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Sow culling and mortality in commercial swine breeding herds
Preventive Veterinary Medicine, 9 (1990) 85-94 Elsevier Science Publishers B.V., Amsterdam
85
Sow culling and mortality in commercial swine breeding herds T.E. Stein, A. Dijkhuizen*, S. D'Allaire**, and R.S. Morris*** Department of Large Animal Clinical Sciences, Collegeof Veterinary Medicine. Universityof Minnesota, St. Paul, MN 55108 (U.S.A.) (Accepted for publication 19 February 1990 )
ABSTRACT Stein, T.E., Dijkhuizen, A., D'Allaire, S. and Morris, R.S., 1990. Sow culling and mortality in commercial swine breeding herds. Prey. Vet. Med., 9: 85-94. Six months of production data from 18 commercial swine breeding PBherds were used to describe reason-specific removal rates and the amount of time lost between farrowing and removal for each removal reason. There were 774 females removed during the 6-month analysis period. The removal rate for the study period was 25.0%, equivalent to an annual rate of 50%. Four major reason groups accounted for 70% of all removals: reproductive failure, degenerative problems, locomotor problems and death. Reproductive failure was the most likely removal reason across all parity groups except parity >/7, in which degenerative problems were much more likely and reproductive failure ranked second. The average time required from farrowing to removal across all reason groups was 78 days. Reproductive failure clustered at > 10 weeks, with an average time to removal of 128 days. Removals for degenerative problems clustered at Weeks 4-6 and at > 10 weeks, with an average time to removal of 57 days. Death clustered at Week 1 and at > 10 weeks, with an average time to removal of 72 days. Locomotor problems clustered at Weeks 5-8, with a second peak at > 10 weeks; the average time to removal was 58 days. Most females were removed involuntarily, and as the replacement rate increased a higher proportion of all removals were involuntary.
INTRODUCTION
Throughout the world, annual removal rates in commercial swine breeding herds are high: 54.8% in Denmark, 37.2% in England (one herd), 39.4% in Sweden, 40-50% in France, 43% in the Netherlands, and 50% in the U.S.A. *Present address: Department of Farm Management, Agricultural University, Wageningen, The Netherlands.. **Present address: Faculty of Veterinary Medicine, University of Montreal, Saint-Hyacinthe, Quebec, Canada. ***Present address: Department of Veterinary Clinical Sciences, Massey University, Palmerston North, New Zealand.
0167-5877/90/$03.50 © 1990 - - Elsevier Science Publishers B.V.
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(Jones, 1967; Einarsson and Settergren, 1974; Svendsen et al., 1975; Dagorn and Aumaitre, 1979; Kroes and Van Male, 1979; D'Allaire et al., 1987 ). High removal rates affect productivity by shifting the herd age distribution toward younger females with lower production levels and by increasing the number of non-productive sow days (Dijkhuizen et al., 1986 ). High culling rates are associated with a decrease in both litters per sow per year and pigs weaned per sow per year, and an increase in cost per pig weaned (Dagorn and Aumaitre, 1979; Kroes and Van Male, 1979; Pattison et al., 1980). A knowledge of both the reasons for removal and the time lost between farrowing and removal is important for understanding the precise ways culling and death loss affect overall herd productivity. The objectives of this study were to describe the reason-specific removal rate, the parity-specific removal rate, and the amount of lost time associated with each removal reason in commercial swine breeding herds that use a computerized recording system. MATERIALS AND METHODS
In September 1985, data were obtained from 18 commercial swine breeding herds located in the U.S.A. ( 16 farms) and Canada (2 farms) that used a computerized record-keeping system (PigCHAMP TM, University of Minnesota; Stein, 1985 ). Herds were selected because they met the following criteria: (1) complete recorded data from February to July 1985; and (2) less than 10% difference between actual female inventory (number of sows and gilts) and calculated average female inventory over the 6-month study period. Because the PigCHAMP TM program had only recently been introduced (in late 1984), there was a small but convenient group of herds from which to choose. The 18 herds were selected from a group of 40 herds using the system at the time of this study; of the 40 herds, 15 did not have complete recorded data for the study period and seven had fluctuating herd inventories. Herd owners provided their production record data on a floppy diskette to the investigators. All herds farrowed on a weekly or continuous basis except one, which farrowed nine groups of sows per year. All herds used veterinary services on a regular basis (4-12 visits per year). All herds had separate facilities for lactation, breeding and gestation. All herds farrowed sows in crates in total confinement buildings. There was wide variation in breeding and gestation housing systems and management of these systems between herds. Fifteen farms selected all replacement gilts from their farm stock; three farms purchased F1 or F2 gilts. The predominant breeds were Landrace, Large White, Yorkshire, and Duroc. All farms used corn and soybean meal in their rations except one, which used barley as the primary energy source. The PigCHAMP TM software program required the herd manager to record both a removal type and a removal reason whenever an animal left the herd. A list of three main removal types and 50 removal reasons with definitions
SOW CULLING AND MORTALITY IN SWINE BREEDING HERDS
