Unveiling causes for growth retardation in piglets

The evolution of hyper‐prolific sow breeds has led to a higher number of piglets born per sow per year. This increase in litter size has enlarged the number of light weight (or growth retarded) piglets, increased pre‐weaning mortality and heterogeneity at the end of the nursery phase (ten weeks of age). These poorly performing piglets represent a challenge to the swine industry as their presence in the herd has economic and welfare implications. Reducing the heterogeneity at the end of the nursery phase is relevant, as it influences the efficiency of use of the grower and finisher facilities, and/or it reduces penalties for delivering underweight piglets to the slaughterhouse. The focus of this thesis was the end of the nursery phase, as this is the time point where piglets are transferred to the grower and finisher facilities. The aim of this thesis was to identify and describe the causes of growth retardation in the nursery phase to provide a basis to look for alternative nutrition or management solutions. The database analysis described in <strong>Chapter 2 </strong>provides a phenotypic definition of growth retardation based on the risk factor analysis approach, and describes season of birth, body weight at birth, at weaning and at six weeks of age as the main factors to predict piglet BW at the end of the nursery phase. Based on the algorithm developed to predict piglets’ BW at the end of the nursery phase, our target population was defined as piglets with a birth weight above the mean ‐2 times the SD from the total population and a predicted BW at the end of the nursery phase below the mean ‐1 time the SD from the mean of the total population, considered Low Performing piglets <strong>LP</strong>). We aimed to characterize differences between LP piglets and their heavier counterpart (piglets with a predicted BW at the end of the nursery phase above the mean +1 time the SD from the mean of the total population <strong>HP</strong>). Compared to the HP, the LP piglets grew slower, ate less and were lighter but have an equal gain:feed ratio at ten weeks of age. The LP piglets tended to take more time to touch a novel object and spent more time eating. The LP and HP piglets have an equal macronutrient digestibility, with the exception of NSP. When fed a high fibre diet, both groups have a lower starch and fat apparent total tract digestibility. Yet, the LP piglets have a reduced fermentative capacity, which might place them in disadvantage. Also LP piglets are unable to engage into compensatory gain or compensatory feed intake, as efficiency of nutrient utilization and feed intake per kg BW0.75 was unaffected. In terms of skeletal muscle development, the LP piglets have a low muscularity (total fiber number and fiber cross sectional area), which might be of disadvantage for lean mass accretion in further life and for meat quality. The LP piglets exhibit insulin resistance and a lower pancreatic amylase activity, which might be related to the lower performance. Lastly, in the general discussion a comparison of the main findings with literature characterizing growth retarded piglets is presented. Also based on modelled data, the economic impact of growth retarded piglets is estimated from 10 weeks of age until slaughter weight (110 kg BW). The studies reported in this thesis describe a novel method for selecting growth retarded piglets in the nursery phase and provide insight into possible mechanism for growth retardation in the piglet phase.