Reproductive performance of outdoor sow herds

Livestock Production Science 78 (2002) 233–243 www.elsevier.com / locate / livprodsci

Reproductive performance of outdoor sow herds V.Aa. Larsen*, E. Jørgensen Department of Agricultural Systems, Research Centre Foulum, Danish Institute of Agricultural Sciences, P.O. Box 50, 8830 Tjele, Denmark Received 2 January 2001; received in revised form 15 April 2002; accepted 29 April 2002

Abstract The differences in conditions in outdoor systems compared to indoor systems were expected to influence the reproduction performance of outdoor sow herds. The differences include e.g. loose-housing, groups of animals and climatic factors as well as registration routines. Sow records were analysed from three Danish and one Scottish outdoor herd. In the analyses the reproduction cycle (days between farrowings (DFAF)) was divided into the three sub-periods: days from farrowing to weaning (DFAW), days from weaning to first recorded service (DWSE), and days from first recorded service to farrowing (DSFA). In addition, the analyses included days from farrowing to culling (DFCU). Explanatory variables were herd, parity, year, serving system and season as well as relevant interactions. The average herd level was 149.9 days for DFAF, 28.0 days for DFAW, 5.6 days for DWSE, and 116.2 days for DSFA, which is similar to levels observed in indoor systems. The average level for DFCU was 27.7 days. The identified variation was limited and comparable with the usual range in sow herds. The effect of the explanatory variables could often be ascribed to specific registration and management practices. There was a tendency for DFAW to be lower in April (26 days) compared to October (27 days). In addition, there was a tendency for DWSE to be higher in herds with uncontrolled servings (4.9 days) compared to herds with controlled servings / artificial inseminations (AI) (6.5 days). In herds with uncontrolled servings, DFAW tended to increase in July to February. In herds with controlled servings /AI, DWSE tended to increase in January to April.  2002 Elsevier Science B.V. All rights reserved. Keywords: Pig reproduction; Season; Parity; Herd management; Field data; Herd; Registrations; Production system

1. Introduction Outdoor production systems for sows is a new way to house sows, at least in a number of European

*Corresponding author. Present address: The National Committee for Pig Production, Vinkelvej 11, 8620 Kjellerup, Denmark. Tel.: 145-8771-4036; fax: 145-8771-4005. E-mail address: [email protected] (V.A. Larsen).

countries. Until the 1990s, only small-scale outdoor pig production existed in Denmark (Mortensen et al., 1994) and in other European countries such as Germany (Spitschak, 1997) and France (Berger et al., 1997). In Great Britain, outdoor production accounts for 25% of the breeding herds (MLC, 2001). It is characteristic for the outdoor production systems that all sows are outdoors and loose during lactation. The facilities for serving are either outdoors or indoors and the servings are based on

0301-6226 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0301-6226( 02 )00099-4

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uncontrolled natural services, controlled services, artificial inseminations (AI), or on a combination of these practices. All gestating sows are loose-housed and in groups. On some farms, facilities for gestation are outdoors, on other farms these facilities are indoors. So, the sows in outdoor systems are housed under different conditions in their complete reproduction cycle or at least in significant parts of it, compared to indoor sows. This means that in the outdoor systems the sows are exposed to the behaviour of other sows (Arey and Edwards, 1998), to changes in number of hours with daylight (Perera and Hacker, 1984; Prunier et al., 1994), and to changes in temperatures (Stansbury et al., 1987; Prunier et al., 1994). In addition to the possible impact of the outdoor production systems on the reproduction of the sows, the outdoor production systems affect the management. In the outdoor systems, observations and identification of the sows are more difficult and this affects the accuracy of registrations. The experience from outdoor production indicates variations in reproduction results obtained (Mortensen et al., 1994; Steen, 1994). Previous investigations of reproduction in outdoor herds have used data from a single herd only (Spitschak, 1997), used data from a database where the production systems were not known (Dial and Xue, 1993) or have not focused on the entire reproduction cycle and the stages within it (Hemsworth et al., 1982; Stansbury et al., 1987; Dove and Haydon, 1994; ten Napel et al., 1995; Spitschak, 1997). So, there is a need to investigate reproduction results and variation in reproduction results obtained in outdoor production systems, where records of the entire reproduction cycle are available from a number of herds. The purpose of this paper is to describe and analyse the level of and the variation in reproduction results obtained in several outdoor production systems for which detailed information on management practices exists.

