Blood-based Biomarkers Of Aggressive Behavior In Weaned Pigs After Weaning

Mixing is a common practice after weaning or after transferring to the fattening units in modern pig industry.To establish dominance hierarchy after mixing,pigs usually express more aggressive behavior,which affects animal health and welfare,as well as growth performance and profitability in pig production.Behavioral traits of pigs,unlike growth performance,have complex causation,development and expression which poses challenges for direct measurement.Therefore,identification of aggression biomarkers helps us to understand biological mechanisms of aggression and breed against harmful social behaviors in pigs.Our hypothesis was that blood-based 5-HT,DA,and TH levels of pigs have relationships with the aggressive behavior after mixing.Therefore,the objective of this study was to investigate the relationships between the parameters of aggressive behavior during different time phases after mixing and the 5-HT level in platelets,5-HT,DA,T3 and T4 levels in plasma of weaned pigs before and after mixing for screening blood-based biomarkers of aggressive behavior of pigs after mixing.1.Effects of aggression behavior of weaned pigs after mixing on the growth performance,skin temperature and lesionsA total of 500 weaned Suhuai pigs from 65 litters were selected in this study.Piglets were weaned at 35 d of age and moved to empty pens with their original pen-mates in a nursery room at two days before mixing.Behavior of weaned pigs was recorded for 36 h after mixing using a digital video recording system.Pig aggressive behavior was assessed by observing the video and recording the frequency and time of fighting behavior for each pigs in a pen in 36 h after mixing.In a fighting behavior,the initiator of the fight,the frequency and time of active attack,bullying and standoff behaviors were recorded.A composite aggressive score(CAS)was calculated as follows:(Frequency of attack+0.2*(Time of attack[s])The temperature of skin surface near ear root was used in this study as the indicator of body temperature:which was measured using an infrared thermometer at 24 h before mixing,2,24,48,and 72 h after mixing.Lesion scores for a pig were assessed on 24 h before mixing,2,24,48,and 72 h after mixing in different parts of body.The saliva samples were collected at 24 h before mixing,2,24,48,and 72 h after mixing.The result showed that the body temperature of pigs have increased after mixing(P<0.05),but there was no significance in body temperature between aggressive pigs and docile pigs(P>0.05).The salivary cortisol concentrations of pigs at the pen level were increased at 2,24,48 h after group mixing compared with that at 24 h before group mixing(P<0.05).The skin lesion score on the front body of aggressive pigs at 2 h after mixing was greater than that in docile pigs(P<0.05),while the skin lesion on the middle and hindquarter were not significantly different(P>0.05).Pigs showed more aggressive behavior during the day time(89.67%)than the night(10.33%)at the first day after mixing.The aggression had no effect on the daily gain in the three days after mixing(P>0.05).2.Correlation between the aggressive behavior parameters and the growth performance and physiological indicators of weaned pigs after mixingThe behavioral tests including backtest and tail test of 500 Suhuai pigs were measured individually before mixing.Age,body weight,parity,gender of pigs was recorded in the experiment.The skin lesions and temperature were measured at 2,24,48,and 72 h after mixing.Backtest and tail test were conducted individually.The individual factors include age,body weight,parity and gender.Skin temperature and the score of skin lesions were listed as physiological factors.Simple correlation and canonical correlation analysis were analyzed between three factors and the parameters of aggressive behavior.The result showed that the first and second correlation coefficients of aggression traits and individual factors were significant(P<0.01).Behavioral tests including backtest and tail test were not significant in the canonical correlation analysis(P>0.05).The analysis indicated that main traits affecting aggression in the canonical correlation were age,gender,score of skin lesions,and body temperature.Correlation analysis between the aggressive behavior and the score of skin lesions at different phases found that the aggressive behavior was related to the skin lesions at 2 h after mixing(P<0.01).Aggressive behavior and skin temperature at different phases were not significant in the canonical correlation analysis(P>0.05).3 Canonical correlation analysis between the aggressive behavior parameters and the blood-based biochemical indicators of weaned pigs after mixing.The most aggressive or docile pigs were selected by a composite aggressive score in the experiment 1.To ascertain the effects of mixing on physiological and biochemical indicators,about 5 mL of blood was sampled via julular venipuncture into tubes(containing 0.5 mL EDTA)from all the experimental pigs at 24 h before mixing and 24 h after mixing.Plasma was obtained by centrifulation at 3000 rpm for 15 min at 4℃.The concentrations of TRP,5-HT and DA were measured using a method of high performance liquid chromatography(HPLC).A commercial multispecies 125 I radioimmunoassay(RIA)Kit(Beijing North Institute of Biological Technology,China)was using to measure the concentrations of THs in the plasma.A significance difference was found in the concentration of T3 between the most aggressive and docile pigs(P<0.01).The frequency of fights was positively associated with plasma TRP concentrations(r=0.45,P=0.002)and platelet number(r=0.42,P=0.082)in canonical correlation analysis,but was negatively associated with the level of platelet 5-HT(r=-0.21,P=0.338).It is likely that selection on physiological and biochemical indicators targets a behavioral strategy,rather than a single physiological and biochemical indicators trait such as 5-HT.In summary,no differences were found in skin temperature between the most aggressive and docile pigs.The score of skin lesions was the greatest at 2 h in front and middle of the body after mixing.The diurnal rhythms of porcine aggressive behavior within 36 h after mixing suggested that the porcine aggression was mainly happened on day.The greater body weight of the pig,the stronger aggressive behavior.In addition,the most aggressive pigs had lower plasma T3 concentration than the most docile pigs.These results suggest that platelet 5-HT level maintain greater values due to lower values of aggression,but level of plasma TRP and platelet number induce higher values due to greater values of the aggression traits.They could be used as blood-based bio-markers to screen individual aggressive behavior of pigs after weaning.