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was provided to producers when they purchased the software. The three removal types were cull, death, or transfer. A cull was a live animal sold from the herd with the intention of slaughter. A death was an animal that either died naturally or was destroyed while on the farm. A transfer was an animal removed from the herd for breeding purposes. For analysis, the 50 reasons were reduced to 10 major categories. ( 1 ) Reproductive failure was used to define no observed puberty in gilts, no postweaning estrus, regular and irregular returns to estrus, negative pregnancy diagnosis, failure to farrow and abortion. Failure to farrow is a conventional description in the swine industry (syn: not-in-pig) for a sow that either appears pregnant or tests positive but then is not pregnant when actually due to farrow. In addition, it could include a sow that is served and assumed to be pregnant yet never checked until farrowing when she is found to be non-pregnant. ( 2 ) Peripartum difficulties included dystocia, prolapse (vaginal, rectal, uterine), dead litter at birth, deformed litter, or a poor litter (i.e. pigs with low birthweight, unthrifty, or low viability). (3) Lactation problems included disease in litter, mastitis, metritis, milk failure and mammary problems. (4) Inadequate performance included small litter at birth ( < 5 born live), low numbers of pigs weaned, and high preweaning mortality. (5) Locomotor problems was used to group unsoundness, lameness, leg or foot injury, 'downer sow' syndrome and musculoskeletal disease. Unsoundness is a well-recognized term in the swine industry that describes a structural problem in the musculoskeletal system, particularly in the feet or legs. The downer sow syndrome represented any condition causing a sow to be unable to walk or stand. (6) Degenerative problems included the reasons injury, abscess, ulcer, unthrifty, thin, too big, old age, and chronic mange. (7) Systemic disease included cardiovascular, urogenital, skin, respiratory, gastrointestinal, and nervous system diseases. ( 8 ) Miscellaneous grouped sudden death, behavior problems, market conditions, tax considerations, test and removal, growth rate or carcass performance of progeny, and no reasons specified. (9) Death and ( 10 ) Transfer, described above, were used as reasons in the analysis. Removal rate and replacement rate were calculated as proportions of the average female inventory during the period February to July. This method is equivalent to the incidence density technique used to estimate an average rate (Kleinbaum et al., 1982, pp. 100-101 ). The numerator is the number of females removed (removal rate) or entered (replacement rate). The denominator is the accumulated time in sow days for the entire population (entrants at any point in the study period, removals at any point in the study period, and those females present throughout the study period) divided by the time period in days. For example, with no entrants or removals, 100 sows for 6 months contribute (100) (30.5) (6) or 18 300 days, divided by the number of days in the period (183) equals an average female inventory of 100 sows.
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Parity is the number of litters produced by a female. A parity begins with a farrowing date and ends with another farrowing or a removal. Data were transferred from floppy diskettes into a mainframe computer, verified and analyzed by cross-tabulations and breakdowns (Statistical Analysis System, 1985 ). The unit of analysis for the herd-specific removal and replacement rate data (cf. Results and Table 5 ) is the herd, with a total sample size of 18. The unit of analysis for Tables 1-4 is the sow, with a total sample size of 774. RESULTS AND DISCUSSION
Beginning and ending average female inventory was similar ( 195 vs. 196 ), as were the replacement (24.9) and removal rates (25.0), indicating stability in the data set. Among the 18 herds, average female inventory ranged from 66 to 407 females. The overall semi-annual removal rate was 25.0%, equivalent to an annual rate of 50%. There was a wide range in herd-specific removal rates: 9.8-43.3% (equivalent to annual rates of 19.6-86.6%). There were 774 females removed during the 6-month analysis period. Four major reason groups accounted for 70% of all removals: reproductive failure, degenerative problems, locomotor problems, and death (Table 1 ). One female was removed as a transfer and only six females were culled for specific systemic disease. Reproductive failure was the most likely reason, accounting for 29.6% of all removals. In all previous investigations, reproductive failure also made up TABLE 1 Reason-specific proportionate removal rates and associated days to removal Removal reason
No. females removed
Reproductive failure Degenerative problems Locomotor problems Death Inadequate performance Lactation problems Miscellaneous Peripartum difficulties Systemic disease Transfer
229 138 86 83 73 68 51 39 6 1
Overall
774
Proportionate rate
Average parity at removal
Average farrow to removal interval, (days)
29.6 17.9 11.0 10.7 9.4 8.8 6.6 5.0 0.8 0.3
3.4 6.3 3.9 3.9 5.1 4.3 2.5 3.7 2.3 2.0
128 57 58 72 54 36 77 49 33 141
100.0
4.2
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SOW CULLINGAND MORTALITYIN SWINEBREEDING HERDS
89
the highest proportion of removals, varying from 27 to 41% (Einarsson and Settergren, 1974; Svendsen et al., 1975; Dagorn and Aumaitre, 1979; Pattison et al., 1980). Within the reproductive-failure group, 45% were removed for negative pregnancy test. Old age accounted for 11.1% of the total removals; others have reported figures varying from 15 to 27.2% (Pomeroy, 1960; Dagorn and Aumaitre, 1979; Pattison et al., 1980). Within the degenerative group, old age accounted for 62% of removals. Locomotor problems accounted for 11.0% of all reasons, and within this group lameness comprised 43% of reasons. In other studies, locomotor disturbances have accounted for 8 to 12% of removals (Dagorn and Aumaitre, 1979; Pattison et al., 1980). Death accounted for 10.7% of all removal reasons, similar to previous reports which ranged from 6.5 to 11.8% (Jones, 1967; Jovic et al. 1975; Svendsen et al., 1975; Dagorn and Aumaitre, 1979). The parity-specific risk of removal was rather similar among Parity groups 1-5, increased slightly in Parity groups 6-8, and increased substantially in Parities 9 and >t 10 (Table 2 ). Svendsen et al. ( 1975 ) found a similar pattern of equal culling rates for Parity groups 1-7 (25%) with a significant increase in Parity groups >i 8 (34.5%). Reproductive failure was the most likely removal reason across all parity groups except Parity >/7, in which degenerative problems were much more likely and reproductive failure ranked second (Table 3). The likelihood of removal for reproductive failure and locomotor problems declined as parity TABLE 2 Parity-specific removal rates Parity
1 2 3 4 5 6 7 8 9 10 Overall
No. of sows farrowed
PrRemi
Average farrow to removal interval (days)
842 673 593 489 470 295 159 111 71 134
0.192 0.166 0.165 0.215 0.189 0.234 0.233 0.225 0.324 0.403
90 78 78 78 78 78 67 67 67 67
3837
0.202
78
iProbability of removal = no. of sows removed/no, of sows farrowed.
