Farms and animals
In total, 29 breeding farms were recruited for participation in the study. Of these 29 farms, 19 were farrow-to-finish herds. Another nine farms sold the weaners to fattening farms. One breeding farm sold the piglets to a weaning farm, which again sold the weaners to a fattening farm. Therefore, 29 different production chains were observed (13 in Bavaria, 16 in North Rhine-Westphalia). Two production chains ended in the same fattening herd. Overall, the 29 production chains involved 39 different herds. Seven pigs from each of two focus litters were selected in each breeding farm.
Antibiotic treatment
Antibiotic treatments to the focus litter pigs were recorded in all 29 production chains. Focus pigs were mainly treated at the weaning stage and often the pigs received more than one treatment, i.e. antibiotic agent per production stage (Table 1; more detailed information presented in (5)). In all 29 chains, treatments were applied to all pigs of both focus litters. As an exception from this rule, two sucklers of one breeding herd and one fattening pig received an antibiotic agent individually (via injection) while none of the other pigs of their focus litters were treated. Despite these individual treatments, the litters were classified as “untreated” at the respective production stage. At suckling, all treatments were administered individually (via injection or drench) once or twice. At weaning exclusively oral group administration via water or feed was used for five to ten treatment days. During fattening, injection on one or two days or feed administration for three to 20 days was used. Treated indications and administered agents were described in Burow, et al. (5).
Table 1
Kind and number of antibiotic treatments to focus herds and litters per production stage1
Production stage | Number of | Applied antibiotic classes |
treated focus herds2 | treatments of focus herds | treated focus litters3 | treatments of focus litters |
Suckling | 4 | 9 | 8 | 16 | Aminoglycosid, Penicillins |
Weaning | 15 | 18 | 30 | 36 | Penicillins, Tetracyclines, Polymyxins, Macolides, Other |
Fattening | 4 | 4 | 8 | 8 | Tetracycline, Macrolide |
1in the 29 production stages |
2both focal litters treated |
3at least half of the piglets per litter treated |
Input variables of housing, management, health and performance
Questionnaire information on housing, management, health and performance was received for all production chains. However, for one of the chains, information from the fattening stage was missing. For other chains, responses to individual questions were missing (Tables 2–4).
Causal diagrams and classification trees
According to the causal diagrams drawn for each production stage, all potential risk factors with p < 0.2 in the univariable analysis appeared largely independent with little confounding and were considered in the multivariable analysis.
The classification trees for each production stage, based on variables with p < 0.05 in the univariable analysis, was in agreement with the result from the multivariable analysis.
Impact of housing, management, health and performance on antibiotic use
Suckling stage
At the suckling stage, eight variables were associated with antibiotic use at p < 0.2 in descriptive tests (Table 2). After univariable analysis, all were considered in the multivariable analysis, except ‘Time interval to farrowing when sows enter the nursery’ since its p-value was > 0.2 (p = 0.9992).
‘Cleaning agents used for cleaning the pens’ (p = 0.0298) and ‘Rodent control carried out by a company’ (probability, p = 0.0312) showed p-values below 0.05 in the univariable analysis. In the multivariable analysis, ‘Rodent control carried out by a company’ was the only factor that was significantly associated with antibiotic use (odds ratio, OR, 6.86, confidence limits, CL, 1.12–41.83, p = 0.023) after backward elimination and forward selection strategy. The value of the Akaike Information Criterion (AIC) was 22.63.
Table 2
Potential risk factors (probability, p < 0.2) for antibiotic use at the suckling stage in 29 breeding herds
Variable | Categories | Number of production chains with antibiotic use | P-value in Fishers‘ Exact test | Categories univariably and multivariably analysed | P-value in type 3 analysis of univariably analysed categories |
No | Yes |
Distance to next livestock farm | 1: <1 km 2: 1–3 km 3: >3 km | 21 2 2 | 2 2 0 | 0.095 | 1 2 2 | 0.153 |
Cattle housed in the next livestock farm | 0: No 1: Yes | 8 | 3 | 0.139 | 0 | 0.103 |
17 | 1 | 1 |
Cleaning agents used for cleaning the pens | 0: No 1: Yes | 12 13 | 0 4 | 0.121 | 0 1 | 0.030 |
Rodent control carried out by a company | 0: No 1: Inside or outside 2: Inside and outside | 20 | 1 | 0.079 | 0 | 0.031 |
3 | 2 | 1 |
2 | 1 | 1 |
5 | 0 | 1 |
1 | 2 | 2 |
1 | 0 | 2 |
2 | 0 | 3 |
4 | 0 | 3 |
2 | 0 | 3 |
1 | 2 | 3 |
4 | 0 | 3 |
Breed | 1: Hybrid DK1 | 5 | 0 | 0.013 | 1 | 0.051 |
2: Hybrid DK2*Pietrain | 5 | 0 | | 1 | |
3: Hybrid GE | 1 | 2 | | 2 | |
4: Hybrid GE*Pietrain | 1 | 0 | | 2 | |
5: DL3 | 2 | 0 | | 3 | |
6: DL*Pietrain | 4 | 0 | | 3 | |
7: DL*DE4 | 2 | 0 | | 3 | |
8: DL*D/B E*Pietrain | 1 | 2 | | 3 | |
9: Diff. breeds/ crossing | 4 | 0 | | 3 | |
Time interval to farrowing when sows enter the nursery | 1: <1 week 2: 1 week 3: >1 week | 12 8 5 | 0 4 0 | 0.052 | 1 2 3 | 0.999 |
Week of life for Porcine circovirus 2 vaccination | 0: no indication 1: 2.-3. week 2: 4.-6. week | 4 11 10 | 0 4 0 | 0.167 | 0 1 2 | 0.053 |
Week of life for Mycoplasma hyopneumoniae vaccination | 1: 1 week 2: 2 week 3: 3 week | 14 4 5 | 1 0 3 | 0.156 | 1 2 3 | 0.095 |
1Danish |
2German |
3German Landrace |
4German Large Wight |
Weaning stage
At the weaning stage, another eight factors were associated with antibiotic use in descriptive tests (p < 0.2; Table 3). All were considered in the subsequent univariable analyses.