2. Material and method Three Danish herds and one Scottish herd were included in this project. The herds represented a broad spectrum of production systems, herd sizes and production levels (Table 1).

2.1. The Danish herds For each sow, the farmers registered parity, dates of services, dates of farrowings and the number of piglets born alive and stillborn, dates of weanings and the numbers of weaned piglets, and date of culling. For units with servings outdoors, date of serving was recorded if servings were observed, otherwise date of serving was recorded as the date on which the sows were moved into a paddock with boars. The registrations were recorded in a computerised herd-management programme (‘Bedriftsløsning’  (www.lr.dk)) where one file contains all data from the individual herd. Sow-records exist as of the end of 1996 and until the summer of 2000. Early in 1999, the herd (IIIA) on farm III was culled because of dysentery and it was replaced in the summer of 1999 (IIIB). The Danish herds were either PIC-animals (herds I and II) or from the Danish Breeding Herds (herd IIIA / B). It was Duroc, Large White and Landrace crosses.

2.2. The Scottish herd For the Scottish herd (herd V), data included sow records from the start of the herd in 1996 till the herd was culled at the beginning of 1999. The sow records from the Scottish herd consisted of four sets of data. One set of data contained records of servings and tests for pregnancy. A second set of data contained records of farrowings. A third set of data contained records of weaning and a fourth contained records of cullings. The records in each of the data sets included identification of the sow and date of the event. However, none of the records included parity of the sow. The first record of farrowing for each sow was assigned to first parity, because the herd was initiated in 1996, at the same time as the recording. For the following farrowings, the parity was increased by one for each farrowing. The servings recorded prior to a farrowing were assigned the same parity as the farrowing, and a weaning recorded later than a farrowing was assigned the parity of this farrowing. For some of the records, the manager had not assigned a sow number, and for some records the date was not possible when compared to the rest of the history of the sow, so not all records could be used in the final analysis.

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Table 1 Production system, herd size and production level for three Danish (Farms I, II, and III (A and B) and one Scottish outdoor herd (Farm V) Herd

I Project period

a

Production system Single lactation paddocks Group lactation paddocks Outdoor uncontrolled natural services Outdoor controlled natural services Outdoor artificial inseminations Indoor uncontrolled services Indoor controlled services Indoor artificial inseminations Pregnancy test Dynamic groups (gestation) Herd size (no. of sows) Production level First parity sows (%) Total born, no. per litter Total dead (stillborn1dead during lactation; % of total born) a

II

III

V A

Pro

End

Pro

End

Pro

Only None Only None None None None None None Only

Only None Some Some None None None None Some Only

Only None Only None None None None None None Only

Few Most Few None None None None Most None None

None Only None None None Few Most Few Some None

130

120

220

390

300

End

B

Pro

End

Few Most None None None None None Most Some None

Only None Few None None Most None None Most Most

Only None Few None None Most None None Most Most

300

540

600

21 12.4

18 13.0

22 13.0

34 11.7

86 10.3

86 11.3

60 12.1

0 12.8

23

27

24

18

19

16

15

19

Pro: 1996; End: 1999 / 2000.

Different crossbreeds have been used, e.g. Landrace 3 Large White as maternal breed of the sow and Landrace 3 Duroc as paternal. At the end of the recording period, the herd used Large White 3 Duroc as maternal breed and Landrace as paternal.

2.3. Data analysis This included analysis of the length of the oestrus cycle, and records for sows culled. Moreover, experience and information from visits to the farms and dialogues with the farmers were significant components in the interpretation of the results. The oestrus cycle of the sows (the number of days between two successive farrowings (DFAF)) was divided into the following sub-periods and these sub-periods were used as response variables in the statistical analyses. 1. Days from farrowing to weaning (DFAW). 2. Days from weaning to first recorded service (DWSE). 3. Days from first recorded service to farrowing (DSFA).

