Wednesday, 14 November 2012 15:39

Raising replacement heifers

Written by  Dwayne Summach, Sask. Agriculture, Kindersley
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The theory behind raising replacement heifers for the last 50 years has been to get them to 65 per cent of mature weight by breeding.


Following this rule of thumb meant the heifers should have reached puberty and begun cycling prior to being exposed to a bull.
Recent research has challenged the 65 per cent of mature body weight concept, recognizing over the course of time, genetic selection has resulted in earlier onset of puberty.
Work by Dr. Rick Funston and his colleagues at the University of Nebraska – West Central Research and Extension Centre, indicates a target of 55 per cent of mature body weight and on an inclining plane of nutrition will result in similar pregnancy rates, and may even improve lifetime productivity.
In practical terms, this means producers have an opportunity to target a lower rate of gain for most of the winter feeding period, with a higher rate of gain for two months prior to breeding. A practical example of the application of this theory would be to compare the development of a 500 pound heifer calf that will be 1,400 pounds at maturity. Using the traditional target of 65 per cent of mature weight, means the target weight at breeding is 910 pounds, and will require an average daily gain of 1.75 pounds per day from Nov. 1 to June 22 for a calving start date of April 1 the following year. Using an alternative method of 55 per cent of mature weight, means the target weight would be 770 pounds at breeding. This weight can be achieved by targeting a rate of gain of 1.0 pound per day from Nov. 1 to Apr. 21, and providing a rising plane of nutrition to move the rate of gain to 1.75 pounds per day from Apr. 22 to June 22. Please note the rations will need to be formulated and balanced to provide the appropriate nutrients to achieve the targeted gains in order for the strategy to be successful.
Additional strategies to improve heifer pregnancy rates include the use of an ionophore, such as monensin sodium.
It is unknown as to why monensin sodium influences conception rates but it routinely reduces the onset of puberty by 10 to 14 days of age. The use of dried distiller’s grains as a supplement has been shown to improve conception to artificial insemination in heifers by 22 per cent.
It is speculated the amino acid composition of the rumen undegradable protein absorbed directly by the small intestine plays a role.
Detailed information regarding rearing replacement heifers can be found at: http://www.iowabeefcenter.org/Beef%20Cattle%20Handbook/Replacement_Heifer.pdf and http://digitalcommons. unl.edu/cgi/viewcontent.cgi?article= 1260&context=rangebeefcowsymp or by contacting a regional livestock specialist with the Saskatchewan Ministry of Agriculture.
(Dwayne Summach is the Regional Livestock Specialist Saskatchewan Ministry of Agriculture, Kindersley)

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