The Professional Animal Scientist 26 (2010):404–411
©2010 American Registry of Professional Animal Scientists
U stoingDetermine the Genetic Lag Value the Optimal
Maximum Parity for Culling in Commercial Swine Breeding Herds1 C. E. Abell,* G. F. Jones,† K. J. Stalder,*2 PAS, and A. K. Johnson† *Department of Animal Science, Iowa State University, Ames 50011; and †Department of Agriculture, Western Kentucky University, Bowling Green 42104
ABSTRACT 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. To estimate this value, a spreadsheet was developed incorporating culling rate by parity, generation interval, genetic improvement rate, and economic values for each trait. Number born alive, 21-d litter weight, and days to market were the traits examined in this study. The genetic improvement per generation (economic values assigned) for these traits were 0.3 pigs ($22.00/pig), 1.36 kg ($1.54/kg), and 3.0 d ($0.17/d), respectively. Backfat was not included because little, if any, genetic improvement is being made for this trait, suggesting that most maternal lines have a near ideal backfat level. The genetic lag value associated with retaining a sow to parity 3, 5, and 7 was $18.23, $32.01, and $47.99, respectively, This journal paper of the Iowa Agriculture and Home Economics Experiment Station (Ames), Project No. 3614, was supported by Hatch Act and State of Iowa funds. 2 Corresponding author:
[email protected] 1
in a herd whose seedstock supplier has a generation interval of 1.5 yr. The average genetic lag value was calculated when the breeding herd parity structure differed because producers allowed differing maximum parities at culling (automatic cull for old age). The average economic value for each sow in a herd with forced culling at parity 3, 5, and 7 and an 18.8% culling rate by parity (annualized 42.3% culling rate) was $11.93, $16.47, and $20.70, respectively. The value of the genetic lag represents lost opportunity, and when this value exceeds the gilt development variable costs, it represents the optimal time for culling the sow from the breeding herd and replacing it with a gilt. Key words: generation interval, genetic lag, parity, sow
INTRODUCTION The production system commonly used in the swine industry involves a 3-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 pyramid. 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 in the pyramid. 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 efficiency, 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 years (Sellers, 1994). Genetic lag is driven by the generation interval or the average age of parents when their offspring that will replace them are born (Falconer and Mackay, 1996;
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Genetic lag value for optimal maximum parity for culling
Bourdon, 1997) in the nucleus and multiplication levels of production. While reducing the generation interval is desirable at the nucleus level of production to increase the rate of genetic progress, the result on profitability of reducing the generation interval at the commercial level has not been examined. A sow should not be voluntarily culled before it has paid for itself, typically around the third or fourth parity (Stalder et al., 2000, 2003). If replacement occurs in the early parities, it is possible that relatively little genetic progress has been made in the production system; hence, the animal culled and the new replacement are likely to have a similar genetic merit for economically important 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.
MATERIALS AND METHODS An Excel (Microsoft Corporation, Redmond, WA) spreadsheet was developed to determine the optimal parity for a sow to be replaced in the breeding herd, taking into consideration the generation interval of the seedstock supplier as well as the genetic progress for economically important maternal traits. The spreadsheet involved a sensitivity analysis for generation interval and genetic progress for 4 economically important swine traits: 1) number of piglets born alive (NBA), 2) 21-d litter weight (W21), 3) days to 113 kg (D113), and 4) 10th-rib backfat (BF10). In the present analysis, genetic improvement for BF10 is assumed to be zero because the current genetic trends indicate no genetic improvement is occurring in maternal lines for this trait (National Swine Registry, 2009). This implies that the maternal lines are at or very near the desired BF10 levels and that no improvement in this trait is needed in the maternal lines. The generation interval values used in the spreadsheet were chosen based on those provided by swine breeding companies that supply US commercial herds with
their seedstock supply (personal communication). The spreadsheet uses the parity structure outlined by Dhuyvetter (2000) to determine the remaining calculations. The different herd parity structures are distinguished by the maximum number of parities a sow is allowed to reach before being culled from the herd and the replacement rate per parity. It was assumed that when the sows reached the maximum parity, they would be voluntarily culled. The spreadsheet was designed to determine a herd parity structure based on the maximum parity a sow would be allowed to reach and the rate at which sows would be replaced in the herd.