In addition, the number of days from farrowing to culling (DFCU) was calculated. Data were checked for consistency. For the sows, for which the order of events differed from the categories above, the total records were printed and checked. If the order of events followed the categories above, but the days between events varied significantly from the expected value, if for example the days between service and farrowing (DSFA) were lower than 100, the total records for these sows were printed and checked. Preliminary results showed that data consisted of several populations, e.g. DSFA consisted of sows which farrowed after the first service, and of sows which returned to service one time, two times, etc. Thus the assumption of normal distribution in the statistical analysis was questionable. To take the lack of normal distribution into account, averages per month per herd by parity were calculated and the response variables as well as the residuals were weighted in the analysis of variance by the number of observations in the average per month. However, the weighted response variables and the residuals did not correspond to the normal distribution either, so the response values were transformed with a variance

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stabilising function (Box et al., 1978) before the statistical analyses. For DFAF and DSFA, the reciprocal value of the response was the response value of the model. For DFAW, DWSE and DFCU, the response value used in the model was the square root of the original value. Following the transformation of the data, the response values used in the analyses were the weighted average of each value per month within a given herd and where the month was the month of the first of the two successive events. Secondly, data were analysed by means of the PROC UNIVARIATE procedure (SAS Institute Inc., 1990). Outliers were identified as observations where the value differed from the mean6three times the standard deviation. The outliers were omitted if establishment of the herd or the recording system caused them or, if the outliers related to time of culling of a herd. Next, the PROC MIXED procedure (SAS Institute Inc., 1990) was used to analyse if the response variable differed between years (1995–2000), herds, parities, seasons and / or serving systems. The parity was defined as ‘first parity’ or ‘older’ sows. The serving system was uncontrolled servings (group 1) or controlled servings (group 2). The following model was used: Yijkln 5 m 1 s i 1 Hij 1 ak 1 hl 1 b1i x 1ijkln 1 b2i x 2ijkln 1 (a H ) ijk 1 (hH ) ijl 1 b1ij x 1ijkln 1 b2ij x 2ijkln 1 ´ijkln where Yijkln is the monthly average by parity of the transformed values of DFAF, DFAW, DWSE, DSFA, and DFCU, respectively; m is a general intercept, s i is the systematic effect of serving system, Hij the random effect of herd j within serving system i distributed as N(0, s 2herd ), ak the systematic effect of parity k, hl the systematic effect of year l. The covariates x 1ijkln , x 2ijkln are defined as x 1ijkln 5 sin((m 2 0.5) / 12(2p )), x 2ijkln 5 cos((m 2 0.5) / 12(2p )), where m is the month of the nth monthly average in service system i, herd j, parity k, year l. These covariates are used for capturing seasonal variation (Nelson et al., 1979; Jørgensen, 2002), and ( b1i , b2i ) are the corresponding regression coefficients for each serving system. b1ij , b2ij are the corresponding regression coefficients for seasonal variation within each herd. Within each serving

