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Original research
Peer reviewed
A retrospective study of mortality in grow-finish pigs in
a multi-site production system
Dominiek Maes, DVM, MS, MSc, PhD; Alejandro Larriestra, DVM,
MSc; John Deen, DVM, MSc, PhD; Robert Morrison, DVM, MBA, PhD
DM: Faculty of Veterinary Medicine, Department of Reproduction,
Obstetrics and Herd Health, Ghent University, Salisburylaan 133
9820 Merelbeke, Belgium
AL, JD, RM: College of Veterinary Medicine, Department of Clinical
and Population Sciences, University of Minnesota, St Paul, MN
55108
Maes D, Larriestra A, Deen J, et al. A retrospective study
of mortality in grow-finish pigs in a multi-site production system.
J Swine Health Prod. 2001;9(6):267-273. Also
available as a PDF.
Summary
Objectives: To investigate overall and weekly mortality
in grow-finish pigs in a large multi-site production system, and
to assess the associated financial losses.
Materials and methods: Between January 1996 and January
2000, mortality was investigated retrospectively in 14 swine complexes,
including 146 closeouts comprising 1,345,127 pigs. Overall mortality
during the entire grow-finish period was expressed as deaths per
1000 pig weeks. Weekly mortality was the number of pigs that died
during a week divided by the average inventory of pigs during
that week. Opportunity costs of mortality were assessed for each
year of the study.
Results: Mean overall mortality during the 4 years was
3.23 per 1000 pig weeks. Mortality increased steadily from 2.6
(1996) to 3.6 per 1000 pig weeks (1999) (P<.001). Late
mortality was consistently greater than early mortality (P<.001),
and increased from 3.1 (1996) to 5.5 pigs per 1000 pig weeks (1999)
(P<.001). Opportunity costs due to overall mortality
increased from $2.86 per marketed pig in 1996 to $5.22 in 1999,
with late mortality, which was more pronounced during fall months,
accounting for about two thirds of these total costs ($US).
Implications: Increased mortality rates occur mainly
in older, more valuable finishing pigs and cannot be predicted
by early mortality. Mortality records should be analyzed to investigate
patterns. Weekly mortality rates may be calculated using the record
keeping systems used in many US swine farms. Causes of death and
risk factors for the seasonal change in mortality pattern warrant
further research.
Keywords: swine, mortality pattern,
grow-finish pigs, multi-site production,
opportunity cost
Received: October 20, 2000
Accepted: March 27, 2001
Modern swine production is characterized
by confinement housing of large groups of pigs, usuallyon large
specialized farms or production systems. This intensification
creates an ideal environment for transmission of infectious agents
and may negatively influence the health of the pigs. To overcome
or limit these disadvantages, new technologies, such as multi-site
production and early weaning, have been introduced in North America
and in some European countries.1 The new technologies
appear to reduce transmission of infectious pathogens2
and improve the productivity and profitability of pig production.3,
4 The pattern of disease (ie, severity of disease, age groups
affected, and specific diseases that occur) differs between multi-site
and traditional production systems.1 Early weaning
and the strict separation of groups of pigs with all in-all out
(AIAO) production generally minimize infections during the early
production phases in multi-site production systems.1
However, the low incidence of infections may not be maintained
until the end of the finishing period. If disease problems are
postponed until later in the finishing period, the result is poorer
performance and more disease in older finishing pigs.5
Mortality rate, ADG, and FCR are the major parameters used
to measure performance in grow-finish pigs.6,7 When
mortality in the finishing period is high, the pigs that die represent
a considerable investment.8 Many studies have investigated
pre-weaning mortality 9,10 and mortality in sows.11,
12 However, only a few have focused on mortality in grow-finish
pigs.13-15 Some investigated mortality in pigs in test
stations,13 and others were conducted on European farms
with traditional management systems.14 Consequently,
the results may not apply to multi-site production systems in
the United States. Moreover, most studies on mortality in grow-finish
pigs were designed to discern risk factors for increased overall
mortality rates16,17 without considering the age at
which pigs die during the grow-finish period. Detailed information
is available about the pattern of mortality in pre-weaning pigs
and sows, but we are not aware of similar studies focused on grow-finish
pigs. Financial losses due to mortality are likely to vary greatly
depending on whether pigs die at the beginning or at the end of
the finishing period. Therefore, analysis of the cost of mortality
will be a more accurate estimate if the age at which pigs die
is taken into account.
This study was conducted in a large multi-site production system
facing an increase in mortality in grow-finish pigs. The major
objectives were to investigate overall mortality and patterns
of mortality during the grow-finish period. In addition, an assessment
was made of the financial costs associated with the increased
mortality.
