancy. Changes in physical activity include a reduction in the display of heat behaviour during oestrus, which is so heavily relied on to detect when she is ready to be bred. Heat displays may be both weaker and of shorter duration. This is not only due to the physical effect of excessive heat changing the normal behaviour of a cow.

The granulosa cells, which line follicles and release the hormone Oestradiol, which drives the act of heat behaviours just prior to ovulation, are very sensitive damage from high temperatures. Developing follicles which contain the oocyte from which a pregnancy will eventually establish, are susceptible to damage from high heat up to 42 days prior to when they are ovulated.

The first month of pregnancy is the most sensitive part of gestation where damage from heat stress, may result in the loss of pregnancy.

The first month of pregnancy is the most sensitive part of gestation where damage from heat stress, may result in the loss of pregnancy. This means those hot days in early to mid-summer could potentially be affecting a substantial number of early pregnancies in spring calving cows.

Bull fertility must not be forgotten either as heat stress resulting in a high peripheral body temperature damages spermatozoa, and can render a bull infertile for many weeks following the heat event.

Mitigating the effects of heat stress is challenging and requires short and long term planning. The provision of enough shade and fresh water to meet the needs of the whole herd at peak demand has practical limitations on most farms.

With a long term view, planting trees to provide large areas of shade is both good for the cows and the environment. It would be good to see someone design a portable shade system which is able to withstand the variable climate of New Zealand’s pastoral farms.

Reductions in non-voluntary physical activities should be considered. This may include:

• Grazing the herd closer to the shed to reduce commuting distances to afternoon milkings or night paddocks

• Reducing crowding in the collecting yard

• Controlling of nuisance flies from early in summer, right through the end of the hot season. Flies not only create irritable cows, but will drive crowding behaviour, tail swishing, stamping and head shaking. NZ farms struggle with the practicalities of reducing heat stress in large open spaces. There is a gap in the market for a durable, portable shelter for use in large open spaces where many animals need access at once.

Perhaps light coloured, breathable covers could help our black cows?

In areas where infrastructure is established, systems can be put in place such as:

• Creating air flow enclosed sheds by opening doorways or using industrial fans

• Using misting systems over yards to create evaporation

• Providing water troughs on races for cows walking long distances In summary, heat stress has known negative impacts on cattle reproduction. It is a practically challenging problem, and there is scope for the development of shelter and cooling systems which allow for the management of groups of cows in open spaces which are exposed to high ambient temperatures. Thought should be invested early on to changing management practices which exacerbate heat stress.

• Lisa Whitfield MVM (Distinction) BVSc - Production Animal Veterinarian.