Downer pigs are a problem for packers and producers. While actual levels of pigs who are unable to move when they arrive at U.S. packing plants are typically reported at 0.25 percent to 0.75 percent, levels as high as 10 percent have been seen for individual loads.
The incidence of downers has been on the rise — from 0.08 percent to 0.30 percent during the past 10 years. “The stress gene is not a prerequisite for the condition,” notes Harold
Gonyou, Prairie Swine Centere, Saskatchewan, Canada. For example, data show that 90 percent of dead pigs arriving at Canadian packing plants do not carry the stress gene.
High blood-lactate and ammonia levels, low blood pH, blotchy skin, open-mouth breathing, vocalizations, muscle tremors and a refusal to walk are typical symptoms. Those behavioral and physiological symptoms are characteristic of a hyperthermic animal under stress.
Rapid energy release from the muscle and liver causes lactate and ammonia to build up in the blood. This causes metabolic acidosis, which may trigger the animal’s refusal to move.
For research purposes, stressful handling was based on handling groups of animals, with the understanding that social stress and crowding would be typical experiences during the moving process. Electric prods were used as they are considered a stress source in aggressive handling.
Two studies, using a total of 336 pigs, were conducted to determine if altering the acid/base balance through diet manipulation (electrolytes) would affect the animal’s response to stressful handling.
Study 1 had a downer rate of 38 percent. Results show a drop in blood pH and an increase in lactate, ammonia, glucose and glycerol levels in the blood as well as an increase in rectal temperatures in downer pigs compared to non-downers. Those changes are indicative of rapid energy mobilization in downer pigs as a response to aggressive handling.
Group-run pigs had lower blood oxygen and carbon dioxide levels and higher blood glycerol post-handling than pigs handled individually. A 54 percent downer rate suggests that handling pigs in groups was more likely to induce downers.
Altering the diet did not affect the pigs’ response to aggressive handling. The high-electrolyte diet raised the pre-handling blood pH in the first study, but it did not prevent typical physiological responses to handling nor did it affect the downer incidence.
In the second study, downer rates of 2 percent, 15 percent and 34 percent occurred for pigs 1) handled gently, 2) aggressively but not prodded, 3) aggressively with electric prods, respectively.
Among the aggressively handled pigs, using an electric prod caused a high metabolic response to handling. Pigs handled aggressively but not prodded had higher blood lactate and glycerol levels post-handling than gently handled pigs. This suggests that aggressive handling may contribute to the downer response, but electric prods exacerbate this.
So what’s the bottom line? Gonyou summarizes it as follows:
Use of electric prods during aggressive handling contributes to the incidence of downer pigs.
Aggressive handling can result in the metabolic response associated with downer pigs.
Increasing the dietary electrolyte balance did not reduce pigs’ metabolic responses to aggressive handling nor the incidence of downers.