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TABLE 3 Reason-specific removal proportions cross-classified by parity Removal reason
Reproductive failure Degenerative problems Locomotor problems Death Inadequate performance Lactation problems Miscellaneous Peripartum difficulties Systemic disease Transfer Total ~
Parity (%) 1
2
3-6
i>7
41 9 14 14 2 4 12 4 l 0
29 8 19 6 7 11 10 7 2 2
30 15 7 12 13 12 5 6 1 0
14 45 11 9 12 6 I 3 0 0
101
101
101
101
~Differences due to rounding error.
increased. The likelihood of degenerative problems increased markedly as parity number increased. Pattison (1980) also found that the likelihood of removal because of reproductive failure or locomotor disturbances decreased as parity number increased. The average time required from farrowing to removal across all reason groups was 78 days (Table 1 ). The time distribution was bimodal; one peak occurred during Weeks 4-5 and another after 10 weeks (Table 4 ). There were clear differences in the time distribution to removal associated with different reason groups. Reproductive failure clustered at > 10 weeks. Removals for degenerative problems clustered at Weeks 4-6 (around weaning) and at > 10 weeks. Death clustered at Week 1 and at > 10 weeks, i.e. during late gestation and at the time of farrowing. Locomotor problems clustered at Weeks 5-8 (post-weaning and early breeding periods ), with a second peak at > 10 weeks. One of our most important findings was the amount of time lost between farrowing and removal, particularly for reproductive failure and Parity 1 females. Clearly, reproductive failure was the most important removal reason because of its high rate and long time delay (128 days). This was especially important in Parity 1 females since in this parity group 41% of all removals were due to reproductive failure. Across all removal reasons, Parity 1 females averaged 11.9, 11.8 and 22.6 days longer than Parity groups 2, 3-6 and >17. respectively (Table 2). Although no other parity-specific data are available for comparison, several investigators have studied the time delay prior to removal. Pattison et al. (1980) reported an 88-day interval from first service to removal for all reasons combined; however, non-pregnant females were
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SOW CULLINGAND MORTALITY IN SWINEBREEDINGHERDS TABLE 4 Time distribution of interval to removal by reason category ~ No. weeks after farrowing (%)
Reproductive failure Degenerative problems Locomotor problems Death Inadequate performance Lactation problems Miscellaneous Peripartum difficulties % of total removals
1
2
3
4
5
6
7
8
9
10
> 10
Total 2
1 4 5 23 8 13 6 0
0 1 8 5 5 9 2 10
0 2 4 2 10 12 16 3
1 23 8 4 16 15 2 26
2 22 15 5 11 19 10 23
4 13 11 5 5 12 14 8
0 2 8 9 7 4 4 5
4 3 11 4 4 6 4 3
4 7 5 4 8 4 2 3
3 4 2 1 1 3 2 0
82 18 24 38 23 3 38 21
101 99 101 100 98 100 100 102
6
4
4
10
12
8
4
5
5
2
40
100
tNot included: 1 transfer and 6 sows culled for systemic disease. 2Differences in row totals due to rounding errors.
removed at an average of 121 days after first service. D'Allaire et al. ( 1987 ) found an interval of 90 days from breeding to culling when the removal reason was reproductive failure. Litters per sow per year (LSY) measures the reproductive rate in a swine breeding herd, and thus measures a major component of herd productivity. It is defined as the number of annual farrowings divided by the average female inventory. It is mainly affected by a herd's average lactation length and average non-productive days per female (Duffy and Stein, 1988). Non-productive days occur when a female is not pregnant or lactating, e.g. weaning-toconception, weaning-to-culling for females that do not come into estrus, and first service-to-culling for sows that fail to conceive or are found to be nonpregnant. The failure to identify and remove sows that do not come into estrus or are found to be not-pregnant has an important effect on herd productivity because the increase in non-productive sow days reduces LSY (Kroes and Van Male, 1979; Pattison et al., 1980). In addition, high removal rates could affect herd productivity by causing a shift in the herd age distribution. Younger breeding females have fewer pigs born live per litter and increased weaning-to-first service intervals (Stein and Duffy, 1988 ). As shown here and elsewhere (Pattison et al. 1980; D'Allaire et al., 1987), they also are more likely to be culled for reproductive failure. Therefore, one could hypothesize that herds with high removal rates would, on average, have fewer pigs born live per litter, lower LSY, and decreased pigs weaned per sow per year (PWSY). Although Svendsen et al. ( 1975 ) found no direct relationship between cull-
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T.E. STEINET AL.
ing rate and PWSY, they noted that PWSY was highest in three herds with the highest culling rates. In contrast, Dagorn and Aumaitre (1979) and Einarsson and Settergren (1974) found higher productivity associated with lower culling rate. We attempted to consider this association by dividing the 18 herds into terciles based on their annual replacement rates (Table 5). Sample size (n = 6 in each tercile) was too small for a statistical test of significance, but contrary to our hypothesis above there appeared to be a trend of increased pigs born live and weaned per litter in herds with higher replacement rates (and shorter average sow life times). This is in direct contradiction to Dagorn and Aumaitre (1979) who showed clearly that average number of pigs born (total or live) and PWSY increased as average parity per culled sow increased. This discrepancy may be due to a small sample effect, random variation, or it could be that ( 1 ) herds with higher replacement and culling rates have an improved genetic foundation (females with higher average prolificacy); or (2) culling rate and replacement rates act as proxies for underlying management effects which bias herds toward higher productivity. In addition, the relationship is confounded between LSY, removal rate (RemRate), the overall herd-specific probability of removal (PrRem), and average parities per sow life time (APSL). The overall herd-specific probability of removal is calculated as (RemRate/LSY); it can also be conceptualized as the removal rate per litter farrowed or annual number of farrowings divided by the annual number of removals (cf. overall PrRem in Table 2 ). The higher a herd's LSY, the higher will be the removal rate at any given PrRem. For example, consider two herds: Herd 1 has LSY 2.0, Herd 2 has LSY 1.5, and RemRate (Herd 1 ) = R e m R a t e (Herd 2 ) = 5 0 % . Thus, PrRem is 0.25 for Herd 1 and 0.33 for Herd 2. Although the two herds have identical TABLE 5 Mean herd-specific production measures by three levels of mean annual replacement rate No. of farms Annual replacement rate
6 33.0
6 51.2
6 65.0
Sow life-span (years) Average female inventory Pigs born live per litter Pigs weaned per litter Involuntary removal rate ~ (%) Voluntary removal rate (%) Inadequate performance (%) Miscellaneous (%) Total removal rate (%)
3.1 159 9.9 8.6 23.7 13.3 7.7 5.6 37.0
2.0 233 10.0 8.8 43.0 7.1 6.0 1.1 50.1
1.6 201 10.4 9.1 60.5 2.1 1.6 0,5 62,6
qncludes reproductive failure, degenerative problems, locomotor problems, death, lactation problems, peripartum difficulties and systemic disease.