system a test for significant amplitude was carried out. (That is, a simultaneous test that either b1i or b2i was different from 0.) If neither group showed significant amplitude, the model was reduced to include an identical seasonal component for each mating system ( b11 5 b12 and b21 5 b22 ). (a H ) ijk and (hH ) ijl are interaction terms distribut2 2 ed as N(0, s herd*Year ), and N(0, s herd*parity ). Similar terms were included in the model for test of interaction between parity and season with parameters b1ij , b2ij . The interaction terms were included in an initial model and omitted afterwards if P.0.1. The interaction between herd within serving system and parity was included because the proportion of first parity sows differed between herds. The interaction between ‘season’ and ‘herd within serving system’ was included because the facilities for servings and gestation were exposed to different climatic conditions in the different herds. The interaction between ‘year’ and ‘herd within serving system’ was included because of differences between herds in periods of establishment and culling of the entire herd. In the section with results tendencies are referred to if 0.05,P#0.10. In continuation of the statistical analyses, the deviation of the individual sow result from the predicted level from the model (based on monthly averages) was calculated. The probability distribution of these deviations was estimated using a kernel density estimator (Scott, 1992). 2 2 2 The total variance (s H 5 s herd 1 s herd*Year ) of the 2 variance between herds (s herd ) and the variance between years within a herd (s 2herd*Year ) was estimated. Lower and upper confidence intervals were calculated as m 96ksH , where k 5 1.96 and m 9 5 mˆ 1 ]12 3(First1Older). In addition, the proportion of the variance between herds of the total variance was 2 estimated (s herd /s 2H ). The number of litters per sow per year was calculated. The calculated value included three components. First, the proportion of first parity sows multiplied by the sum of days from insertion to first recorded service and days from first recorded service to farrowing (DSFA). Second, the proportion of older sows multiplied by days between farrowings (DFAF). Third, the proportion of culled sows multiplied by days from farrowing to culling. The days from insertion to the first service recorded were set

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to 60 days for all the herds. The number of days in a year divided by the sum of the three components resulted in a calculated value of the number of litters per sow per year.

3. Results

3.1. Variation between herds in the length of the sub-periods of the reproduction cycle Table 2 indicates that the variance between herds accounted for the major part of the total variance in yearly averages, except for days from weaning to culling (DFCU). As most of the traits are dependent on management strategy, this was to be expected. The trait least affected from management strategy, days from weaning to service (DWSE), showed a slightly lower relative variance. The results in Table 2 are based on analyses of the data before the herds were grouped according to service system.

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1999 from approx. 24 days to approx. 29 days. In addition, there was a tendency to an effect of year (P,0.10) on days from first recorded service to farrowing (DSFA). From 1995 to 1997, the number of DSFA increased by 2 days (from approx. 117 to approx. 119 days) in all herds, and from 1997 and onwards, the number of DSFA decreased in all herds by approx. 2–3 days. On the other hand, the statistical analyses showed no effect of either year in general on days between farrowings (DFAF) or days from weaning to service (DWSE). For days from farrowing to culling, there was no overall effect of year (P.0.10) but there was considerable variation within herds over the years indicating that the herds had changed strategy for culling.

3.3. Parity There was no effect of parity on DFAF, DFAW, DWSE, DSFA or DFCU (Table 3).

3.2. Year

3.4. Season

There was a tendency (P,0.10) (Table 3) for days from farrowing to weaning (DFAW) to show an effect of year, so DFAW increased from 1995 to

There was a tendency (P,0.10) (Table 3) for days from farrowing to weaning (DFAW) to show an effect of season, where the lowest number of days

Table 2 Expected average herd level, lower and upper 95% confidence interval for herd averages for the five response variables and relative herd variance ((s 2herd /s 2H ) 3 100)

Average, no. of days Lower 95% confidence interval Upper 95% confidence interval Relative herd variance (%)

DFAF

DFAW

DWSE

DSFA

DFCU

149.9 142.7 157.9 91.6

28.0 24.2 32.2 81.3

5.6 3.7 7.9 60.7

116.2 114.2 118.2 96.5

27.7 10.9 52.4 0

Table 3 P-values for the effect of the four fixed effects year, parity, season ( b1 , b2 ) and serving system Effect

DFAF

DFAW

DWSE

DSFA

DFCU

Year Parity ‘Season’ Serving system

NS NS NS NS

,0.10 NS ,0.10 NS

NS NS NS ,0.05

,0.10 NS NS NS

NS NS NS NS

Season* Serving system Season*Group 1 Season*Group 2

NS NS

,0.10 NS

NS ,0.10

NS NS

– –

Group 1, uncontrolled servings; group 2, controlled servings /AI; NS if P.0.1.

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(approx. 26 days) occurred at the end of April and the highest number of days (approx. 27 days) occurred at the end of October.

