At Iowa State University, ongoing research has focused on selective breeding pigs with low residual feed intake. After 10 generations, it has become clear that the benefits extend those of feed efficiency – these pigs also deal better with stress.
Feed is the largest cost in pork production; therefore, improving feed efficiency can increase producer profitability. Furthermore, improved feed efficiency can support industry competitiveness, decrease the demand on global feed resources, and complement environmental sustainability.
Genetically, selective breeding for residual feed intake (RFI) shows promise in meeting these increased demands. However, it is important to balance the benefits of feed efficiency selection with the pig’s ability to cope with stress and its welfare. Therefore, this article will discuss physiological and behavioural stress research on swine selected on the basis of RFI.
Residual feed intake is one method of measuring feed efficiency. Residual feed intake is defined as the difference between a pig’s observed and expected feed intake. Expected feed intake is determined for each pig, based on its growth rate and backfat thickness. Pigs that consume less feed than expected have a lower RFI, are more feed efficient, and they are therefore economically better for lean production compared to pigs with high RFI.
Factors that contribute to variation in RFI are the same as those that affect feed efficiency:
• Energy used during activity;
• Efficiency of digestion;
• Metabolic balance;
• Thermoregulation; and
Using RFI, two pig lines have been developed at Iowa State University; an efficient line that has been selected for low RFI over ten generations and an inefficient line that was randomly selected for the first five generations and then selected for high RFI for another five generations. Now in their tenth generation, pigs from the low RFI line require 12-15% less feed to reach market weight than pigs from the high RFI line,also see Figure 1.
In 2013, Judson K. Grubbs and colleagues investigated the protein profile of mitochondria from liver, semitendinosus (red portion), semitendinosus (white portion) and longissimus dorsi to determine differences between pigs from the high RFI and low RFI lines. More feed efficient (low RFI) pigs indicated an increase in anti-oxidant defenses and potential modifications of metabolic pathways leading to oxidative stress, metabolism, and cellular repair. Results indicated that pigs divergently selected for low RFI may be less prone to muscular oxidative stress, and the liver may have a greater metabolic capacity when compared to their less feed efficient (high RFI) contemporaries.
In 2015, Jessica Colpoys investigated how selecting for RFI may impact the pig’s ability to cope with stress.
Naturally, following a stressful event, pigs secrete adrenocorticotropic hormone (ACTH) from the anterior pituitary gland, which then stimulates the secretion of cortisol from the adrenal cortex. Therefore, this hormone can be exogenously administered to induce a stress response, and the pig’s responsiveness can be measured through cortisol. Cortisol is important for reacting to a stressor, as it prepares the body for a fight or flight response and therefore influences animal welfare.
Colpoys also evaluated the physiological stress response of gilts from the two RFI lines by administering ACTH and measuring the stress response through cortisol concentrations. The results showed that low RFI (more efficient) gilts tended to have lower baseline cortisol and had lower cortisol concentrations in response to the hormone stimulation compared to high RFI gilts.
In 2015, Jenelle R. Dunkelberger and colleagues, assessed the effect of infection with Porcine Reproductive and Respiratory Syndrome virus (PRRSv) on pigs from the two RFI lines. In total, 100 weaner piglets from each line were infected and followed for 42 days. There was a tendency for pigs from the low RFI line to have lower levels of viraemia in blood, to have a faster antibody response, and the growth rate of the low RFI pigs was less affected by infection than pigs from the high RFI line. Growth of the low RFI line was less affected by the PRRSv-challenge than growth of the high RFI line. Additionally, the increase in antibody levels was greater for the low RFI line than for the high RFI line.
The lipopolysaccharide challenge can also be used to determine whether selection for improved feed efficiency impacts sickness behaviour expression. A study by Samaneh Azarpajouh and colleagues in 2015, using gilts from the RFI lines did not show differences in sickness behaviours in response to a lipopolysaccharide challenge, see Figure 2.
In 2012, Anoosh Rakhshandeh and colleagues evaluated the impact of the lipopolysaccharide challenge and divergent selection for RFI on apparent ileal digestibility and apparent faecal digestibility of nutrients and intestinal nutrient active transport and barrier function. The immune system stimulation decreased apparent ileal digestibility of nutrients. The reduction in apparent faecal digestibility of crude protein as a result of immune system stimulation was greater in the low RFI line compared to the high RFI line. Genetic selection for low RFI increased the apparent faecal digestibility but had no effect on apparent ileal digestibility of nutrients. It also reduced the total tract digestive capacity of growing pigs during immune system stimulation. The immune system stimulation affected both apparent ileal digestibility and apparent faecal digestibility of dietary crude protein.
In 2011, Larry J. Sadler and colleagues evaluated the extent to which RFI selection impacted behaviour and lesion presence using the RFI selection lines. On the day of placement there were no postural, behaviour, or general activity differences between the two lines, but over subsequent rounds, behavioural differences were observed, with low RFI gilts becoming less active. Gilts from the low RFI line also had lower lesion scores on the day after placement. However, over subsequent rounds there were no differences in lesion scores between the 2 lines.
Novel object- and human-approach tests are commonly used to test a pig’s behavioural response to fear eliciting stimuli as pigs are innately neophobic. Research conducted by Jessica Colpoys and colleagues in 2014 compared behavioural reactions of barrows from the two RFI selection lines. Barrows were evaluated using a human-approach and novel object approach test. During both the human approach test and the novel object approach test, low RFI barrows expressed fewer fear behaviours than high RFI barrows.
In summary this work is very encouraging as it relates to physiological, immunological and behavioural responses in pigs that have been selected for improved feed efficiency. Further work is needed to evaluate pigs from the more feed efficient line in commercial situations to further ascertain their robustness.