SOWCULLINGANDMORTALITYIN SWINEBREEDINGHERDS
93
removal rates, there is an important difference in the risk of removal, i.e. the biological processes associated with removal, between the two herds. This is reflected as differences in average sow age between these two herds. APSL is calculated as ( 1 0 0 / ( R e m R a t e / L S Y ) ). Thus, APSL is 4.0 for Herd 1 and 3.0 for Herd 2; sows are older in Herd 1 even though the herd removal rates are equal. The example may explain the conflicting results discussed above for culling rate effects on herd productivity. Thus, when trying to associate between-herd productivity differences with removal and replacement effects (i.e. herd age structure effects), one should use a measure of herd-specific average female age rather than compare directly on removal rates. For example, in a recent study Duffy and Stein (1988) found no association between herd-specific culling rate and PWSY or LSY, but did find that culling rate per litter farrowed was associated negatively with both. In this sample of herds, most females were removed involuntarily, and as the replacement rate increased a higher proportion of all removals were involuntary. This can cause the 'structural' maintenance of a young herd age distribution, thus reducing the economic efficiency of an enterprise (Dijkhuizen et al., 1986). This study has several limitations. Convenience sampling was used, and the sample was biased, in that these were herds who had self-selected the use of a sophisticated production record-keeping program. These results are perhaps not representative of North American swine herds but may represent commercial herds using computerized recording systems. In addition, farmer-recorded removal reasons were not validated, and multiple removal reasons were not allowed. However, even with these limitations, our results are quite similar to previous investigations. These similarities suggest that the information obtained from computerized recording systems is valid, and justifies the use of such data in future research. In future investigations, an attempt should be made to replicate the work of Dagorn and Aumaitre ( 1979 ) and assess the relationship between herd age and overall productivity. In addition, reason-specific risk factors for removal need to be assessed, as has been done recently in dairy cattle (Milian-Sauzo et al., 1989 ). This is especially true for reproductive failure and locomotor problems in young females, degenerative problems in older females, and death in all females. Breeding female wastage in the swine enterprises studied here is at unacceptable levels. REFERENCES Dagorn, J. and Aumaitre, A., 1979. Sow culling: reasons for and effect on productivity. Livest. Prod. Sci., 6: 167-177. D'Allaire, S., Stein, T.E. and Leman, A.D., 1987. Culling patterns in selected Minnesota swine breeding herds. Can. J. Vet. Res., 51: 506-512.
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Dijkhuizen, A.A., Morris, R.S. and Morrow, M., 1986. Economic optimization of culling strategies in swine breeding herds, using the "Porkchop" computer program. Prev. Vet. Med., 4: 341-353. Duffy, S. and Stein, T.E., 1988. Correlations between production, productivity and population factors in swine breeding herds. Proc. 10th Int. Pig Vet. Soc., p. 345 (Abstract). Einarsson, S. and Settergren, I., 1974. Fertility and culling in some pig breeding herds in Sweden. Nord. Vet. Med., 26: 576-584. Jones, J.E.T., 1967. An investigation of the causes of mortality and morbidity in sows in a commercial herd. Br. Vet. J., 123: 327-339. Jovic, M., Varadin, M. and Nikolic, P., 1975. The length of reproduction of breeding sows at intensive piglet production and the chief reasons for their elimination from the breeding herd. Veterinaria, 24: 17-23. Kleinbaum, D.G., Kupper, L.L. and Morgenstern, H., 1982. Epidemiologic research: Principles and Quantitative Methods. Van Nostrand Reinhold, New York, NY, pp. 100-103. Kroes, Y. and Van Male, J.P., 1979. Reproductive lifetime of sows in relation to economy of production. Livest. Prod. Sci., 6:179-183. Milian-Sauzo, F., Erb, H.N. and Smith, R.D., 1989. Risk factors for reason-specific culling of dairy cows. Prev. Vet. Med., 7:19-29. Pattison, H.D., 1980. Patterns of sow culling. Pig News Inf., 1: 215-218. Pattison, H.D., Cook, G.L. and Mackenzie, S., 1980. A study of culling patterns in commercial pig breeding herds. Proc. Br. Soc. Anim. Prod., Harrogate, pp. 462-463. Pomeroy, R.W., 1960. Infertility and neonatal mortality in the sow. I. Lifetime performance and reasons for disposal of sows. J. Agric., Sci., 54: 1-17. Statistical Analysis System, 1985. User's Guide: Basics and Statistics. SAS Institute, Cary, NC. Stein, T.E., 1985. The computer as the core of health management in food animal populations. PhD dissertation, University of Minnesota. Stein, T.E. and Duffy, S.J., 1988. Parity-specific production values for 68 North American swine breeding herds. Proc. l 0th Int. Pig Vet. Soc., p. 346 (Abstract). Svendsen, J., Nielsen, N.C., Bille, N. and Riising, H.J., 1975. Causes of culling and death in sows. Nord. Vet. Med., 27: 604-615.
