Key Points:

• Use of POSILAC^® does not eliminate genetic variation.
• Genetic progress is expected to increase because both the mean and variance are expected to increase in an environment with POSILAC.

The effect of bovine somatotropin is similar to the selection goal of genetic programs and other dairy technologies which is to lower farm fixed costs over units of milk produced. Use of POSILAC bovine somatotropin will not eliminate genetic variation, and selection will continue to be important. There will be challenges in distinguishing between "AI proven bulls" and "bST proven bulls," which is no different from established practices that have always existed within the industry. Dairy producers who use POSILAC will pay more attention to the contribution genetics can make to increasing productivity.

Genetic progress is likely to increase with POSILAC from the current gain of 125 lbs per year.¹ Genetic progress is expected to increase because both the mean and variance are expected to increase in an environment with POSILAC. Genetic progress could be less under an environment with POSILAC compared to current progress if manipulation of records occurs. Manipulated records will decrease the accuracy of cow and sire evaluations. Dairy records rely on the integrity of individual producers. Under current conditions, records can be manipulated by simply offering additional feed, for example. Biases due to POSILAC can be handled if records are properly coded.²

Several computer simulation models^1,3,4 indicated that if administration of bST is accurately recorded, then effects on genetic progress will be minimal. The more random the use of bST in the population, the less will be the effect on genetic evaluations, especially for bulls.³ A major problem would arise if individual cows are supplemented with bST and the highest responders are used as bull dams. However, as stated previously, research has not been successful in predicting individual cow responses so manipulation of potential bull dams would be difficult. POSILAC is not recommended for use on any bull dams as we do not have any specific trial data on the effects on male offspring. If the response would be an individual trait, this would lead to a preference for high responders.⁵ High-yielding cows and daughters of bulls with high PDs will respond to bST equally as well as cows of lower genetic merit.^6,7

Dairy farmers with top genetic and production herds will have top genetic and production herds under POSILAC bovine somatotropin.¹ The selection goal, increased yield, will remain the same, and will continue to be highly associated with net income within a herd. The top genetic herds will still sell bulls to AI and producers will continue to benefit from sampling young sires.¹ The importance of selection will continue to grow with increased emphasis on milk production efficiency. With POSILAC, variation within herds will increase and the top genetic cows will be the best daughters of the best bulls. If all available cows are supplemented, the variance will increase because the mean will increase. If only a part of the herd is supplemented with POSILAC, then there will be a significant increase of within the herd variation because the difference between the highs and the lows will increase.⁸ This can be handled by adjusting the records for the effect of POSILAC.²

The impact of POSILAC on sire proofs is unknown until widespread adoption within the industry occurs. POSILAC may neutralize the effects of some genes, but is not likely to reverse their actions and cause bulls to rank much differently.¹ There might be slight changes in sire rankings and sire proofs, but the net effect is expected to be minimal and the best bulls available today will also rank at the top in a POSILAC environment.

References

1. Everett RW: How will bST affect dairy genetics in the 1990's? Hoards Dairyman 1987(April 10), p 301.
   
2. Everett RW, Galton DM, Kachman SD: Dairy genetics in a bST environment. Proc: Advanced Technologies Facing the Dairy Industry: bST. Cornell Cooperative Extension Meeting, Rochester, NY, 1989.
   
3. Burnside EB, Meyer K: Potential impact of bovine somatotropin on dairy sire evaluations. J Dairy Sci 1988;71:2210.
   
4. Frangione TA, Cady RA: A simulation study of the effects of somatotropin usage on sire evaluations and milk yield and yield heritabilities. J Dairy Sci 1988;71(Suppl. 1):239.
   
5. Gravert HO: bST and breeding-overview. Monsanto bST-Symposium, Brussels, May 22-24, 1988.
   
6. Ferguson JD: Strategies of bST utilization. Proc: Advanced Technologies Facing the Dairy Industry: bST. Cornell Cooperative Extension Meeting, Rochester, NY, 1989.
   
7. McDaniel BT, Hayes PW: Absence of interaction of merit for milk with recombinant bovine somatotropin. J Dairy Sci 1988;71(Suppl 1):240.
   
8. Everett RW: Personal communication, 1991.

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