No abstract

The immune system is a life history trait that can be expected to trade off against other life history traits. Whether or not a trait is considered to be a life history trait has consequences for the expectation on how it responds to natural selection and evolution; in addition, it may have consequences for the outcome of artificial selection when it is included in the breeding objective. The immune system involved in pathogen resistance comprises multiple mechanisms that define a host's defensive capacity. Immune resistance involves employing mechanisms that either prevent pathogens from invading or eliminate the pathogens when they do invade. On the other hand, tolerance involves limiting the damage that is caused by the infection. Both tolerance and resistance traits require (re)allocation of resources and carry physiological costs. Examples of trade-offs between immune function and growth, reproduction and stress response are provided in this review, in addition to consequences of selection for increased production on immune function and vice versa. Reaction norms are used to deal with questions of immune resistance vs. tolerance to pathogens that relate host health to infection intensity. In essence, selection for immune tolerance in livestock is a particular case of selection for animal robustness. Since breeding goals that include robustness traits are required in the implementation of more sustainable agricultural production systems, it is of interest to investigate whether immune tolerance is a robustness trait that is positively correlated with overall animal robustness. Considerably more research is needed to estimate the shapes of the cost functions of different immune strategies, and investigate trade-offs and cross-over benefits of selection for disease resistance and/or disease tolerance in livestock production.

The increasing size of the human population is projected to result in an increase in meat consumption. However, at the same time, the dominant position of meat as the center of meals is on the decline. Modern objections to the consumption of meat include public concerns with animal welfare in livestock production systems. Animal breeding practices have become part of the debate since it became recognized that animals in a population that have been selected for high production efficiency are more at risk for behavioral, physiological and immunological problems. As a solution, animal breeding practices need to include selection for robustness traits, which can be implemented through the use of reaction norms analysis, or though the direct inclusion of robustness traits in the breeding objective and in the selection index. This review gives an overview of genotype × environment interactions (the influence of the environment, reaction norms, phenotypic plasticity, canalization, and genetic homeostasis), reaction norms analysis in livestock production, options for selection for increased levels of production and against environmental sensitivity, and direct inclusion of robustness traits in the selection index. Ethical considerations of breeding for improved animal welfare are discussed. The discussion on animal breeding practices has been initiated and is very alive today. This positive trend is part of the sustainable food production movement that aims at feeding 9.15 billion people not just in the near future but also beyond.

Because feed is the major cost to pork production, management practices and breeding strategies are aimed at optimizing feed intake. Knowledge about the shape of feed intake and feeding behavior curves may be of interest for optimization of lean meat production. This study investigated trends based on daily measurements of feeding behavior in 200 Duroc barrows, originating from 5 sires and 200 dams, during growth. Daily values were examined between 88 and 188 d of age. Furthermore, phenotypic correlations between feeding length and feeding rate, and feeding frequency, feed intake, residual feed intake, growth rate, and rate of fat deposition were investigated for a period between 95 and 175 d of age. No differences were observed between sires for parameter estimates of a curvilinear function fitted to data on feeding length as a function of age, but the effect of sire was significant (P < 0.01) for values at individual ages up to 132 d of age. Feeding rate (feed ingested for each minute spent eating) increased in a linear fashion with age (average R(2) = 0.80) and differently so for different sires (P < 0.05 for the intercept and P < 0.01 for the regression coefficient). Because the increase in BW is linear over this time period (average R(2) = 0.99), the results suggest that feeding rate increased with increased BW and is related to the physical capacity for feed intake. Results indicate that pigs that ate faster also ate more (r = 0.29, P < 0.001), grew faster (r = 0.40, P < 0.001), and grew fatter (r = 0.28, P < 0.001), but had no greater or lower residual feed intake (r = -0.01). The linear regression slope of feeding rate on age seemed inherent to the individual and was correlated with feed intake but not with residual feed intake. Feeding length may be selected for in order to regulate absolute feed intake at different stages of growth.

Coping styles in response to stressors have been described both in humans and in other animal species. Because coping styles are directly related to individual fitness they are part of the life history strategy. Behavioral styles trade off with other life-history traits through the acquisition and allocation of resources. Domestication and subsequent artificial selection for production traits specifically focused on selection of individuals with energy sparing mechanisms for non-production traits. Domestication resulted in animals with low levels of aggression and activity, and a low hypothalamic–pituitary–adrenal (HPA) axis reactivity. In the present work, we propose that, vice versa, selection for improved production efficiency may to some extent continue to favor docile domesticated phenotypes. It is hypothesized that both domestication and selection for improved production efficiency may result in the selection of reactive style animals. Both domesticated and reactive style animals are characterized by low levels of aggression and activity, and increased serotonin neurotransmitter levels. However, whereas domestication quite consistently results in a decrease in the functional state of the HPA axis, the reactive coping style is often found to be dominated by a high HPA response. This may suggest that fearfulness and coping behavior are two independent underlying dimensions to the coping response. Although it is generally proposed that animal welfare improves with selection for calmer animals that are less fearful and reactive to novelty, animals bred to be less sensitive with fewer desires may be undesirable from an ethical point of view.