Age of Sows The age at first breeding for replacement gilts was assumed to be approximately 7.5 mo (225 d) or by the time it would likely have reached the second estrus, and first farrowing would occur at 12 mo of age (Whittemore, 1998). The age of sows at subsequent parities was calculated
by first dividing a year (365 d) by the litters per female per year to arrive at the number of days between parities (farrowing interval). The average litters per female per year in this analysis was assumed to be 2.25 (PigCHAMP, 2008). This quotient was added to the age at first breeding to determine the age at parity 2 and was then added again to determine the age of the sow at parity 3, and so forth until the age at parity 15 (the maximum parity evaluated in this study) was reached. The generation interval age of the sow at each parity was calculated as follows: age at each parity/generation interval. Table 1 shows the age of the sow in generation interval units by parity when using 4 different generation interval values. For example, a sow at parity 3 (1.89 yr of age) would be 1.26 generations old in a herd in which the generation interval of the seedstock supplier is 1.5 yr and would be 0.63 generations old in a herd in which the generation interval is 3.0 yr. Hence, as the generation interval increases, the age
Table 1. Age of sows by each parity (in generation units) from a study evaluating the genetic lag effect on gilt replacement decisions in commercial sow breeding herds1 Generation interval at the seedstock level, yr Parity 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1
Age at farrowing, yr 1.00 1.44 1.89 2.33 2.78 3.22 3.67 4.11 4.56 5.00 5.44 5.89 6.33 6.78 7.22
1.5
2.0
2.5
0.672 0.96 1.26 1.56 1.85 2.15 2.44 2.74 3.04 3.33 3.63 3.93 4.22 4.52 4.81
0.50 0.72 0.94 1.17 1.39 1.61 1.83 2.06 2.28 2.50 2.72 2.94 3.17 3.39 3.61
0.40 0.58 0.76 0.93 1.11 1.29 1.47 1.64 1.82 2.00 2.18 2.36 2.53 2.71 2.89
3.0 0.33 0.48 0.63 0.78 0.93 1.07 1.22 1.37 1.52 1.67 1.81 1.96 2.11 2.26 2.41
Assumed 2.25 litters/yr, and age at first farrowing of 1 yr.
The age in terms of generation units is determined by dividing the age of the sow in years by the generation interval.
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determine the genetic lag for each of the 4 traits involved in the maternal line index (recall that BF10 is being ignored) associated with each parity structure. An average culling rate of 18.8% at each parity (PigCHAMP 2007, 2008) was used to determine the parity distributions. This is the equivalent of an annual culling rate of 42.3%. The spreadsheet developed by Stalder et al. (2000) was used to determine the average value for breeding herd females expressed in generation units under different maximum parity, and hence herd parity, structure scenarios. The average generation of a given parity structure can be used to determine the economic value for the temporary loss in genetic improvement (sow at some parity × aggregate genetic value − the aggregate genetic value of the replacement gilt) attributable to retaining a sow beyond its first parity.
Table 2. Genetic lag by parity for the maternal trait number of pigs born alive in a study evaluating the genetic lag effect on gilt replacement decisions in commercial sow breeding herds1 Generation interval at the seedstock level, yr Parity
1.5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0.20 0.29 0.38 0.47 0.56 0.64 0.73 0.82 0.91 1.00 1.09 1.18 1.27 1.36 1.44
1
2
2.0
2.5
3.0
0.15 0.22 0.28 0.35 0.42 0.48 0.55 0.62 0.68 0.75 0.82 0.88 0.95 1.02 1.08
0.12 0.17 0.23 0.28 0.33 0.39 0.44 0.49 0.55 0.60 0.65 0.71 0.76 0.82 0.87
0.10 0.14 0.19 0.23 0.28 0.32 0.37 0.41 0.46 0.50 0.54 0.59 0.63 0.68 0.72
Genetic improvement per generation assumed 0.3 pigs born alive.
Genetic lag per parity was determined by multiplying the age of the sow in generation units by the genetic improvement made per generation.
2
Genetic Lag of a sow in terms of generation units is less given the same parity.
calculated in generation interval units. In turn, these values were used to
The genetic lag was calculated for each parity and parity distribution.