Results in the univariable analyses suggested association (p < 0.2) of all eight variables, except for ‘Week of life for vaccination against Mycoplasma hyopneumoniae‘ (p = 0.9512). Beside ‘Production type’, ‘Cleaning pens with warm water’ (OR 8.67 CL 0.89–8.67, p = 0.031) showed significant association with antibiotic use. ‘Number of life births’ (OR 0.22, CL 0.05–1.09 p = 0.055) and ‘Cleaning feeding system after antibiotic administration’ (OR 0.15, CL 0.01–1.58, p = 0.079) also tended to be associated with a decreased risk of antibiotic use (p < 0.1). However, confidence limits of these three variables included the value 1, which contradicts a significant association. In the multivariable analysis (including seven variables in the initial model), ‘Production type’ was the only significant factor in the final model (OR 0.15, CL 0.02–0.89, p = 0.023) after backward elimination or stepwise forward selection.
Table 3
Potential risk factors (probability, p < 0.2) for antibiotic use at the weaning stage in 29 production chains
Variable | Categories | Number of production chains with antibiotic use | P-value in Fishers‘ Exact test |
no | yes |
Production type1 | 1: Farrow-to-finish 2: Piglets/ weaners – fattening 3: All stages separated | 12 1 1 | 7 8 0 | 0.014 |
Housing places fattening | 1: ≤1000 2: >1000 | 6 5 | 7 1 | 0.177 |
Cleaning of pens with warm water | 0: no 1: yes | 13 1 | 9 6 | 0.080 |
Water source | 1: Municipal 2: Own well or both | 6 8 | 10 5 | 0.1982 |
Cleaning feeding system after mixing of antibiotics | 0: none 1: yes | 12 1 | 7 4 | 0.142 |
Average weight [kg] at weaning4 | Continuous | | | 0.1443 |
Week of life for vaccination against Mycoplasma hyopneumoniae | 1: 1. week 2: 2. week 3: 3. week | 7 0 5 | 8 4 2 | 0.101 |
Live births per sow and year | 1: <32 2: ≥32 | 5 9 | 10 4 | 0.128 |
1Production type analysed in univariable and multivariable analysis as 1: farrow-to-finish vs. 2: movement of pigs to another farm after suckling or weaning (p = 0.05 in Fisher’s exact test) |
2Chi-square |
3GENMOD procedure |
4N = 28, mean 7.1 standard deviation 1.0 kg |
Fattening stage
Antibiotic treatment was applied to four of the 29 production chains at the fattening stage. Seven variables were associated with antibiotic use (p < 0.2) at fattening (Table 4) in the descriptive evaluation and included in the further analyses.
In the univariable analyses, ‘Hygiene program for water pipe’, ‘Number of weaned piglets per sow and year’ and ‘Number of rounds of fatteners per year’ were significantly associated with antibiotic use (p < 0.05), whereas ‘Weight at entrance to growing’ and ‘Mortality of weaners in the last completed business year’ showed tendency of association (p < 0.1).
In the multivariable analysis, ‘Number of weaned piglets per sow and year’ (OR 2.5, CL 0.58–29.58, p = 0.0172) was the only significant factor in the final model (lowest value of AIC (10.9) in stepwise forward selection). However, the confidence limits included the value 1, which contradicts a significant association.
Table 4
Potential risk factors (probability, p < 0.2) for antibiotic use at the fattening stage in 29 production chains
Variable | Categories | Number of production chains with antibiotic use | P-value in Fishers‘ Exact test |
No | Yes |
Hygiene program for water pipe | 0: no 1: yes | 15 10 | 0 4 | 0.042 |
Average age [weeks] at entrance to growing1, 2 | Continuous | | | 0.0591 |
Weight [g] at entrance to growing | 1: <30 kg 2: 30 kg 3: >30 kg | 3 7 10 | 2 1 0 | 0.082 |
Mortality of weaners in the last completed business year [%] | 1: <1 2: =2 3: >2 | 7 7 4 | 0 0 2 | 0.079 |
Number of weaned piglets per sow and year | 1: ≤28 2: >28 | 11 5 | 0 2 | 0.137 |
Number of rounds of fatteners per year | 1: <3 2: ≥3 | 12 13 | 0 4 | 0.121 |
Live weight of fatteners at slaughter [kg]3 | Continuous | | | 0.1421 |
1GENMOD procedure in SAS |
2N = 22, mean 11.0 standard deviation 1.2 kg |
3N = 29, mean 119.1 standard deviation 2.9 kg |