Physiological characteristics of slow growing pigs from birth to market

Abstract Text: In all-in/all-out production systems, pigs that cannot reach desired market weight according to pig flow schedules are sold at much lower value than their contemporaries. The objective of this study was to identify characteristics of these pigs in order to understand physiological causes of slow growth. Growth performance of 440 pigs was monitored from birth to marketing. Pigs were weighed individually at birth, weaning (4 wk), nursery exit (9 wk), and at marketing (22 to 25 wk). To evaluate the competitive ability of pigs at the feeder, feed consumption rate was measured on 96 focal pigs at 8 wk of age, consisting of the 2 heaviest and 2 lightest pigs from each of 24 randomly selected pens. Blood samples were collected from 48 focal pigs in 12 pens at nursery exit and marketing to analyze for essential amino acids, growth hormone, IGF-I and leptin concentrations. In addition, loin muscle area (LMA) and backfat thickness (BF) were measured for all pigs using a real-time ultrasound imaging scanner at 22 wk. Pigs were categorized as slow, average, and fast growers based on market weight adjusted to 170 d of age (slow growers < 105 kg, average growers between 105 and 125 kg, and fast growers > 125 kg). The Mixed Procedure of SAS was used to compare differences among the pig categories. Compared with fast growers, slow growers had lighter weight at birth (1.13 vs. 1.64 kg, SE = 0.18; P < 0.01), weaning (5.1 vs. 8.1 kg, SE = 0.51; P < 0.01) and nursery exit (18.6 vs. 28.0 kg, SE = 1.97; P < 0.01), ate slower (14.2 vs. 18.1 g/min, SE = 1.79; P = 0.01), had less BF (1.8 vs. 2.3 cm, SE = 0.05; P < 0.01) and smaller LMA (32.0 vs. 41.8 cm², SE = 1.58; P < 0.01). The concentration of IGF-I at nursery exit was positively correlated with final BW (R² = 0.36, P < 0.01) and ADG during the grower-finisher period (R² = 0.19, P < 0.01), indicating low IGF-I concentrations may be associated with slow growth. Concentrations of essential amino acids, growth hormone and leptin did not differ among pig categories. These results suggest that slow growing pigs can be identified before nursery exit by light weights, slow eating rates, and low concentrations of IGF-I.

Keywords:  pigs, hormones, slow growth