Generation Interval The generation interval for the seedstock supplier was used to determine the genetic improvement made after each parity. The longer the generation interval resulting at the nucleus and multiplier levels of seedstock production, the longer the genetic lag before the improvement is realized at the commercial herd level. This study compared the improvement made at differing generation intervals with the production from retaining sows to examine the profitability associated with the interaction between genetic improvement and differing generation intervals. Genetic lag of the commercial breeding herd associated with maintaining sows in the herd for additional parities was calculated using the following generation intervals for the seedstock suppliers: 1.5, 2.0, 2.5, and 3.0 yr. In this study, the age of the sow at each parity and the average sow age in a given herd parity structure were
Table 3. Genetic lag by parity for the maternal trait 21-d litter weight in a study evaluating the genetic lag effect on gilt replacement decisions in commercial sow breeding herds1 Generation interval at the seedstock level, yr Parity
1.5
2.0
2.5
3.0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0.912 1.31 1.71 2.12 2.52 2.92 3.32 3.73 4.13 4.53 4.94 5.34 5.74 6.15 6.55
0.68 0.98 1.28 1.59 1.89 2.19 2.49 2.80 3.10 3.40 3.70 4.00 4.31 4.61 4.91
0.54 0.79 1.03 1.27 1.51 1.75 1.99 2.24 2.48 2.72 2.96 3.20 3.45 3.69 3.93
0.45 0.65 0.86 1.06 1.26 1.46 1.66 1.86 2.07 2.27 2.47 2.67 2.87 3.07 3.27
1
Genetic improvement per generation assumed 1.36 kg of 21-d litter weight.
Genetic lag per parity was determined by multiplying the age of the sow in generation units by the genetic improvement made per generation.
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Genetic lag value for optimal maximum parity for culling
Table 4. Genetic lag by parity for the terminal trait days to 113 kg in a study evaluating the genetic lag effect on gilt replacement decisions in commercial sow breeding herds1 Generation interval at the seedstock level, yr Parity 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
1.5 2.00 2.89 3.78 4.67 5.56 6.44 7.33 8.22 9.11 10.00 10.89 11.78 12.67 13.56 14.44
2
2.0
2.5
3.0
1.50 2.17 2.83 3.50 4.17 4.83 5.50 6.17 6.83 7.50 8.17 8.83 9.50 10.17 10.83
1.20 1.73 2.27 2.80 3.33 3.87 4.40 4.93 5.47 6.00 6.53 7.07 7.60 8.13 8.67
1.00 1.44 1.89 2.33 2.78 3.22 3.67 4.11 4.56 5.00 5.44 5.89 6.33 6.78 7.22
Genetic improvement per generation assumed 3.0 d to market. Backfat was not included in this evaluation because most maternal lines are at or near their desired phenotypic backfat level; hence, little or no genetic change for backfat is occurring in most maternal lines.
1
Genetic lag per parity was determined by multiplying the age of the sow in generation units by the genetic improvement made per generation.
2
The genetic lag associated with each parity is shown in Tables 2, 3, and 4. The genetic lag for each trait was determined by multiplying the assumed genetic improvement per generation by the age of the sow in generation units at each parity. For example, in a herd with a generation interval of 3.0 yr, keeping a sow until parity 3 would result in a genetic lag of 0.19 NBA, 0.86 kg of W21, and 1.89 D113.
Economics To determine the economic value of the genetic lag, it was assumed that a maternal line genetic index was used for selection and included NBA, W21, D113, and BF10 as the economically important traits under selection, as described previously for this maternal line example (BF10 was excluded from the calculation, as described previously). 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 associated with each trait were obtained from the Swine Testing and Genetic Evaluation System (National Swine Registry, 2009). The economic values given for each trait were $22 per pig born alive, $1.54 per kg of W21, and $0.17 per day to market. The value of the genetic gain or genetic improvement is the aggregate genetic improvement that occurs per generation for the traits under selection for a given breed or line of swine. The genetic improvement per generation for the 3 remaining traits in which improvement is desired in the maternal lines was assumed to be a 0.3-piglet increase for NBA, a 1.36-kg increase in litter weight for W21, and 3 fewer days for D113. The genetic improvement per generation was used to determine the value of the genetic lag associated with keeping a sow in the herd for additional parities when compared with the opportunity to replace an older sow with a replacement gilt.
