It’s not uncommon to hear producers or employees talk about an obstinate or “knuckle-headed” hog, but it’s not a trait you select for or against in your herd. 

The priority is primarily on how fast a pig grows, with temperament not being a serious consideration. But that thought process has some consequences and may deserve another look.

Not only has temperament been overlooked, producers may have unwittingly selected for aggression. For example, selecting for average daily gain could be selecting for aggressive animals that create the most opportunities at the feeder.

While such animals might thrive, it doesn’t necessarily mean the whole pen is better off. Mild-mannered pigs are likely to be intimidated by aggressive pigs and not get their needed time at the feeder, leading to poor daily gain.

But a group of docile hogs housed together can have positive gains. “We have been using group selection to incorporate behavior traits and our genotypes have increased pigs per sow per year and average daily gain over the last year,” says Fields Gunsett, Newsham Genetics’ vice president of genetics and product development.

While no one seems certain about how much aggressive pigs can cost you, the ways they can be detrimental are clear.  Aggressive or high-strung pigs can cause increases in such things as tail biting, fighting and reduced growth for some of the more docile pigs. “Aggressive pigs impact the environment for other pigs,” says Joe Cassady, assistant professor, North Carolina State University. “We don’t want to select for growth and performance at the expense of a pig’s pen mates by inadvertently selecting for aggression.”

Such pigs can increase the number of downer or fatigued pigs, or pigs that are dead on arrival at the packing plant. There’s also the potential for carcass impacts such as increased abrasions and poor meat quality by producing more pale soft exudative pork.

“Producers will get docked for having high-strung pigs if downers increase,” says Dave Murray, in the procurement division of Indiana Packers Corporation. “Some packers euthanize downers, some give only half value and others dock on a percent of the carcass value.”

Another potential cost is in health impacts. Having aggressive pigs can cause more open wounds, which can be a source of infection. This could increase your herd-health costs and eventually increase carcass trim.

Aggressive pigs can cost producers in less direct ways, like impacting employees’ time, attitude and safety. If pigs are pushing employees around or employees are afraid of your pigs, you risk employee turnover. They may get to the point where they don’t want to deal with the pigs or worse, become injured.

“Aggressive animals can have a big effect on workers,” says Ed Pajor, Purdue University animal behaviorist. “You need to be careful and aware that the employees don’t avoid working with the pigs because they’re aggressive. That can only compound the problem.”

High-strung animals tend to travel in large groups or packs making them harder to move, while docile pigs are more likely to move in smaller groups, says Murray.

There are generally three factors involved in swine aggression: genetics, pig’s life experience and management.

There’s little doubt that there is a genetic component to swine behavior. However, quantifying it can be tricky because there’s no definitive measurement.

Cassady looks at the competitive effect, which is a pig’s genetic ability to influence his pen-mates’ performance. Competitive effect is about 20 percent heritable, meaning that it’s affected 20 percent by genetics and 80 percent by environment.

Researchers are working on developing a BLUP index number for the competitive effect, that could be used in genetic selection indexes. Cassady says that while work is under way, behavioral traits aren’t likely to be added to selection indexes anytime soon.

There is some evidence that suggests high-lean-genetic lines tend to be more aggressive than other lines, says Pajor.  A pig’s life experience also plays a role in how aggressive the pig becomes. “There can be differences within genetic lines, not all animals are the same,” he adds. “If a pig wins fights within its litter early in life, it’s quite likely it will become aggressive.”

The next thing to consider is management. Ensuring that you and your workers interact with pigs can have a major effect on aggression levels.

“At the packing plant we can usually tell if the producer walked the pens with the pigs,” says Murray. “Pigs that are used to humans are just not as excitable, and they move at a more relaxed, pace.”

Murray says that walking the pens makes the pigs more docile and leads to fewer fatigued and downer pigs. Every downer can cost you $60 in today’s hog market, he says.

Of course, the proper space allotment for pigs also can reduce fighting. Each finishing pig should have 8 square feet of space to avoid crowding. Make sure there is enough feeder and waterer space as well. “We tend to want to feed pigs all together and provide very few holes at the feeder,” says Pajor. “In the nursery you should have as many feeder spaces as you have pigs in the pen. In the finisher that’s not likely, but the more spaces you can provide the better.”

Make sure your ventilation and climate-control systems are working, because if a pen is drafty, pigs will fight over the pen’s comfortable spots.

Make sure you walk the pens, provide ample space and access to food, water and comfortable areas. If there’s still a problem, it might be worth looking at your genetic line. 

For the Good of the Whole

The broiler industry went through a phase where it selected only for the fastest growing animals, and while that was successful, there was soon trouble with birds fighting. In fact, while the industry thought it was selecting for performance, it was inadvertently selecting for aggression.

Then Bill Muir, Purdue University animal scientist, developed a group selection system. Rather than selecting only birds that would individually perform the best, he selected pens of birds with the best overall performance. This eliminated the ultra-aggressive birds that performed well, but negatively
impacted their cage mates.

Aggression isn’t as big of a problem in the pork industry as it’s been in the broiler industry, but the concepts are the same.

Newsham Genetics has implemented group-selection concepts mainly for productivity gains, but has gotten the side benefit of more docile animals.

“Before we used group selection, we would see great trends in our nucleus herds, but customers weren’t seeing the same effects and in some cases saw negative effects,” says Fields Gunsett,  Newsham’s vice president of genetics and product development. “Our nucleus herds had more space and were given more human attention, but in commercial herds the pigs’ competitive effects overcame the productivity gains.”

Gunsett says Newsham has seen a significant increase in growth performance from the genetic trend of their hogs before they implemented group selection.

Not that group selection is perfect. “Group selection creates challenges, because it can reduce the selection intensity, which makes genetic progress slower,” says Joe Cassady, assistant professor North Carolina State University.

Still, group selection provides some interesting options.