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Sow culling and mortality in commercial swine breeding herds T.E. Stein, A. Dijkhuizen*, S. D'Allaire**, and R.S. Morris*** Department of Large Animal Clinical Sciences, Collegeof Veterinary Medicine. Universityof Minnesota, St. Paul, MN 55108 (U.S.A.) (Accepted for publication 19 February 1990 )
ABSTRACT Stein, T.E., Dijkhuizen, A., D'Allaire, S. and Morris, R.S., 1990. Sow culling and mortality in commercial swine breeding herds. Prey. Vet. Med., 9: 85-94. Six months of production data from 18 commercial swine breeding PBherds were used to describe reason-specific removal rates and the amount of time lost between farrowing and removal for each removal reason. There were 774 females removed during the 6-month analysis period. The removal rate for the study period was 25.0%, equivalent to an annual rate of 50%. Four major reason groups accounted for 70% of all removals: reproductive failure, degenerative problems, locomotor problems and death. Reproductive failure was the most likely removal reason across all parity groups except parity >/7, in which degenerative problems were much more likely and reproductive failure ranked second. The average time required from farrowing to removal across all reason groups was 78 days. Reproductive failure clustered at > 10 weeks, with an average time to removal of 128 days. Removals for degenerative problems clustered at Weeks 4-6 and at > 10 weeks, with an average time to removal of 57 days. Death clustered at Week 1 and at > 10 weeks, with an average time to removal of 72 days. Locomotor problems clustered at Weeks 5-8, with a second peak at > 10 weeks; the average time to removal was 58 days. Most females were removed involuntarily, and as the replacement rate increased a higher proportion of all removals were involuntary.
INTRODUCTION
Throughout the world, annual removal rates in commercial swine breeding herds are high: 54.8% in Denmark, 37.2% in England (one herd), 39.4% in Sweden, 40-50% in France, 43% in the Netherlands, and 50% in the U.S.A. *Present address: Department of Farm Management, Agricultural University, Wageningen, The Netherlands.. **Present address: Faculty of Veterinary Medicine, University of Montreal, Saint-Hyacinthe, Quebec, Canada. ***Present address: Department of Veterinary Clinical Sciences, Massey University, Palmerston North, New Zealand.
0167-5877/90/$03.50 © 1990 - - Elsevier Science Publishers B.V.
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T.E. STEIN ET AL.
(Jones, 1967; Einarsson and Settergren, 1974; Svendsen et al., 1975; Dagorn and Aumaitre, 1979; Kroes and Van Male, 1979; D'Allaire et al., 1987 ). High removal rates affect productivity by shifting the herd age distribution toward younger females with lower production levels and by increasing the number of non-productive sow days (Dijkhuizen et al., 1986 ). High culling rates are associated with a decrease in both litters per sow per year and pigs weaned per sow per year, and an increase in cost per pig weaned (Dagorn and Aumaitre, 1979; Kroes and Van Male, 1979; Pattison et al., 1980). A knowledge of both the reasons for removal and the time lost between farrowing and removal is important for understanding the precise ways culling and death loss affect overall herd productivity. The objectives of this study were to describe the reason-specific removal rate, the parity-specific removal rate, and the amount of lost time associated with each removal reason in commercial swine breeding herds that use a computerized recording system. MATERIALS AND METHODS
In September 1985, data were obtained from 18 commercial swine breeding herds located in the U.S.A. ( 16 farms) and Canada (2 farms) that used a computerized record-keeping system (PigCHAMP TM, University of Minnesota; Stein, 1985 ). Herds were selected because they met the following criteria: (1) complete recorded data from February to July 1985; and (2) less than 10% difference between actual female inventory (number of sows and gilts) and calculated average female inventory over the 6-month study period. Because the PigCHAMP TM program had only recently been introduced (in late 1984), there was a small but convenient group of herds from which to choose. The 18 herds were selected from a group of 40 herds using the system at the time of this study; of the 40 herds, 15 did not have complete recorded data for the study period and seven had fluctuating herd inventories. Herd owners provided their production record data on a floppy diskette to the investigators. All herds farrowed on a weekly or continuous basis except one, which farrowed nine groups of sows per year. All herds used veterinary services on a regular basis (4-12 visits per year). All herds had separate facilities for lactation, breeding and gestation. All herds farrowed sows in crates in total confinement buildings. There was wide variation in breeding and gestation housing systems and management of these systems between herds. Fifteen farms selected all replacement gilts from their farm stock; three farms purchased F1 or F2 gilts. The predominant breeds were Landrace, Large White, Yorkshire, and Duroc. All farms used corn and soybean meal in their rations except one, which used barley as the primary energy source. The PigCHAMP TM software program required the herd manager to record both a removal type and a removal reason whenever an animal left the herd. A list of three main removal types and 50 removal reasons with definitions
SOW CULLING AND MORTALITY IN SWINE BREEDING HERDS
87
was provided to producers when they purchased the software. The three removal types were cull, death, or transfer. A cull was a live animal sold from the herd with the intention of slaughter. A death was an animal that either died naturally or was destroyed while on the farm. A transfer was an animal removed from the herd for breeding purposes. For analysis, the 50 reasons were reduced to 10 major categories. ( 1 ) Reproductive failure was used to define no observed puberty in gilts, no postweaning estrus, regular and irregular returns to estrus, negative pregnancy diagnosis, failure to farrow and abortion. Failure to farrow is a conventional description in the swine industry (syn: not-in-pig) for a sow that either appears pregnant or tests positive but then is not pregnant when actually due to farrow. In addition, it could include a sow that is served and assumed to be pregnant yet never checked until farrowing when she is found to be non-pregnant. ( 2 ) Peripartum difficulties included dystocia, prolapse (vaginal, rectal, uterine), dead litter at birth, deformed litter, or a poor litter (i.e. pigs with low birthweight, unthrifty, or low viability). (3) Lactation problems included disease in litter, mastitis, metritis, milk failure and mammary problems. (4) Inadequate performance included small litter at birth ( < 5 born live), low numbers of pigs weaned, and high preweaning mortality. (5) Locomotor problems was used to group unsoundness, lameness, leg or foot injury, 'downer sow' syndrome and musculoskeletal disease. Unsoundness is a well-recognized term in the swine industry that describes a structural problem in the musculoskeletal system, particularly in the feet or legs. The downer sow syndrome represented any condition causing a sow to be unable to walk or stand. (6) Degenerative problems included the reasons injury, abscess, ulcer, unthrifty, thin, too big, old age, and chronic mange. (7) Systemic disease included cardiovascular, urogenital, skin, respiratory, gastrointestinal, and nervous system diseases. ( 8 ) Miscellaneous grouped sudden death, behavior problems, market conditions, tax considerations, test and removal, growth rate or carcass performance of progeny, and no reasons specified. (9) Death and ( 10 ) Transfer, described above, were used as reasons in the analysis. Removal rate and replacement rate were calculated as proportions of the average female inventory during the period February to July. This method is equivalent to the incidence density technique used to estimate an average rate (Kleinbaum et al., 1982, pp. 100-101 ). The numerator is the number of females removed (removal rate) or entered (replacement rate). The denominator is the accumulated time in sow days for the entire population (entrants at any point in the study period, removals at any point in the study period, and those females present throughout the study period) divided by the time period in days. For example, with no entrants or removals, 100 sows for 6 months contribute (100) (30.5) (6) or 18 300 days, divided by the number of days in the period (183) equals an average female inventory of 100 sows.