Materials and methods
The production system
The study was conducted in a three-site production system consisting
of one sow complex, five similar nursery complexes, and 14 grow-finish
complexes. The sow complex consisted of 12 sow barns housing 2240
sows each. Each nursery complex consisted of 24 barns with a capacity
of 1360 pigs per barn. Each grow-finish complex consisted of eight
barns with a capacity of 1150 pigs per barn (9200 pigs per complex).
The grow-finish facilities, built in 1994 to 1995, contained 46
pens per barn.
Piglets were weaned three times a week at about 18 days of
age and transferred to one of the nursery complexes. In order
to fill nursery complexes completely, some pigs originating from
a different sow complex were transferred to the nursery complexes.
This amounted to approximately 10% of the pigs present in the
nurseries (1100 per week). At about 10 weeks of age, nursery pigs
were moved into the finishing barns, with 25 to 26 pigs per pen,
and an initial stocking density of approximately
0.641 m2 per pig. Barrows and gilts were not housed
separately. Three pens per barn were used as hospital pens.
Feeding practices and housing conditions were identical in
all grow-finish complexes for the entire duration of the study.
The barns were tunnel-ventilated (ie, plastic ducts with a fan
in one end of the barn) and had fully slatted concrete floors.
Manure was flushed daily to a lagoon. Pigs were fed a corn-soybean
feed (meal) ad libitumin a wet-dry feeding system. Six different
feeding phases were used during the grow-finish period, with bacitracin
added to all phases as a growth promotant.
Nursery facilities were managed AIAO by barn, and grow-finish
facilities were managed AIAO by complex. The stand-empty period
between two closeouts, when barns were cleaned by power washing
and disinfected with a commercial disinfectant, was on average
4 days. This period was similar in all complexes and remained
constant over time.
The whole herd was free of pseudorabies, but was infected with
Mycoplasma hyopneumoniae, Actinobacillus pleuropneumoniae
serovar 3, and porcine reproductive and respiratory syndrome (PRRS)
virus. Sows were vaccinated before breeding against PRRS and Escherichia
coli, and against influenza H1N1 until mid-1998, when the
influenza vaccination was discontinued. The same vaccines were
used for incoming gilts. Pigs in the nursery unit were vaccinated
against M hyopneumoniae at 6 and 8 weeks of age. No other
vaccination programs were employed during the study period.
Study population and study design
Mortality was investigated retrospectively in all 14 grow-finish
complexes between January 1996 and January 2000 (4 years), including
146 closeouts comprising 1,345,127 placed pigs. A closeout consisted
of all grow-finish pigs placed into one complex before it was
emptied (approximately 9200 pigs). For practical reasons, the
barns within a complex were usually populated within 1 week, and
the finishing pigs in a complex were sent to slaughter in different
shipments, usually during a period of about 5 weeks. Pigs were
marketed as early as week 13 of the grow-finish period, but in
most closeouts, the first groups were marketed at about week 15.
The last groups were marketed week 19 (1999) or 21 (1996, 1997,
and 1998).
On a weekly basis, the farm manager recorded the number of
pigs in each complex that died and the numbers culled and transferred.
Dead pigs included those that died naturally and sick or injured
pigs euthanized for welfare reasons. Culled pigs included pigs
weighing less than 160 lb at closeout, pigs that suffered from
inguinal or umbilical hernias, and pigs that were thin, severely
lame, or suspected of having gastro-esophageal ulcers. Because
these pigs were unlikely to reach market weight either within
the farm or in an outside production system, they were sent to
a separate market. Transferred pigs were low-weight pigs that
were expected to reach market weight only with extra time and
were moved out of the farm to continue their finishing period
outside the system.
Overall and weekly mortality
Overall mortality during the entire grow-finish period and
early mortality (weeks 1 to 10 in the finishing period) and late
mortality (week 11 to end of finishing period) across the 4 years
of the study were calculated as incidence rates and expressed
as the number of deaths per 1000 pig weeks.18 Weekly
mortality was calculated as the number of pigs that died during
a week divided by the average inventory of pigs during that week.
The average weekly inventory of a closeout was a function of the
number of pigs placed and the number of pigs lost during the grow-finish
period because of death, culling, transfer, and shipment to slaughter.
To check the validity of the data, we compared the number of dead,
culled, transferred, and marketed pigs with the number of pigs
placed in the grow-finish unit. Necropsies were performed on approximately
600 pigs that died during the summer of 1999.
Statistical analyses
The complex closeout was the unit of analysis. Differences
in overall mortality across the 4 years were analyzed using ANOVA.