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Value of Genetic Loss To estimate the average value of genetic lag ($) per sow in the herd at each parity, the genetic lag for each of the 3 traits involved in the study (NBA, W21, and D113) was multiplied by the economic value associated with each trait, and these 3 values were then summed together. Using these values, we determined the average value of the genetic lag ($) per sow in the herd with each herd parity distribution. Using the percentage of sows at each parity with each herd parity structure, as reported by Dhuyvetter (2000), we calculated the value by averaging the genetic lag obtained with each sow in the herd. The value of the genetic lag (V) was calculated as follows: V = (GNBA(P) × ENBA) + (GW21(P) × EW21) + 1/2{[(GD113(P) × ED113) + (GBF10(P) × EBF10)]NP + [(GD113(P−1) × ED113) + (GBF(P−1) × EBF10)]N(P−1) + … + [(GD113(1)×ED113) + (GBF10(1)×EBF10)]N1}, where G is the genetic lag for each trait at parity P, E is the economic value for each trait, and N is the total number of pigs that were produced by the sow at parity P. The total number of pigs sold at each parity was assumed to be 9.58 pigs at parity 1, 9.92 at parity 2, 10.26 at parity 3, 10.17 at parity 4, 9.92 at parity 5, 9.58 at parity 6, 9.15 at parity 7, 8.81 at parity 8, 8.38 at parity 9, 8.21 at parity 10, 8.04 at parity 11, and 7.61 at parity 12+. These assumed values were based on industry averages for NBA (PigCHAMP, 2008), preweaning mortality, number weaned, and postweaning mortality. Preweaning and postweaning mortality rates were set to be 10 and 5%, respectively. These are standard industry benchmarking values. Slightly higher values have been reported (McBride and Key, 2007; National Swine Registry, 2009), which would suggest that these values should be goals for producers.
408 For this study, it was assumed that no genetic improvement was made in BF10. Therefore, GBF10 is equal to zero and does not affect the value of the genetic lag. The cumulative value of the genetic lag for the previous and current parities is the overall value of the difference in genetic potential of the sow in the herd and the gilt available from the seedstock supplier or from internal multiplication.
Sensitivity Analysis The generation interval and genetic gain combinations were compared across different parity structures. The generation interval and genetic gain values for the present sensitivity analysis were based on mean values from 2 seedstock suppliers who provided information anonymously (K. J. Stalder’s personal communication with suppliers). The sensitivity analysis was used to determine the genetic gain per generation and the resulting genetic lag value for each genetic gain and generation interval combination.
RESULTS AND DISCUSSION Table 5 presents the average value of the difference in genetic potential between the sow in the herd and the potential replacement gilt by parity and each generation interval with 0.3 improvement per generation and $22 per pig for NBA, a 1.36-kg improvement per generation and $1.54/kg for W21, and a 3-d improvement per generation and $0.17/d to 113 kg. For example, a producer would expect $18.31 of genetic lag by keeping a sow to parity 3 in a herd with a generation interval of 1.5 yr at the seedstock level, or only $9.16 in a herd with a generation interval of 3.0 yr at the seedstock level. These values differ because of the sow being able to have multiple parities before a generation has passed. The longer the generation interval for the seedstock supplier, the less the genetic lag would be affected by retaining sows in the breeding herd for additional parities because genetic progress is occurring more slowly. In other words, the genetic loss resulting
Abell et al.
Table 5. Value ($) of the difference in genetic potential between sows in the herd and a potential replacement gilt by parity and generation interval1 Generation interval at the seedstock level, yr Parity 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
1.5 7.43 12.44 18.31 24.93 32.19 40.01 48.29 57.02 66.09 75.65 85.66 95.86 106.63 117.97 129.90
2
2.0
2.5
3.0
5.57 9.33 13.74 18.70 24.14 30.01 36.22 42.77 49.57 56.73 64.25 71.89 79.97 88.48 97.42
4.46 7.46 10.99 14.96 19.31 24.01 28.98 34.21 39.65 45.39 51.40 57.51 63.98 70.78 77.94
3.71 6.22 9.16 12.46 16.09 20.01 24.15 28.51 33.05 37.82 42.83 47.93 53.31 58.99 64.95
Economic values assumed $22.00 per pig born alive, $1.54/kg of 21-d litter weight, and $0.17/d to market. Genetic improvement per generation assumed 0.3 pigs born alive, 1.36 kg of 21-d litter weight, and 3.0 d to 113 kg. Backfat was not included in this evaluation because most maternal lines are at or near their desired phenotypic backfat level; hence, little or no genetic change for backfat is occurring in most maternal lines.
1
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 on which the line is selected, the amount of time that has passed between the culling of the sow (tc), and the economic value placed on those traits.
