Here's how to determine optimal maximum culling parity.

By Caitlyn Abell, Gordon Jones and Ken Stalder

The production system commonly used in the swine industry involves a three-tiered genetic pyramid. The nucleus, where most genetic improvement occurs, is at the top of the pyramid and represents the smallest percentage of total animals in the production system. The second tier is called the multiplication level and is where the improvement occurring at the nucleus herd is multiplied or produced in mass. Some genetic improvement can still occur at the multiplication level of the genetic pyramid and this tier generally represents approximately 10 to 15% of the animals production system. Finally, the third and bottom tier of the genetic pyramid is represented by the commercial level of production. This level of the pyramid represents the largest portion of the system. The genetic improvement occurring in the system is targeted to generate improved production and hence profitability at the commercial level. Genetic lag is the time required for genetic merit or improvement to pass from its source (in this example the nucleus through the multiplication level) to the commercial level of production and it is usually measured in years¹. Genetic lag is driven by the generation interval (the average age of parents when their offspring are selected to replace them) in the nucleus and multiplication levels of production^2,3.

GAIN THE BEST GENETIC ADVANTAGE

While reducing the generation interval is desirable at the nucleus level of production in order to increase the rate of genetic progress, reducing generation interval at the commercial level of production results in unnecessarily high replacement rates and reduced profitability. A sow should not be voluntarily culled before she has "paid" for herself, typically around the third or fourth parity^4,5. If replacement occurs in the early parities the animal culled and the new replacement are likely to have similar genetic merit for economically important production traits. The objective of this study was to determine the value of the genetic loss associated with retaining sows in a commercial herd for additional parities.

Four economically important swine traits were considered: 1. Number of piglets born alive (NBA), 2. 21-day litter weight (W21), 3. Days to 113kg (D113), and 4. Backfat (BF10). In the present analysis, genetic improvement for backfat is assumed to be zero as the current genetic trends indicate no genetic improvement is occurring in maternal lines for this trait⁶. This implies that the maternal lines are at or very near the desired backfat levels and no improvement in this trait is needed in the maternal lines. The genetic improvement per generation for the three remaining traits where improvement is desired in the maternal lines was assumed to be 0.3 piglets increase for NBA, 1.36 kg increased litter weight for W21, and 3 fewer days to 113kg for D113.

In this study, the sow's age at each parity and the average sow age in a given herd parity structure was calculated in generation interval units. In turn these values were used to determine the genetic lag for each of the 4 traits involved in the maternal line index (recall that backfat is being ignored) associated with each parity structure. Genetic lag of the commercial breeding herd associated with maintaining sows in the herd for additional parities was calculated using a varying generation intervals (1.5, 2.0, 2.5, and 3.0 years) at the seedstock level.

The genetic lag was calculated for each parity and generation interval. The genetic lag associated with each parity and a 1.5 year generation interval at the seedstock level is shown in Table 1. The genetic lag was determined by multiplying the assumed genetic improvement per generation by the sow's age in generation units at each parity. For example, in a herd with a generation interval of 1.5 years, keeping a sow until P3 would result in a genetic lag of 0.38 NBA, 1.71 kg of W21, and 3.78 D113. The genetic gain was given economic value by multiplying the assumed genetic improvement by the economic value associated with the trait of interest. The economic values given for each trait were $22 per pig born alive, $1.54 per kg of 21-day litter weight, and $0.17 per day to market⁶. To estimate the average value of genetic lag (in dollars) per sow in the herd at each parity the genetic lag for each of the three traits involved in the study (NBA, W21, and D113) was multiplied by the economic value associated with each trait and then these three values were summed together.

1. Genetic Improvement per generation assumed: 0.3 pigs born alive, 1.36 kg of W2, and 3.0 days to market. Backfat was not included in this evaluation because most maternal lines are at or near their desired phenotypic backfat level and hence, little or no genetic change for backfat is occurring in most maternal lines.
   Generation interval assumed: 1.5 years
2. Genetic lag per parity was determined by multiplying the age of the sow in generation units by the genetic improvement made per generation.

SOWS MUST PAY FOR THEMSELVES

When considering the replacement costs of gilts and associated gilt development costs (feed, facilities, breeding, veterinary expense, etc.) and the higher production from sows, sows should not be culled before they reach a positive net present value or, in more lay terms, they have paid for themselves⁴. When sows are retained for additional parities, the cost of developing gilts can be spread over larger numbers of pigs produced, thereby reducing the cost to produce a market hog. The cost of developing gilts that never enter the breeding herd has to be recovered by the remaining gilts that enter the breeding herd and produce for some number of parities. Finally, if a sow is replaced with a gilt before sufficient time has passed for the genetic supplier to make genetic progress, then the replacement gilt will have essentially the same aggregate genetic value or be from the same generation as the sow she is replacing.

The differences in production by parity must be considered when making culling decisions. Not only are there improvements of NBA and W21 with increasing parity, progeny from P2 versus P1 females have higher average daily gain⁸.