Where overall differences were significant, pair-wise comparisons
between years were performed using the Scheffé test.
Life table analysis was used to investigate weekly mortality
patterns of the grow-finish period across the years.19
This method was chosen because censoring occurred in the data
set due to withdrawals (culling, transfer, and shipment of pigs
to slaughter). The cumulative probability that an animal had not
died some time after the start of the kth week of the
grow-finish period was the product of the probabilities that an
animal did not die beyond the start of that week and through each
of the k-1 preceding weeks. Possible differences in survival curves
across the years were analyzed with the log-rank test.19
The correlation between early mortality (weeks 1 to 10 of finishing
period) and late mortality (week 11 to end of finishing period)
for the entire study period was analyzed using the Spearman rank
correlation test. Differences in early and late overall mortality
for the entire study period were analyzed using ANOVA. In case
of a significant overall difference, pair-wise comparisons between
early and late mortality within each year were made using the
Scheffé test. Differences in early and late overall mortality
across the 4 years were analyzed by ANOVA. Statistical tests were
considered significant at the 5% level. Calculations, graphs,
and statistical analyses were performed using the software package
Statistica(TM).
Financial analyses
Financial losses due to mortality during the grow-finish period
were calculated using a simple economic model (all values in $US).
First, the value of slaughter pigs was assumed to be $100 per
pig, assuming an average pig price of $40 per 100 lb live weight
and a final weight of 250 lb per pig. Next, feed costs were calculated
for pigs that died, for feed that they would have consumed if
they had reached slaughter weight. These costs were a function
of the amount of feed consumed and feed price. The amount of feed
that pigs would have consumed during the remainder of the grow-finish
period was calculated using the date of death, the duration of
the finishing period, and standard average daily feed intake curves.20
The end date of the grow-finish period was the weighted average
of the different shipment dates of slaughter pigs, which were
18.8 (1996), 19.1 (1997), 17.3 (1998) and 16.5 weeks (1999). An
average price of feed was used in the analyses, which was $120
per 2000 lb, corresponding to $0.06 per lb. Feed costs for pigs
that died before reaching slaughter weight were obtained by multiplying
the average price of feed by the amount of feed not consumed.
Finally, financial losses for a dead pig (ie, opportunity costs)
were calculated as the value of a slaughter pig minus the costs
for the feed not consumed. Loss due to mortality was calculated
for each year of the study, and expressed as the cost per pig
marketed.
Results
Overall mortality
Overall mortality rate during the 4 years was 3.23 deaths per
1000 pig weeks (Table 1). Overall mortality steadily increased
every year from 1996 to 1999 (P<.001). Overall mortality
was significantly lower in 1996 compared to 1997 (P<.01),
and in 1996 compared to 1998 and 1999 (P<.001). Overall
mortality was significantly lower in 1997 than in 1999 (P<.01).
There were no other differences in overall mortality between years
(P>.05). There were no differences in overall mortality
among the grow-finish complexes (P>.05). The percentage
of culled pigs showed some variation across the 4 years, but no
pattern could be identified (Table 1). The percentage of transferred
pigs decreased slightly in the first 3 years of the study. In
1999, no pigs were transferred because analysis had shown that
transfer was not financially justified. The total number of culled
and transferred pigs showed no clear pattern across the 4 years:
the highest percentage was observed in 1997, followed sequentially
by 1998, 1996, and 1999. The ratio of dead, culled, transferred,
and marketed pigs and the number of pigs placed in the grow-finish
unit each year was consistently between 0.99 and 1.
Necropsy data indicated that pneumonia was the most frequent
lesion, followed by gastric ulcers. However, no conclusive diagnosis
could be established.
No major changes in management practices related to pig flow,
genetics, feeding, or housing were reported during the study period,
except in 1999, when pigs were no longer transferred and groups
were closed out approximately 2 weeks earlier than in previous
years. Several methods of intervention were attempted to combat
the increasing mortality (for example, in-feed treatment with
chlortetracycline or chlortetracycline-tiamulin at the start of
the grow-finish phase, vaccination of piglets for PRRS, vaccination
of feeder pigs with a commercial vaccine against Salmonella
serovar Choleraesuis, and a change in feed). None of these measures
were associated with a change in mortality or with a change in
the pattern of mortality varying from year to year during the
fall months. No clear relationship could be identified between
mortality and the number of weeks pigs spent in each grow-finish
complex.
Weekly mortality
Weekly mortality rates remained constant during the first weeks
of the grow-finish period, but increased in the last weeks (Figure
1). In addition, weekly mortality showed much more variation in
the second half of the grow-finish period. This was evidenced
by much wider 25% and 75% quartiles, ie, values that include 50%
of the data.