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Parity is the number of litters produced by a female. A parity begins with a farrowing date and ends with another farrowing or a removal. Data were transferred from floppy diskettes into a mainframe computer, verified and analyzed by cross-tabulations and breakdowns (Statistical Analysis System, 1985 ). The unit of analysis for the herd-specific removal and replacement rate data (cf. Results and Table 5 ) is the herd, with a total sample size of 18. The unit of analysis for Tables 1-4 is the sow, with a total sample size of 774. RESULTS AND DISCUSSION
Beginning and ending average female inventory was similar ( 195 vs. 196 ), as were the replacement (24.9) and removal rates (25.0), indicating stability in the data set. Among the 18 herds, average female inventory ranged from 66 to 407 females. The overall semi-annual removal rate was 25.0%, equivalent to an annual rate of 50%. There was a wide range in herd-specific removal rates: 9.8-43.3% (equivalent to annual rates of 19.6-86.6%). There were 774 females removed during the 6-month analysis period. Four major reason groups accounted for 70% of all removals: reproductive failure, degenerative problems, locomotor problems, and death (Table 1 ). One female was removed as a transfer and only six females were culled for specific systemic disease. Reproductive failure was the most likely reason, accounting for 29.6% of all removals. In all previous investigations, reproductive failure also made up TABLE 1 Reason-specific proportionate removal rates and associated days to removal Removal reason
No. females removed
Reproductive failure Degenerative problems Locomotor problems Death Inadequate performance Lactation problems Miscellaneous Peripartum difficulties Systemic disease Transfer
229 138 86 83 73 68 51 39 6 1
Overall
774
Proportionate rate
Average parity at removal
Average farrow to removal interval, (days)
29.6 17.9 11.0 10.7 9.4 8.8 6.6 5.0 0.8 0.3
3.4 6.3 3.9 3.9 5.1 4.3 2.5 3.7 2.3 2.0
128 57 58 72 54 36 77 49 33 141
100.0
4.2
78
SOW CULLINGAND MORTALITYIN SWINEBREEDING HERDS
89
the highest proportion of removals, varying from 27 to 41% (Einarsson and Settergren, 1974; Svendsen et al., 1975; Dagorn and Aumaitre, 1979; Pattison et al., 1980). Within the reproductive-failure group, 45% were removed for negative pregnancy test. Old age accounted for 11.1% of the total removals; others have reported figures varying from 15 to 27.2% (Pomeroy, 1960; Dagorn and Aumaitre, 1979; Pattison et al., 1980). Within the degenerative group, old age accounted for 62% of removals. Locomotor problems accounted for 11.0% of all reasons, and within this group lameness comprised 43% of reasons. In other studies, locomotor disturbances have accounted for 8 to 12% of removals (Dagorn and Aumaitre, 1979; Pattison et al., 1980). Death accounted for 10.7% of all removal reasons, similar to previous reports which ranged from 6.5 to 11.8% (Jones, 1967; Jovic et al. 1975; Svendsen et al., 1975; Dagorn and Aumaitre, 1979). The parity-specific risk of removal was rather similar among Parity groups 1-5, increased slightly in Parity groups 6-8, and increased substantially in Parities 9 and >t 10 (Table 2 ). Svendsen et al. ( 1975 ) found a similar pattern of equal culling rates for Parity groups 1-7 (25%) with a significant increase in Parity groups >i 8 (34.5%). Reproductive failure was the most likely removal reason across all parity groups except Parity >/7, in which degenerative problems were much more likely and reproductive failure ranked second (Table 3). The likelihood of removal for reproductive failure and locomotor problems declined as parity TABLE 2 Parity-specific removal rates Parity
1 2 3 4 5 6 7 8 9 10 Overall
No. of sows farrowed
PrRemi
Average farrow to removal interval (days)
842 673 593 489 470 295 159 111 71 134
0.192 0.166 0.165 0.215 0.189 0.234 0.233 0.225 0.324 0.403
90 78 78 78 78 78 67 67 67 67
3837
0.202
78
iProbability of removal = no. of sows removed/no, of sows farrowed.
