Analysis of the Factors Affecting Boar culling in commercial Boar Studs in Southern China

On commercial boar stud, boar culling and mortality were of great relevance from the production efficiency and return of investment (Koketsu et al., 2009; Robinson et al., 2005; Knox et al., 2016). Therefore, a prerequisite for future sustainable production would be put on keep improving culling program (Robinson et al., 2005; Sasaki et al., 2010; Zhao et al., 2015). The knowledge of boar culling frequency and its reasons could be beneficial in breeding herd management and identifying underlying problems. Earlier author studies suggested that high culling rates in commercial boar studs could seriously impact production efficiency, the primary causes contributed to the unplanned boar culling were reproductive disorders and lameness (Koketsu et al., 2009; Sylvie et al., 1990).
        
Authors have documented the primary reasons for boar replacement as genetic improvement (approximately 22%), poor semen quality (21%) and feet and leg issues (19%) in Canada and the USA(Knox et al., 2008). Other researchers in European countries reported that the culling reasons were genetics (20-45%), semen quality (10-30%), libido (1-21%), physical soundness (13-60%) and others (10-20%) (Robinson et al., 2005). In Japan the reasons identified were old age (27.3%), reproductive problems (23.9%), lameness (8.6%), death (8.0%) and others (32.2%) (Koketsu et al., 2009). By determining the frequency as well as the causes of mortality and culling, prevention strategies could be established to optimise production, increase profit levels on commercial boar studs, improve biosecurity standards and increase animal welfare (Safranski et al., 2008). However, a comprehensive study of the frequency of boar culling and factors effects under subtropical climate with Chinese model of pig management and housing has not been reported.
        
In recent years in China, more and more commercial boar studs with different sizes were built, driven by the national “Pig breeding subsidies” which facilitate the genetic and breeding improvement of swine. Against this background, the main objectives of the present approach were to (1) determine the reason of boar culling, (2) improve the boar service life under subtropical climate and (3) insight the relationship of boar culling frequency with the seasons and breeds.

Herds and housing systems
 
The study was carried out on nine commercial boar studs owned by a single large integrated pork production company that located between latitude 20°54'-26°24’NL and longitude 104°26'-112°04’EL in Southern China. The area has a subtropical climate, with abundant rainfall and hot seasons, with short, cold and wet winters (lasts from February to April) and long, hot and wet summers (lasts from May to October).
        
The boars harvested were purebred Landrace, Yorkshire and Duroc that had the similar procedures in feeding, semen collections and culling program. The boars were mechanically fed with an individual drop tube to each boar twice a day (approximately 2.5kg of feed/ boar/d) with specifically designed diets, and water delivery was ad libitum essentially by nipples and troughs. The semen were collected through the gloved- hand method.
 
Definitions and category
 
The cull referred to here in all categories including the death boar, which consist of planned types (mainly contain old age and poor performance) and unplanned types with the reasons were categorized into seven groups and a range of subgroups showed in Table 1.
 

 
Data collection and statistical analyses
 
The present study encompassed a databank (KFNets software, IPIG Management & Consulting Co.,Ltd.) originally consisting of 2342 culling boar records from nine boar studs collected over the period from July 2013 to June 2016. All statistical analyses were performed with SPSS11.0 software (SPSS Inc., Chicago, IL).If there were more than one recorded reason for culling, the primary reason would be analyzed in this study.

Reasons for boar culling and seasonal distribution
 
Descriptive statistics of reasons for boar culling were summarized and presented in Table 1. In the total 2342 culled boars, the frequency of unplanned culling of boar accounted for a large proportion, reaching 88.04% in the total culls, reproductive disorders and lameness were the most frequently cited reasons, the rate were 40.61% and 27.2% respectively. Performance and old agewerethe two subgroups of planned culling, accounted for 7.34% and 4.61% of all culling risk, respectively.
        
The average distribution was illustrated to study the effect of seasons on boar culling (The result was showed in Fig 1A and 1B). The frequency of boar culling reached its peak in May (396, 16.91%) while the season was characterized by high ambient temperature and humidity accompanied by a wide-range of temperature fuctuations. The frequency of boar culling reached its lowest point in September (116, 4.95%) with dry and comfortable climate.
 

        
The highest frequency of boar culling due to reproductive disorders rore in May (171, 17.98%) while the lowest in November (40, 4.21%). The highest frequency of boar culling due to lameness was appeared in May (90, 14.13%), followed by January (68, 10.68%) and December (66, 10.36%). The highest frequency of boar culling due to death was arisen in January (27, 12.11%), followed by May (26, 11.66%), February (23, 10.31%) and November (22, 9.87%), while the lowest in September (5, 2.24%). The highest frequency of boar culling due to performance was arisen in December (72, 41.86%) followed by May (32, 18.60%), while the lowest in September (1, 0.58%). Over all, the boar culling was relatively less during September to October with the climatic characteristics was dry and comfortable in Southern China while the most frequency appeared at or during high temperature and humidity, especially from May to August, followed by winter (including November, December and January) both with cold and wet.
 