Increased risk for mortality occurred mainly after week 10
of the grow-finish period in all 4 years (Figure 2). Consequently,
two distinct episodes of mortality were distinguished: early mortality
occurring during weeks 1 to 10 of the grow-finish period and late
mortality during week 11 until the end of the finishing period.
In addition, there was a divergence in survival curves among the
years, especially between 1996 and the other years, with survival
decreasing in successive years. For clarity, the 95% confidence
intervals surrounding the survival functions are not presented.
The average ratio of late to early mortality for the 4 years
was 2.5. In 95% of the close-outs, this ratio was >=1.20, indicating
that late mortality was at least 20% higher than early mortality.
Early and late mortality
Early and late mortality across the 4 years of the study were
calculated in the same way as overall mortality. During the entire
study period, late mortality (4.92 deaths per 1000 pig weeks)
was greater (P<.001) than early mortality (2.16 deaths
per 1000 pig weeks), and there was no correlation between early
and late mortality (r=0.038; P=.64). Early and late mortality
did not differ in different grow-finish complexes (P>.05).
Early mortality remained constant across the years except in
1998, when early mortality was higher than in 1996 (P<.01).
Early mortality, expressed in number of pig deaths per 1000 pig
weeks, was 1.93+/-0.69 in 1996, 2.12+/-0.67 in 1997, 2.50+/-0.83
in 1998, and 2.10+/-0.51 in 1999 (Figure 3).
In each year of the study, late mortality was greater than
early mortality (P<.001) and increased steadily from
3.39 in 1996 to 5.91 in 1999 (P<.001). Late mortality
was lower in 1996 than in 1997 (P=.053) and lower in 1996
than in 1998 or 1999 (P<.001). Late mortality was lower
in 1997 than in 1998 (P<.01) and 1999 (P<.001).
Late mortality did not differ between 1998 and 1999 (P>.05).
A peak in late mortality consistently occurred each year in
September, October, and November for groups placed in June, July,
and August (Figure 4). The seasonal pattern for early mortality
was less pronounced (Figure 4). The peaks were lower and were
more dispersed over the months. However, groups placed during
fall months had a higher early mortality, corresponding with a
higher mortality in younger grow-finish pigs during the fall and
early winter.
Economic losses
The opportunity cost due to overall mortality increased from
$2.86 per marketed pig in 1996 to $5.22 in 1999 (Table 2). The
cost of early mortality increased slightly between 1996 ($1.12
per marketed pig) and 1999 ($1.72 per marketed pig). The cost
of late mortality showed a more pronounced increase from 1996
($1.74 per marketed pig) to 1999 ($3.50 per marketed pig). On
average, late mortality accounted for about two thirds of the
total costs of overall mortality.
Discussion
Unique characteristics of this study include the method of
recording mortality on a weekly basis, the long period of observation
(4 years), the large number of pigs, and the study population,
which was homogeneous with respect to breed, management practices,
housing conditions, feeding, disease prevention procedures, and
biosecurity measures. Studying a homogeneous population has the
advantage that results are not influenced by variation in the
listed variables, which may be the case when mortality is investigated
across different swine farms. On the other hand, using a homogeneous
study population also implies that the results may not be applicable
to all swine farms, or even to other multi-site production systems.
Clermont and Désilets21 reported differences
in mortality across different types of farms, with higher overall
mortality in pigs from grow-finish farms compared to pigs from
farrow-to-finish farms. Further studies are required to investigate
whether the overall mortality and mortality pattern observed in
this study also occur in single-site farms or other multi-site
production systems.
Overall mortality increased from 2.57 deaths per 1000 pig weeks
in 1996 to 3.57 in 1999. A recent NAHMS study in 393 US swine
operations, each with more than 300 finishing pigs, showed a mean
mortality of 2.3% for pigs placed in grow-finish pigs facilities.15
Overall mortality higher than 4% was experienced by 13.5% of the
operations. Straw et al13 found an overall mortality
of 5.3% and 6.3% in two groups of grow-finish pigs raised in a
test station.
Because the inventory decreased throughout the grow-finish
period due to death, culling, transfer, and shipment to slaughter,
overall mortality was not expressed as the percentage of the number
of pigs initially placed in the finishing units, but as the number
of pigs per 1000 pig weeks. In this way, changes in the total
number of pigs were taken into account in the calculations. Because
the number of pigs in the barns decreased towards the end of the
finishing period, the absolute number of pigs that died late in
the finishing period may be somewhat lower than appear based on
the mortality risk at this time. The percentage of pigs culled
and transferred varied across the years. Because culled or transferred
pigs may be more likely to die, this variation may have introduced
some minor bias into the mortality results.