2
from retaining a sow in the herd for an additional parity would be smaller with the associated increased generation interval from the seedstock supplier providing replacement gilts for this scenario. The value of the difference in genetic potential between sows in the herd at each parity structure and available replacement gilts with different generation intervals is shown in Table 6. For example, in a herd in which the maximum parity is set at 5 and with a generation interval of 1.5 yr, the average value of the genetic difference between a gilt from the seedstock suppliers and a sow currently in the commercial breeding herd is $16.54. This shows the value of the genetic gain that would have been achieved if sows in the herd were replaced after the first parity. The economic value
of the genetic gain would not even cover the feed cost ($28.06) associated with developing a gilt (Stalder et al., 2000). The initial replacement gilt costs typically are valued as the current market price multiplied by the weight of the gilt plus some genetic premium. The breeding costs are calculated as twice the cost per dose of semen (assumes each gilt is serviced twice). The gilt isolation and acclimation feed costs are calculated by multiplying the kilograms of feed expected to be eaten during this period by the feed cost ($/kg). Previous literature cost estimates associated with replacing a sow with a gilt include the initial cost ($200), breeding cost ($15.00), and housing and feed for the isolation and acclimation of gilts ($35.56; Stalder et al., 2000, 2003). Other costs that
Genetic lag value for optimal maximum parity for culling
Table 6. Average value ($) of the difference in genetic potential between sows in the herd and an available replacement gilt by parity structure and generation interval1 Generation interval at the seedstock level, yr Parity of forced culling 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
1.5 7.43 9.67 11.98 14.28 16.54 18.73 20.80 22.75 24.57 26.25 27.80 29.21 30.49 31.65 32.69
2
2.0
2.5
3.0
5.57 7.26 8.98 10.71 12.41 14.04 15.60 17.07 18.43 19.69 20.85 21.91 22.87 23.74 24.52
4.46 5.80 7.19 8.57 9.93 11.24 12.48 13.65 14.74 15.75 16.68 17.53 18.29 18.99 19.62
3.71 4.84 5.99 7.14 8.27 9.36 10.40 11.38 12.29 13.13 13.90 14.60 15.24 15.82 16.35
Economic values assumed $22.00 per pig born alive, $1.54/kg of 21-d litter weight, and $0.17/d to market. Genetic improvement per generation assumed 0.3 pigs born alive, 1.36 kg of 21-d litter weight, and 3.0 d to market. Backfat was not included in this evaluation because most maternal lines are at or near their desired phenotypic backfat level; hence, little or no genetic change for backfat is occurring in most maternal lines.
1
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 on which the line is selected, the amount of time that has passed between the culling of the sow (tc), and the economic value placed on those traits.
2
are typically incurred include veterinary services involving blood testing, vaccine purchases, and so on. When sows are retained for additional parities, the development costs (including isolation and acclimation) can be spread over greater numbers of pigs produced, thereby reducing the cost to produce a market pig. The number of gilts being developed must be greater than the number of sows being taken out of the breeding herd because not all gilts purchased as replacements actually farrow a litter (Hughes and Varley, 2003; Moeller et al., 2004). Additionally, 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 defined 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 essentially be from the same generation and have the same aggregate genetic value as the sow it is replacing. When determining whether to replace a sow with a gilt, producers must consider the value of a cull sow. Cull sow values, particularly for lighter weight sows, are generally not as high as the value of market hogs. Increasing the BCS of the sow will improve its cull sow value (Fitzgerald et al., 2008). This implies that it may be advantageous to retain sows in the herd for additional parities to increase their cull sow value. Sows become heavier with increasing parities, up to at least parity 4 (Rozeboom et al., 1996; Moeller et al., 2004). Because older sows have greater BW, their cull
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value would be greater than the cull value for younger sows. Based on the data in Table 6, 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 sufficient to justify the purchase or 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 NBA and growth rate of the pigs. The differences in production by parity must be considered when making culling decisions. Not only are there improvements in NBA and W21 with increasing parity, but also progeny from parity-2 versus parity-1 females have higher ADG (Burkey et al., 2008). Furthermore, it has been reported that progeny from parity-3 females have higher immunoglobulin levels than progeny from parity-1 females (Burkey et al., 2008). Higher immunoglobulin levels suggest that disease susceptibility is lower. 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 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 to compare differences, assuming the very best improvement occurs at the seedstock level. Table 7 shows the average value of the different genetic potential between the sows in the herd in each parity structure, with different genetic improvement made per generation. The value of the genetic difference becomes smaller as less genetic improvement is made per generation. A greater number of traits included in any index reduces the maximum obtainable improvement in any one trait. Today, most maternal lines are selected for both maternal and terminal traits. This reduces the achievable genetic improvement in maternal