Seasonal and breed effects on reproductive disorders
 
The average distribution of the total 951 boars culled due to reproductive disorders was illustrated to study the seasonal effects on boar culling (Fig 2). The highest frequency of boar culling due to reproductive disorders was arisen in May (171, 17.98%) and the relatively high culling lasted for 16 weeks continued until August (95, 10.00%) due to the heat stress. In addition, Yorkshire boar owned highest frequency, the culling rate reached 47.09% (275/584) and the culling number reached its peak in May (60, 21.82%), followed by July (33, 12.00%). The date among Landrace boar and Duroc boar were 38.97% (385/988) and 37.80% (291/770) respectively, which also occurred in the summer while lowest frequency of culling appeared in November (13, 3.38%) and December (9, 3.27%).
 

        
More detailed description was displayed to study the subgroups in boars culled due to reproductive disorders (Fig  3A and Fig 3B). Sperm-related problems accounted for the largest proportion (668, 70.24%) in the total 951 boars culled due to reproductive disorders, followed by the abnormal genital system (169, 17.78%), while the lowest was the poor libido (114, 11.99%). More precisely, Yorkshire boar owned the highest frequency of culling due to sperm-related problems peaked in hot seasons where the rate reached 34.08% (199/584),which was higher than Duroc boar (209, 27.24%) and Landrace boar (260, 26.32%). Landrace boar had a higher frequency of culling due to poor libido (86, 8.70%) than Duroc boar (55, 7.14%), while the Yorkshire boar had the lowest frequency of culling due to poor libido (28, 4.80%). However, Yorkshire boar  had shown the highest frequency of culling due to abnormal genital system with a rate of 8.22% (48/584) that was higher than Duroc boar (27, 3.51%) and Landrace boar (39, 3.95%).
 

 
Seasonal and breed effects on lameness
 
The average distribution of the total 637 boars culled due to lameness was illustrated to study the seasonal effects (The result was presented in Fig 4). The highest frequency of boar culling due to lameness appeared in May (90, 14.13%) with heat and humidity, followed by January (68, 10.68%) and December (66, 10.36%) with cold and wet. More precisely, the frequency of culling due to lameness in the Duroc boar was highest, reached 29.22% (225/770), followed by Landrace 28.14% (278/988) and the Yorkshire with the lowest, 22.95% (134/584). For the Duroc boar, the culling number was arisen in May (27, 12.00%), followed by December (25, 11.11%). For the Landrace boar, the culling was arisen in May (44, 15.83%) followed by January (33, 11.87%). For the Yorkshire boar, the highest frequency date was showed in May (19, 14.18%), followed by December (17, 12.69%) and January (17, 12.69%).
 

 
Reasons for boar culling
 
The study elucidated the reasons for the boar culling under the subtropical climate in the Southern China. In the study, the proportion of unplanned boar culling was 88.04% (N=2342 culled boars), higher than other report (71.8%) (Koketsu et al., 2009). The reproductive disorders (40.61%) and lameness (27.2%) were the main reasons for unplanned culling which were significantly higher than the previous report with reproductive disorders (18.4%) and lameness (11.8%)(Sylvie et al.,1990), and higher than the Japanese report with reproductive disorders (26.4%) and lameness (8.4%) (Robinson et al., 2005). It reminded that progress was required in boar feeding for improving the efficiency of the boar studs and reducing the unplanned boar culling in Southern China.
 
Seasonal effects on reproductive disorders
 
Reproductive failure was a major economic burden for the commercial boar studs (Knox et al., 2016). The success of the boar studs relied on the fertility and high-quality semen in sufficient quantity and the genetic value of the boars (Smital et al., 2009). It has been shown that high temperatures affected sperm production adversely, with impact on sperm motility and percentages of normal sperms (Bao et al., 2016; Zasiadczyk et al., 2015; Suriyasomboon et al., 2004). Our research displayed that reproductive disorders was an essential factor affecting boar culling, while the sperm-related problems accounted for the largest proportion (70.24%) of reproductive disorders (N=951 boars). The highest frequency of boar culling was arisen in May (17.98%) and lasted for 16 weeks until August (10.00%). Researchers had reported semen quality declined significantly during the hot season might be associated with the lower levels of HSP70 in boar spermatozoa (Huang et al., 2000). Others considered that heat stress (HS) could induce autophagy in immature boar sertoli cells by incubating the cultured immature boar SCs at 43°C for 30 minutes (Bao et al., 2016). Due to the subtropical climate, high ambient temperatures and humidity start to appear in late spring in Southern China with the temperature ranged from 21°C to 34°C and 68%-90% relative humidity. Boar studs should pay close attention to temperature and humidity changes and start cooling process at 25.5°C to reduce heat stress in boar stud from late spring in Southern China.
        