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TABLE 3 Reason-specific removal proportions cross-classified by parity Removal reason
Reproductive failure Degenerative problems Locomotor problems Death Inadequate performance Lactation problems Miscellaneous Peripartum difficulties Systemic disease Transfer Total ~
Parity (%) 1
2
3-6
i>7
41 9 14 14 2 4 12 4 l 0
29 8 19 6 7 11 10 7 2 2
30 15 7 12 13 12 5 6 1 0
14 45 11 9 12 6 I 3 0 0
101
101
101
101
~Differences due to rounding error.
increased. The likelihood of degenerative problems increased markedly as parity number increased. Pattison (1980) also found that the likelihood of removal because of reproductive failure or locomotor disturbances decreased as parity number increased. The average time required from farrowing to removal across all reason groups was 78 days (Table 1 ). The time distribution was bimodal; one peak occurred during Weeks 4-5 and another after 10 weeks (Table 4 ). There were clear differences in the time distribution to removal associated with different reason groups. Reproductive failure clustered at > 10 weeks. Removals for degenerative problems clustered at Weeks 4-6 (around weaning) and at > 10 weeks. Death clustered at Week 1 and at > 10 weeks, i.e. during late gestation and at the time of farrowing. Locomotor problems clustered at Weeks 5-8 (post-weaning and early breeding periods ), with a second peak at > 10 weeks. One of our most important findings was the amount of time lost between farrowing and removal, particularly for reproductive failure and Parity 1 females. Clearly, reproductive failure was the most important removal reason because of its high rate and long time delay (128 days). This was especially important in Parity 1 females since in this parity group 41% of all removals were due to reproductive failure. Across all removal reasons, Parity 1 females averaged 11.9, 11.8 and 22.6 days longer than Parity groups 2, 3-6 and >17. respectively (Table 2). Although no other parity-specific data are available for comparison, several investigators have studied the time delay prior to removal. Pattison et al. (1980) reported an 88-day interval from first service to removal for all reasons combined; however, non-pregnant females were
91
SOW CULLINGAND MORTALITY IN SWINEBREEDINGHERDS TABLE 4 Time distribution of interval to removal by reason category ~ No. weeks after farrowing (%)
Reproductive failure Degenerative problems Locomotor problems Death Inadequate performance Lactation problems Miscellaneous Peripartum difficulties % of total removals
1
2
3
4
5
6
7
8
9
10
> 10
Total 2
1 4 5 23 8 13 6 0
0 1 8 5 5 9 2 10
0 2 4 2 10 12 16 3
1 23 8 4 16 15 2 26
2 22 15 5 11 19 10 23
4 13 11 5 5 12 14 8
0 2 8 9 7 4 4 5
4 3 11 4 4 6 4 3
4 7 5 4 8 4 2 3
3 4 2 1 1 3 2 0
82 18 24 38 23 3 38 21
101 99 101 100 98 100 100 102
6
4
4
10
12
8
4
5
5
2
40
100
tNot included: 1 transfer and 6 sows culled for systemic disease. 2Differences in row totals due to rounding errors.
removed at an average of 121 days after first service. D'Allaire et al. ( 1987 ) found an interval of 90 days from breeding to culling when the removal reason was reproductive failure. Litters per sow per year (LSY) measures the reproductive rate in a swine breeding herd, and thus measures a major component of herd productivity. It is defined as the number of annual farrowings divided by the average female inventory. It is mainly affected by a herd's average lactation length and average non-productive days per female (Duffy and Stein, 1988). Non-productive days occur when a female is not pregnant or lactating, e.g. weaning-toconception, weaning-to-culling for females that do not come into estrus, and first service-to-culling for sows that fail to conceive or are found to be nonpregnant. The failure to identify and remove sows that do not come into estrus or are found to be not-pregnant has an important effect on herd productivity because the increase in non-productive sow days reduces LSY (Kroes and Van Male, 1979; Pattison et al., 1980). In addition, high removal rates could affect herd productivity by causing a shift in the herd age distribution. Younger breeding females have fewer pigs born live per litter and increased weaning-to-first service intervals (Stein and Duffy, 1988 ). As shown here and elsewhere (Pattison et al. 1980; D'Allaire et al., 1987), they also are more likely to be culled for reproductive failure. Therefore, one could hypothesize that herds with high removal rates would, on average, have fewer pigs born live per litter, lower LSY, and decreased pigs weaned per sow per year (PWSY). Although Svendsen et al. ( 1975 ) found no direct relationship between cull-
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ing rate and PWSY, they noted that PWSY was highest in three herds with the highest culling rates. In contrast, Dagorn and Aumaitre (1979) and Einarsson and Settergren (1974) found higher productivity associated with lower culling rate. We attempted to consider this association by dividing the 18 herds into terciles based on their annual replacement rates (Table 5). Sample size (n = 6 in each tercile) was too small for a statistical test of significance, but contrary to our hypothesis above there appeared to be a trend of increased pigs born live and weaned per litter in herds with higher replacement rates (and shorter average sow life times). This is in direct contradiction to Dagorn and Aumaitre (1979) who showed clearly that average number of pigs born (total or live) and PWSY increased as average parity per culled sow increased. This discrepancy may be due to a small sample effect, random variation, or it could be that ( 1 ) herds with higher replacement and culling rates have an improved genetic foundation (females with higher average prolificacy); or (2) culling rate and replacement rates act as proxies for underlying management effects which bias herds toward higher productivity. In addition, the relationship is confounded between LSY, removal rate (RemRate), the overall herd-specific probability of removal (PrRem), and average parities per sow life time (APSL). The overall herd-specific probability of removal is calculated as (RemRate/LSY); it can also be conceptualized as the removal rate per litter farrowed or annual number of farrowings divided by the annual number of removals (cf. overall PrRem in Table 2 ). The higher a herd's LSY, the higher will be the removal rate at any given PrRem. For example, consider two herds: Herd 1 has LSY 2.0, Herd 2 has LSY 1.5, and RemRate (Herd 1 ) = R e m R a t e (Herd 2 ) = 5 0 % . Thus, PrRem is 0.25 for Herd 1 and 0.33 for Herd 2. Although the two herds have identical TABLE 5 Mean herd-specific production measures by three levels of mean annual replacement rate No. of farms Annual replacement rate
6 33.0
6 51.2
6 65.0
Sow life-span (years) Average female inventory Pigs born live per litter Pigs weaned per litter Involuntary removal rate ~ (%) Voluntary removal rate (%) Inadequate performance (%) Miscellaneous (%) Total removal rate (%)
3.1 159 9.9 8.6 23.7 13.3 7.7 5.6 37.0
2.0 233 10.0 8.8 43.0 7.1 6.0 1.1 50.1
1.6 201 10.4 9.1 60.5 2.1 1.6 0,5 62,6
qncludes reproductive failure, degenerative problems, locomotor problems, death, lactation problems, peripartum difficulties and systemic disease.