3.5. Serving system There was an effect of serving system on days from weaning to first recorded service (DWSE) (P, 0.05) (Table 3), where the herds which used uncontrolled servings (group 1) had approx. 4.9 days compared to the herds which used controlled servings (group 2), which had approx. 6.5 days from weaning to first recorded service. In addition, there was a tendency to an interaction of season and serving system on DFAW (P,0.10), where the herds with uncontrolled servings (group 1) had increased days from farrowing to weaning from July to February with the maximum in October (approx. 27 days) and the lowest level in April (approx. 26 days). Similarly, there was a tendency to an interaction of season and serving system on DWSE (P,0.10), where the herds with controlled servings (group 2) had increased days from weaning to first recorded service in January to April with the maximum in February / March (approx. 6.3 days) and the lowest level in August / September (approx. 4.7 days).

3.6. Culling pattern On herds I and II, 70% of the culled sows had been registered as culled after weaning and only 30% after service. In contrast, herd V registered 40% as culled after weaning and 60% after service (Table 4). The proportion of culled first parity versus culled older sows reflected the herd structure and differed between herds (Table 4). If the herd structure was

stable, there was no apparent difference between proportion of sows culled in different years or different seasons. Herd IIIA culled all sows in January–February 1999. Of the culled sows, 5–13% died or were destroyed on the farms.

3.7. Variation between individual animals Fig. 1(A–E) illustrates the variation between individual sows and the magnitude of the range is similar to the values indicated by Whittemore (1983), see Fig. 2, except for farrowing to weaning interval (DWSE) where the values in Fig. 2 must include between-herd variation. Fig. 1 clearly demonstrates the non-normality of data, with marked skewness, and multimodality. The multimodality is probably mostly due to the cyclic occurrence of the events, e.g. repeat services, but lack of an expected farrowing may have an influence for the distribution DFCU. Fig. 1(E) illustrates that especially for the response variable DFCU the predicted level differed from the observed level. Comparing Fig. 1A and E, it is clear that the individual differences in DFCU were much larger than in DFAF, even though the average value of DFCU was lower than the average of DFAF. In Table 2, it was shown that the variation between herds accounted for very little or nothing of the total variation. Thus, the results indicated that the random influences on DFCU hid any potential systematic differences in the present study.

3.8. Coherence between the sub-periods of the reproduction cycle of sows housed outdoors In the previous sections, the reproduction period from farrowing to farrowing was separated into three

Table 4 Proportion of sows (first or older parity) culled after service or weaning for four outdoor herds Herd

I II IIIA IIIB V

First parity

Older parity

Weaning (%)

Service (%)

Weaning (%)

Service (%)

1 2 9 32 3

3 8 15 12 11

68 67 40 50 36

28 23 37 6 49

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Fig. 1. Kernel density of difference between observed and predicted level for DFAF (A), DFAW (B), DWSE (C), DSFA (D) and DFCU (E) corrected for effect of herd, parity, year and season.

Fig. 2. Distribution of the observed average values per month as the value of the 50%-quantiles and the differences (in days) between the 50%-quantile and the 5, 25, 75, and 95%-quantiles.

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Table 5 Estimated level of DFAF, DSFA, DFCU (days), litters per sow per year (no.) for four outdoor herds Herd

I II IIIA IIIB V

Days from

Litter /

Farrowing to farrowing (DFAF)

First service to farrowing (DSFA)

Farrowing to culling (DFCU)

sow / year (no.)

153.2 151.0 151.0 160.3 150.6

119.5 117.9 118.0 119.7 117.3

25.8 25.5 25.9 26.5 46.8

2.20 2.22 2.22 2.10 2.16

sub-periods to increase the understanding of the possible causes of variation. The sub-periods differed in nature, where DFAW, to a larger degree than DWSE and DSFA, was a result of management strategies. The estimated average levels of the three sub-periods and of their lower and upper confidence intervals (Table 2) added up to DFAF. The results indicated, to some extent, that the variations in each of the three sub-periods counterbalanced each other instead of being additive. Fig. 2 shows the time-scale ranges for the four variables DFAF, DFAW, DWSE, and DSFA. The figure illustrates that the interval in days was narrow, and that the differences between the value of the 75%-quantile and the value of the 25%-quantile were less than 10 days for all four variables. On the basis of the estimated values of DFAF, DSFA, and DFCU within herds, the number of litters per sow per year was calculated as 2.1 to 2.2 litters per sow per year (Table 5).