Table 2 shows the value of the difference in genetic potential between sows in the herd and a potential replacement gilt by parity and generation interval. Based on the data in Table 2, it can be recommended that sows should not be voluntarily culled when the average value of the genetic loss of the sows in the herd is not greater than or equal to the variable costs for gilt development which is sufficient to justify the purchase/development of a new gilt. Sows should be allowed to stay in the breeding herd as long as they are still producing satisfactorily based on number born alive, number weaned, weaning litter weight, etc. The optimal culling parity is when the value of the genetic improvement made in the gilt population exceeds the variable costs of developing the replacement gilt. In the present study, this occurs somewhere between parity 7 or greater depending on the specific development costs and the genetic progress that an individual commercial pork producer experiences.

1. Economic values assumed: $22.00 pig born alive-1, $1.54 kg-1 21-day litter weight, $0.17 day to market-1, Genetic Improvement per generation assumed: 0.3 pigs born alive, 1.36 kg of W21, and 3.0 D113. Backfat was not included in this evaluation because most maternal lines are at or near their desired phenotypic backfat level and hence, little or no genetic change for backfat is occurring in most maternal lines.
2. Establishing the total value of the genetic difference between a sow at a given parity with a replacement gilt at any time (t0) is a function of the rate of improvement for the economically important traits for which the line is selected upon, the amount of time that has passed between the culling of the sow (tc), and the economic value placed on those traits.

The economic value of the genetic lag associated with retaining a sow for additional parities that were presented in the results represent the upper limits with respect to the amount of genetic progress one would expect to make in a swine breeding program. Hence, the values used for the genetic gain per generation are the very highest one could expect to occur. However, when assigning values to compare making replacement decisions based on the amount of genetic gain using the extreme values is justified in order to compare differences assuming the very best improvement occurs at the seedstock level. Table 3 shows the value of the difference in genetic potential between sows in the herd and a potential replacement gilt by parity and with varying levels of genetic progress.

1. Economic values assumed: $22.00 pig born alive-1, $1.54 kg-1 21-day litter weight, $0.17 day to market-1, 1.5 years generation interval at the seedstock level. Backfat was not included in this evaluation because most maternal lines are at or near their desired phenotypic backfat level and hence, little or no genetic change for backfat is occurring in most maternal lines.
2. Units: NBA (number born alive) - pigs, W21 (21-day litter weight) - kg, D113 (days to 113kg) - days
3. Establishing the total value of the genetic difference between a sow at a given parity with a replacement gilt at any time (t0) is a function of the rate of improvement for the economically important traits for which the line is selected upon, the amount of time that has passed between the culling of the sow (tc), and the economic value placed on those traits.

SPREAD OUT THE COSTS

The findings support that it is not profitable to replace sows in the breeding herd at rates currently employed if the goal is solely to replace sows in order to keep up with genetic improvement that is occurring at the nucleus and multiplication levels of the genetic system used by the genetic supplier. It is imperative that commercial swine producers consider the fact that just because a gilt has a greater genetic potential than the current sow in the breeding herd, it does not mean that the sow should be removed from the herd. Furthermore, producers must remember they will gain the genetic improvement immediately when a replacement gilt is entered into the breeding herd to replace an "old" sow regardless of the number of parities that sow is retained in the breeding herd. The sow must be maintained in the herd for a period of time so that producers can recover the initial investment costs and longer such that the initial investment costs can be spread out over a greater number of pigs produced.

References

1	Sellers, H. I. 1994. Genetic principles of Farmers Hybrid Companies, Inc. Swine Products. Farmers Hybrid Companies, Inc. Des Moines, IA.
2	Falconer, D.S., 1989. Introduction to quantitative genetics, 3rd ed., Longman Scientific and Technical, England.
3	Bourdon, R.M. 1997. Understanding animal breeding, 2nd ed., Prentice-Hall, Upper Saddle River, New Jersey 1997.
4	Stalder, K. J., R. C. Lacy, T. L. Cross, G. E. Conatser, and C. S. Darroch. 2000. Net present value analysis of sow longevity and the economic sensitivity of net present value to changes in production, market price, feed cost, and replacement gilt costs in a farrow-to-finish operation. Prof. Anim. Sci. 16:33-40.
5	Stalder, K. J., R. C. Lacy, T. L. Cross, and G. E. Conatser. 2003. Financial impact of average parity of culled females in a breed-to-wean swine operation using replacement gilt net present value analysis. J. Swine Health Prod. 11:69-74.
6	NSR. 2009. Swine Testing and Genetic Evaluation System (STAGES) National Swine Registry. Available: http://www.ansc.purdue.edu/stages/ Accessed: May 11, 2009.
7	Fitzgerald, R. F., K. J. Stalder, C. D. Johnson, L. L. Layman, and L. A. Karriker. 2008. An economic analysis of feeding cull sows. Prof. Anim. Sci. 24:355-362.
8	Burkey, T. E., P. S. Miller, R. K. Johnson, D. E. Reese, and R. Moreno. 2008. Does dam parity affect progeny health status? 2008 Nebraska Swine Report. University of Nebraska.

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Editor's Note: Caitlyn Abell is a graduate student at Iowa State University; Dr. Gordon Jones, PhD is in the Department of Agriculture, Western Kentucky University; and Dr. Kenneth Stalder, PhD is in the Department of Animal Science at Iowa State University.

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