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Part 4 | rbST: Friend or Foe

In previous articles on rbST, we explained its origin, the reasons why it is used, and what effect it has on the sustainability of dairy farming. In this article, we delve deeper. Farmers usually have a list of questions that they frequently ask when interested in rbST, so we will answer some of them.

Nutrition is one of the most important factors capable of influencing the response to rbST, but management, health, and reproduction could also have an effect.

The following are some of the questions farmers frequently ask regarding rbST:

  • Is rbST harmful to cows?

  • How is management affected and does the use of rbST mask poor management?

  • Does rbST change milk composition and quality?

  • Will I have more mastitis in my herd or a higher somatic cell count?

  • What about my reproduction and conception rate?

  • Does rbST affect longevity?

Let us look at some of the most important questions that often arise, starting with the most controversial question of all.


Ever since it was first used, there have been many studies done on the effect of rbST on reproduction and although there have been slight differences, depending on the type of study conducted, they have all produced similar results. Stanisiewski et al. (1992) showed that cows receiving 5 mg of bST/day, even from 14 to 130 days, had higher

pregnancy rates, conception and first service conception rates, and maintained their body condition scores.

Santos et al. (2004) went into more detail regarding the effect of rbST on reproduction and found that bST and insulin-like growth factor 1 have an effect on the CL differentiation and embryonic development that favour fertility. They showed that

conception rates on day 45 were increased by bST treatment due to increased

pregnancy maintenance. The follicular waves and oestrus cycle lengths were similar in the control and the bST-treated cows.


Injections of rbST are given every 14 days to maintain the high levels required for peak milk production. To maintain these high levels, cows need access to additional feed. It is also important to ensure adequate farm management when injecting rbST, otherwise the full potential of the product will not be attained. As management practices improve, so does the production response of cows receiving rbST.


Huber et al. did an experiment in which they used rbST for four lactations and found that over all the lactations there was an increase in milk production, body weights were higher but there was no real change in body condition score. Chilliard (1988) also reported an increase in milk production of around 1 000 kg per 200-day lactation


However, this can vary depending on the farming system and conditions on the farm. Chilliard (2008) showed that treatment with rbST can increase the lactation yield by a percentage ranging from 5% to over 25%. The effect of rbST in one lactation does not affect the response or yield in the next lactation. If the cow has been genetically bred to produce for only two to three lactations, she will still only produce for those lactations regardless of whether or not rbST is used. rbST only maintains the peak yield of that lactation for a longer period and does not affect the following lactations or the longevity of the cows. More questions will be covered in the next article.

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