4. Discussion Prior to this study, it was expected that the reproduction of sows housed outdoors varied significantly. This expectation was partly based on the variance in the quarterly averages of reproduction performance in the Danish outdoor herds (Kongsted et al., 2000). This apparent variation was accepted because of the expected impact of the housing conditions on the reproductive performance of the sows and because of the results from previous studies (Claus et al., 1983; Dial and Xue, 1993; Prunier et al., 1996; Rydhmer, 2000). There were considerable differences between the

facilities for servings on the farms, where herds I and II (group 1) had the most extensive systems, although herd I changed to controlled servings outdoors and herd II changed to AI indoors in groups at the end of the project. Herd IIIA / B (group 2) had more intensive supervision of the servings and used AI indoors and stable groups of sows. Herd V (group 1) used the dynamic service system indoors and tested the sows for pregnancy before they returned to the paddocks outdoors. The serving facilities in all herds differed from recommendations used in indoor systems in Denmark. Moreover, the sows in all the herds were exposed to the behaviour of other sows, there was no control of number of servings by boars and no specific routine for heat observation, etc. So, it was expected that the level of reproduction results obtained in the outdoor systems might differ compared to indoors and that the results varied significantly.

4.1. The length of the reproduction stages The variation in the response variables was limited (Table 2), however, there were several possible explanations for this limited variation. First, DFAF and DSFA only included sows, which came to farrowing. A proportion of the sows, which returned to oestrus a number of times were likely to have been culled before the next recorded event. Thereby the influence of these sows was not included in DFAF or DSFA, but explained some of the variation seen in DFCU. DFAW and DWSE included all sows, which had farrowed or weaned, so there was no censoring of this group of animals. However, DFAW was based on the farmers’ strategies. The variance (3.7–7.9 days) in DWSE was limited.

4.2. Effect of the year, parity, season, and serving system The tendency for DFAW to increase from 1995 to 1999 could be explained by changes in the strategy for batch intervals in the herds, which required adjustments of the weaning age, which subsequently led to an increase in age at weaning, until a new strategy for batch intervals was incorporated in the management of the herds. Another possible explanation for this tendency was that the farmers increased

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the weaning age to increase the weight at weaning and the strength of the piglets when the use of growth promoters ceased in the production of weaners. Season was included in the model as the sows on all the farms were exposed to changes in climatic conditions within a year. Herds IIIA, IIIB, and V had indoor serving facilities, the climate of the buildings, however, was not controlled by mechanical equipment. Other studies (Sterning et al., 1990) have shown that the ability to show standing oestrus and ovulate within 10 days after weaning was influenced by the season in which weaning occurred. In a study by Weitze et al. (1994), only 10% of the sows had a weaning-to-heat-onset interval of more than 7 days. Claus et al. (1985) showed that the concentrations of steroids in the blood plasma and seminal plasma of boars were lowest in July and August and highest in October to December. So, the hormone level reached optimum at the same time as the decrease in daylength reached optimum. Claus et al. (1985) concluded that the daylength was the main factor responsible for the seasonal variations in steroid production. Berger et al. (1997) showed longer farrowing intervals for outdoor herds than for indoor herds. However, the farrowing interval in the present study was at the same level as found by Tholen et al. (1996a,b) and ten Napel and Johnson (1997). The serving system affected DWSE (P,0.05), where DWSE was at a lower level for the herds with uncontrolled servings outdoors (group 1) compared to the herds with controlled servings (group 2). Group 1 recorded the majority of sows as served on the date on which they were placed in paddocks with boars, a fact which could explain the limited variation shown in DWSE for these herds. Previous studies have indicated that different factors affected weaning-to-oestrus interval (Vesseur et al., 1994). ten Napel et al. (1995, 1998) and Rydhmer (2000) described the possibilities of decreasing the weaningto-oestrus interval by selection. However, the genetics of the herds in the present study varied (Larsen, 2000), and as the records of date of service were not exact, it was unlikely that the farmers had selected for decreased interval from weaning to oestrus. In addition, the results of the present study indi-