The weekly mortality patterns indicated that increased overall
mortality was due mainly to a higher mortality occurring after
week 10 of the finishing period, ie, in pigs older than 20 weeks.
From week 10 onwards, there was no longer an overlap of the 95%
confidence intervals surrounding the survival curves of the different
years. We used this to split the grow-finish period into two periods,
one before 10 weeks, and one after 10 weeks. Straw et al13
showed that more pigs in a test station died in the last 6 weeks
of the 18-week finishing period than in the first 6 weeks. To
the authors' knowledge, the present study is the first that investigated
weekly mortality patterns in pigs raised in a multi-site production
system.
In contrast with clinical, serological, pathological, or even
some performance parameters, mortality can be recorded reliably
by swine barn managers. Although mortality is often recorded on
a weekly basis, the data are usually used to calculate only overall
mortality. The results of this study suggest that farm managers
should analyze mortality on a weekly basis and use this data to
monitor the health status of pigs throughout the grow-finish period.
In addition, weekly mortality data can be linked with other health
parameters (eg, coughing index) that are more difficult to measure
under practical conditions.
The results of the financial analysis in this study show that
death loss in grow-finish pigs may have a great impact on the
profitability of swine operations.8 In addition, from
an economic point of view, Table 2 also demonstrates that it is
important to know whether pigs are dying at the beginning or at
the end of the grow-finish period. The periods of risk for mortality
are well documented for suckling pigs 9,10 and breeding
females;11,12 however, despite the long grow-finish
interval, the periods with the highest risk for mortality have
not yet been identified for grow-finish pigs. In general, two
farms with the same percentage of overall mortality in grow-finish
pigs can suffer quite different financial losses depending on
the age at which pigs die, because older pigs are more valuable.
The increase in financial losses due to late mortality in the
present study would have been even more pronounced if the duration
of the grow-finish period had remained the same across the years.
The grow-finish period was about 2 weeks shorter in 1999, resulting
in a shorter period for late mortality. A simple economic model
was used to calculate the opportunity costs.22 Although
the outcome may differ slightly from the results of detailed economic
calculations on mortality, our model provides a good estimate
of the losses associated with mortality. A fixed price was used
for feed and for slaughter pigs to eliminate the influence of
different market conditions across the years. Therefore, differences
in mortality pattern were responsible for the changes in costs
of mortality across the years, and the comparisons of mortality
costs in different years were not confounded by changing market
conditions. Reducing mortality increases gross revenue for the
producer. However, the entire value of the losses are not recaptured,
because costs of preventive measures to reduce mortality must
be included when the net profitability of reduced mortality is
calculated. Pigs grow more efficiently and economically in multi-site
production systems than in conventional swine farms,3,4
in part because they have a lower pathogen load and better health
status. However, it remains to be investigated whether better
health status can be maintained until the end of the finishing
period.5 If disease problems are delayed until later
in the finishing period, the benefits resulting from better performance
in younger pigs may be diminished by the higher disease prevalence
and mortality in older, more valuable finishing pigs. Further
prospective studies, using clinical parameters, serology, and
additional laboratory investigations, are required to identify
causes of increased mortality and associated risk factors late
in the grow-finish period. The most common causes of mortality
in grow-finish pigs include respiratory disease, cardio-vascular
failure, and diseases of the intestinal and urogenital tract.13,14
While the risk of mortality in this production system did not
increase until pigs reached 20 weeks of age, the problem might
actually have started earlier. However, a very consistent seasonal
pattern could be observed, especially with respect to late mortality.
Schoder et al14 also found a higher risk of mortality
in grow-finish pigs in the month of September. A precise explanation
for the seasonal pattern is currently not available. Studies that
investigated pneumonia lesions in slaughter pigs23,24
showed a higher prevalence during the winter, and the investigators
ascribed this to more adverse weather conditions during that period.
Further research is warranted to investigate whether a similar
mortality pattern occurs in other production systems and to elucidate
possible risk factors for increased mortality.
Implications
- Assessing weekly mortality in grow-finish pigs in large,
multi-site production farms may reveal a change in mortality
pattern.
- An increase in mortality may occur specifically in older,
more valuable finishing pigs and the increase may be more pronounced
during fall months.
- In one multi-site production system studied over 4 consecutive
years, approximately two thirds of the overall costs of mortality
in grow-finish pigs were associated with deaths occurring in
pigs older than 20 weeks.
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