Semen quality of reproductive boars differed throughout the year but significant breed-by-season interactions were reported (Suriyasomboon et al., 2004; Huang et al., 2000; Sonderman et al., 2008; Zaja et al., 2016; Rodriguez et al., 2016; Schulze et al., 2014). Sonderman et al., 2008 considered that crossbred boars usually produce the highest seminal quality, followed by purebred terminal lines and lastly, purebred maternal lines (Sonderman et al., 2008). Our results displayed that the culling frequency due to sperm-related problems was significantly different, where Yorkshire boar owned the highest frequency (34.08%) than Duroc (27.24%) and Landrace (26.32%).
        
Smital (2009) reported that the year-season effect had a clear impact on semen quality (Munsterhjelm et al., 2015). The result showed the lowest values of semen traits were observed in summer while the highest values were found in autumn and winter. Further trials should include a large number of samples carried out for multiple years to fully characterize the repeatability of seasonal changes in semen quality traits.
        
In our result, differences also existed among genetic lines in libido. Our report revealed that Landrace boar had a highest percentage of poor libido (8.70%) than the Duroc boar (7.14%) which was consistent with Sonderman’s report (Sonderman et al., 2008). Poor libido was likely caused by behavioral rather than endocrinological problems (Levis et al., 2005). Genital lesions or musculoskeletal problems, unfamiliar environment and the feared person might have a strong negative effect. Breed and strain differences were also seen (Sonderman et al., 2008; Estienne et al., 2014; Dong et al., 2016).
 
Seasonal effects on lameness
 
Lameness was an important trait included in the pig breeding goal due both to animal welfare and economical aspects (Munsterhjelm et al., 2015; Le et al., 2015; Holinger et al., 2015). General physical examination including the back, legs and locomotor function should be part of fertility evaluation. Osteomalacia, osteoarthrosis and arthritis, which might result in lameness was likely to be associated with pain, distress and was the most common reasons for culling. In our study, lameness was the second reasons for unplanned culling accounted for 27.2% (N=2342) that higher than other reports (8.4% and 11.9%) (Koketsu et al., 2009; Sylvie et al., 1990; Knox et al., 2008). It was speculated extreme weather more likely to cause lameness for the highest frequency was appeared in May (90, 14.13%) with heat and humidity, followed by January (68, 10.68%) and December (66, 10.36%) with cold and wet.
        
As for breeds, the frequency of culling due to lameness in the Duroc boar was highest, where the frequency was 29.22% (N=770), followed by Landrace 28.14% (N=988), and Yorkshire, 22.95% (N=584). Various floor types and also high stocking density affected leg strength. Jensen pointed out that the boars with lameness had reduced daily weight gain compared to boars without lameness in spite of treatment with antibiotics for arthritis (Jensen et al., 2008). Others reported that feed intake decreased in lame or tail bitten animals and the anorexia was prolonged up to about 30 days in culled-to-be lame animals (Munsterhjelm et al., 2015). Thus improving leg quality in pig herds was expected to increase profitability as well as animal welfare.

A comprehensive study of the frequency of boar culling and factors effecting the same, under subtropical climate with Chinese model of pig management and housing were reported. The higher unplanned boar culling reminded the progress required in boar feeding and reducing the unplanned boar culling in Southern China. The subtropical climate had a significant effect on reproductive disorders, lameness, death and diseases mostly due to the high temperatures and humidity or extreme cold and wet. In addition, a large data of semen quality traits for multiple years needed to illustrate and to fully characterize the repeatability with seasonal changes and their relationship to breeds and seasons. The relationships among specific reasons for culling, reproductive data, and gross morphology of genital system of replacement boars culled due to reproductive failure should be further studied to improve boar stud reproductive efficiency.

This work was supported by National Natural Science Foundation of China (No. 31502071) and China Agriculture Research System (CARS-36), project from Ministry of Science and Technology of China (2014 FY120800 and 2013GA790001). We are grateful to Yu Zhou and Xing Peng from Guangxi Yang xiang Animal Husbandry Co.,Ltd. for data collection and to professor Hao Zhang from the College of Animal Science of Southern China Agricultural University for helpful comments on the manuscript.