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cated an effect of season of weaning on DWSE (for group 2), which was in accordance with Prunier et al. (1996), Koketsu et al. (1997), and Knox and Zas (2001). Koketsu et al. (1997) observed that springfarrowing sows had longer weaning-to-oestrus intervals, and Knox and Zas (2001) observed that parities 1 and 2 sows tended to show ovulation failure in the spring months. There was no immediate explanation of the tendency to a higher level of DFAW for group 1 in October. However, the tendencies to interactions (for DFAW and DWSE—interactions between season and serving system) were in accordance with the results presented in Table 2, where the relative herd variation was lower for DFAW and DWSE than for DFAF and DSFA. The analysis showed that some of the possible explanations of the variations were factors which were not directly related to the characteristics of the outdoor production system. In contrast, the variations were caused by factors which were related to management strategies or registration practice on the individual herds. DSFA in the present study could be expected to vary from indoor sow herds partly because of the use of the dynamic service system (Grigoriadis et al., 2000) in three of the herds (group 1). The dynamic service system is characterised by no control of the contact frequency between boars and newly weaned sows (Hughes, 1994), multi-sire matings (Tanida et al., 1989), and no control of the maximum mating frequency per boar (Hemsworth et al., 1983; Chamberlain and Hughes, 1996). For herds which used AI, Weitze et al. (1994) showed that insemination management had to be adapted to the individual oestrus behaviour of the sows. In the present study, even though the conditions for servings were not optimal on any of the farms, the variation in DSFA was limited.

4.3. Within a herd In the present study, the variations in the response values within herd were comparable to the variations presented by Whittemore (1983). As an example (in the present study), 75% of the sows (Fig. 1), which came to farrowing, farrowed within 118 days from the first recorded service. The time-scale ranges

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found by Whittemore (1983) showed an average length of pregnancy of 115 days, ranging from 112 to 118 days. In addition, the observed averages per month of the response variables were compared to the predicted values. The difference was similar to the usual range in sow herds. There was a difference between herds in regard to the last event of a sow before culling. In herds I and II, the majority of the culled sows had been registered as culled after weaning. In contrast, herd V registered less than half of the culled sows as culled after weaning. Possible explanations were the differences between the production systems. The sows in herds I and II were primarily culled because of litter performance, because this was the most accurate knowledge the managers had of their sows, whereas herd V tested the majority of the sows for pregnancy, so the knowledge of conception or failure of conception was included in the culling strategy. For a period, the farmers registered reasons for cullings and these were in accordance with Table 4. So the herds in which sows were culled at weaning, were culled primarily because of production performance, whereas the sows in herds that culled at service were culled because of the production performance and reproduction failures such as ‘not in pig’, ‘return to service’, etc. The proportion of sows culled or removed because of death was similar to the results in the study by Lucia et al. (2000).

5. Conclusion The present study indicated only limited variation in the length of reproduction stages and the identified variation was explained, to a large extent, by management factors. These management factors were not necessarily related to outdoor characteristics, but could be the establishment of a herd, the culling of a herd, or the change in practice for registrations, etc. A seasonal variation was identified, but, it only explained a few days. Another outcome of the analyses was the identification of a possible impact of differences in registration practice on the estimation of the length of the reproduction stages. It could be argued that for recording systems and decision support systems to be

supportive of the decision process on the individual farm, these systems need to take into account the method of recording. That is, whether registrations are based on an observation, standard for all individuals in a batch, or a mixture. The results of the present study indicated that the reproduction obtained in outdoor systems not necessarily differed from reproduction results obtained under controlled conditions in indoor systems. Moreover, the results did indicate that in production systems where the sows are loose-housed and identification and handling of individual animals are difficult, a relevant alternative to individual sow records could be records at batch or group level, similar to the systems used in production of weaners and finishers.

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