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Table 7. Average value ($) of the difference in genetic potential between sows in the herd and available replacement gilts by parity and differing genetic improvement rates per generation1 Genetic improvement2 in no. born alive, 21-d litter weight (kg), and days to 113 kg (d) Parity of forced culling 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0.15, 0.68, 1.5
0.2, 0.91, 2.0
0.25, 1.13, 2.5
0.3, 1.36, 3.0
3.71 4.84 5.99 7.14 8.27 9.36 10.40 11.38 12.29 13.13 13.90 14.60 15.24 15.82 16.35
4.95 6.45 7.99 9.52 11.03 12.48 13.87 15.17 16.38 17.50 18.53 19.47 20.33 21.10 21.79
6.19 8.06 9.98 11.90 13.79 15.61 17.33 18.96 20.48 21.88 23.17 24.34 25.41 26.37 27.24
7.43 9.67 11.98 14.28 16.54 18.73 20.80 22.75 24.57 26.25 27.80 29.21 30.49 31.65 32.69
2
Economic values assumed $22.00 per pig born alive, $1.54/kg of 21-d litter weight, $0.17/d to market, and 1.5-yr 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; hence, little or no genetic change for backfat is occurring in most maternal lines.
1
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 on which the line is selected, the amount of time that has passed between the culling of the sow (tc), and the economic value placed on those traits.
2
traits. The values for genetic improvement per generation are maximum values that could be expected from selection based on one trait. Using more attainable genetic improvement values would show that the values presented for the value of the genetic difference between a sow in the breeding herd and a gilt from a seedstock supplier are the maximum values one would expect. This means that most commercial herds would be able to retain sows in the herd for additional parities because of the lower genetic lag value difference between the sows in the breeding herd and potential replacement gilts produced by the genetic supplier. Even though it takes longer to make genetic improvement at the commercial herd level when sows are retained to older parities, it may not be profitable to decrease the lag time. 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 older sow, regardless of the number of parities for which that sow is retained in the breeding herd. The sow must be maintained in the herd for a period of time so that it continues to produce at a profitable level in the operation for as long as possible. This study examined the value of the genetic lag associated with retaining sows in the commercial breeding herd for additional parities. The findings indicate that it is not profitable to replace sows in the breeding herd at rates currently used if the goal is solely to replace sows to keep up with the genetic improvement that is occurring at the nucleus and multiplication levels of the genetic system used by the genetic supplier. When considering the replacement costs of gilts and associated gilt development costs (e.g., feed, facilities, breeding,
veterinary expenses) and the higher production from older parity sows, sows should not be culled before they reach a positive net present value or, in more lay terms, they have paid for themselves (Stalder et al., 2000). 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 after parity 7, depending on the specific development costs and the genetic progress that an individual commercial pork producer experiences. These results are in general agreement with those of Dhuyvetter (2000), who reported the optimal removal parity of a sow to be 8 or 9 based on the replacement cost of the gilt. However, Rodriguez-Zas et al. (2006) reported that the optimal sow removal parity was 4 or 5, which is lower than the result in the present study. Both of the previously reported studies considered the genetic improvement effects when determining optimal culling parity.
Genetic lag value for optimal maximum parity for culling
IMPLICATIONS The relatively low economic value associated with the genetic loss of retaining a sow in the herd for additional parities does not seem to justify the costs of obtaining and developing replacement gilts. Producers should focus on sound development of their replacements gilts to enhance sow longevity. Maintaining sows in the herd for additional parities is profitable as long as the sows reproduce regularly and wean large litters with nearaverage W21. With this information, commercial producers would be able to optimize genetic gain by developing better parity management strategies in cases in which excessive replacement rates are occurring.
LITERATURE CITED Bourdon, R. M. 1997. Understanding Animal Breeding. 2nd ed. Prentice-Hall, Upper Saddle River, NJ. Burkey, T. E., P. S. Miller, R. K. Johnson, D. E. Reese, and R. Moreno. 2008. Does dam parity affect progeny health status? 2008 Nebr. Swine Rep. University of Nebraska, Lincoln. Dhuyvetter, K. C. 2000. What does attrition cost and what is it worth to reduce? p. 110 in
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