SOWCULLINGANDMORTALITYIN SWINEBREEDINGHERDS
93
removal rates, there is an important difference in the risk of removal, i.e. the biological processes associated with removal, between the two herds. This is reflected as differences in average sow age between these two herds. APSL is calculated as ( 1 0 0 / ( R e m R a t e / L S Y ) ). Thus, APSL is 4.0 for Herd 1 and 3.0 for Herd 2; sows are older in Herd 1 even though the herd removal rates are equal. The example may explain the conflicting results discussed above for culling rate effects on herd productivity. Thus, when trying to associate between-herd productivity differences with removal and replacement effects (i.e. herd age structure effects), one should use a measure of herd-specific average female age rather than compare directly on removal rates. For example, in a recent study Duffy and Stein (1988) found no association between herd-specific culling rate and PWSY or LSY, but did find that culling rate per litter farrowed was associated negatively with both. In this sample of herds, most females were removed involuntarily, and as the replacement rate increased a higher proportion of all removals were involuntary. This can cause the 'structural' maintenance of a young herd age distribution, thus reducing the economic efficiency of an enterprise (Dijkhuizen et al., 1986). This study has several limitations. Convenience sampling was used, and the sample was biased, in that these were herds who had self-selected the use of a sophisticated production record-keeping program. These results are perhaps not representative of North American swine herds but may represent commercial herds using computerized recording systems. In addition, farmer-recorded removal reasons were not validated, and multiple removal reasons were not allowed. However, even with these limitations, our results are quite similar to previous investigations. These similarities suggest that the information obtained from computerized recording systems is valid, and justifies the use of such data in future research. In future investigations, an attempt should be made to replicate the work of Dagorn and Aumaitre ( 1979 ) and assess the relationship between herd age and overall productivity. In addition, reason-specific risk factors for removal need to be assessed, as has been done recently in dairy cattle (Milian-Sauzo et al., 1989 ). This is especially true for reproductive failure and locomotor problems in young females, degenerative problems in older females, and death in all females. Breeding female wastage in the swine enterprises studied here is at unacceptable levels. REFERENCES Dagorn, J. and Aumaitre, A., 1979. Sow culling: reasons for and effect on productivity. Livest. Prod. Sci., 6: 167-177. D'Allaire, S., Stein, T.E. and Leman, A.D., 1987. Culling patterns in selected Minnesota swine breeding herds. Can. J. Vet. Res., 51: 506-512.
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Dijkhuizen, A.A., Morris, R.S. and Morrow, M., 1986. Economic optimization of culling strategies in swine breeding herds, using the "Porkchop" computer program. Prev. Vet. Med., 4: 341-353. Duffy, S. and Stein, T.E., 1988. Correlations between production, productivity and population factors in swine breeding herds. Proc. 10th Int. Pig Vet. Soc., p. 345 (Abstract). Einarsson, S. and Settergren, I., 1974. Fertility and culling in some pig breeding herds in Sweden. Nord. Vet. Med., 26: 576-584. Jones, J.E.T., 1967. An investigation of the causes of mortality and morbidity in sows in a commercial herd. Br. Vet. J., 123: 327-339. Jovic, M., Varadin, M. and Nikolic, P., 1975. The length of reproduction of breeding sows at intensive piglet production and the chief reasons for their elimination from the breeding herd. Veterinaria, 24: 17-23. Kleinbaum, D.G., Kupper, L.L. and Morgenstern, H., 1982. Epidemiologic research: Principles and Quantitative Methods. Van Nostrand Reinhold, New York, NY, pp. 100-103. Kroes, Y. and Van Male, J.P., 1979. Reproductive lifetime of sows in relation to economy of production. Livest. Prod. Sci., 6:179-183. Milian-Sauzo, F., Erb, H.N. and Smith, R.D., 1989. Risk factors for reason-specific culling of dairy cows. Prev. Vet. Med., 7:19-29. Pattison, H.D., 1980. Patterns of sow culling. Pig News Inf., 1: 215-218. Pattison, H.D., Cook, G.L. and Mackenzie, S., 1980. A study of culling patterns in commercial pig breeding herds. Proc. Br. Soc. Anim. Prod., Harrogate, pp. 462-463. Pomeroy, R.W., 1960. Infertility and neonatal mortality in the sow. I. Lifetime performance and reasons for disposal of sows. J. Agric., Sci., 54: 1-17. Statistical Analysis System, 1985. User's Guide: Basics and Statistics. SAS Institute, Cary, NC. Stein, T.E., 1985. The computer as the core of health management in food animal populations. PhD dissertation, University of Minnesota. Stein, T.E. and Duffy, S.J., 1988. Parity-specific production values for 68 North American swine breeding herds. Proc. l 0th Int. Pig Vet. Soc., p. 346 (Abstract). Svendsen, J., Nielsen, N.C., Bille, N. and Riising, H.J., 1975. Causes of culling and death in sows. Nord. Vet. Med., 27: 604-615.