Detection of heat: the challenges of Silent Heat

Timing of artificial insemination is critical, with even 24 hours out from the optimal timing having a major effect on fertility. Failure to AI is costly and some cows are incorrectly identified as infertile due to heat is not being observed. A proportion of missed heats are due to the cow failing to display the behaviours associated with heat – increased activity and standing to be mounted – so called silent heat. Cows showing silent heat are cycling normally and so have a period of oestrus every 21 – 24 days but fail to show behavioural signs.

There are a number of identified causes of silent heats:

  • The type of housing and floor surface, can cause a marked reduction in oestrus-associated activity. For example, slippery floors, dead end corridors and housing in tied stanchion will reduce the willingness or ability of cows to express oestrus behaviour.
  • High yielding cows may show less oestrus behaviour
  • Cows in poor condition
  • Lameness

Causes of Silent heat

Hot weather causing heat stress can result in an increase in silent heats – something that may be seen in 4 or 5 months from now. For more on the effects on heat stress on the display of signs of oestrus. This paper and a second cover the topic well if you would like to know more…

Typical heat behaviour depends to a large extent on interaction between cows, whether to mount or stand. Where cows are crowded there may be insufficient space to express heat-related behaviour. Cows in heat that are housed with pregnant herd mates are less likely to show visible signs of heat as their herd mates are unlikely to interact. Cows most likely to interact are those that are close to oestrus themselves, thus it may become increasingly difficult to see heat signs in the “tail enders” that have not become pregnant in a block calving herd, as the majority of the herd mates are pregnant.

Effect of poor conditions

Cows in poor condition may be cycling normally but failing to show signs of heat at oestrus. However when cows are in poor condition, and in some disease conditions, they may fail to cycle altogether, as they are anoestrus. In this situation the cow has permanently low progesterone (where normally progesterone levels rise after each oestrus and decline before the next oestrus) and there are no signs of heat. There is a natural period of anoestrus which occurs for a period of time after calving, as the reproductive tract retracts. A normal cow will begin cycling 3 or 4 weeks after calving and farms can then begin to identify heat and inseminate after the voluntary waiting period post calving is over.

What if I miss the window?

Following a silent heat there may be a show of mucus at the end of oestrus. Unfortunately this occurs too late for insemination. The observation should be recorded and a recheck scheduled in about 18 days’ time. At that time, milk progesterone levels must be assessed and the cow inseminated on the second day of low progesterone.

Weekly milk samples can be checked for progesterone from a cow that has not had heat observed or been detected by activity monitors. If three weekly samples are taken, one should have low progesterone and then progesterone can be monitored three weeks after the low progesterone observation to identify heat.

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