Understanding the early growth and development of gilt progeny to improve their lifetime performance and survival

In commercial pork production, progeny born to primiparous sows (gilt progeny; GP) are slower growing and of a lower health status than those piglets born to older sows in subsequent parities (sow progeny; SP). Hence, they represent an area of significant loss of productivity. The major aims of the current thesis were to characterise performance differences between GP and SP in commercial production systems, to identify some of the underlying anatomical and physiological factors responsible for these differences, and target some management strategies to address these deficiencies through late gestation and (or) lactation feeding strategies. Therefore, a series of experiments were carried out to test the general hypotheses that GP would be lighter and grow slower than SP at all stages of production and have reduced reproductive performance in the breeding herd. It was expected that GP would have access to colostrum and milk with a lower energetic value due to lower fat, protein and lactose contents, and a lower immunoglobulin G (IgG) concentration. It was therefore anticipated that GP would be compromised in terms of the pre- and postnatal development and function of several organs and tissues (such as skeletal muscle and the gastrointestinal tract) in lactation, have reduced reproductive performance and acquire less maternal immunity, leading to a lower chance of lifetime survival in comparison to SP. It was further hypothesised that feeding conjugated linoleic acid (CLA) and (or) medium-chain fatty acids (MCFA) in late gestation would improve piglet energy levels, improve colostrum and milk composition and therefore improve the growth performance and pre-weaning survival of GP, more so than in SP. Experiment 1 (Chapter 3) involved the retrospective analysis of 3 years of data from first cross (F1; PrimegroTM Genetics) gilts selected for breeding at a commercial facility (Rivalea Australia; Corowa, New South Wales). Results showed that GP are 1 day older at first mating compared to SP, and that 4% more GP fail to reach first oestrus and be mated by 270 days of age. However, GP that were mated at least once had a 4% higher farrowing rate from this mating than the corresponding SP, with comparable reproductive performance and longevity in the first three parities. Gilt progeny selected for the breeding herd were lighter than SP at birth, weaning, 2 weeks after weaning, selection and at first mating. Experiments 2 and 3 (Chapters 4 and 5) further quantified differences in performance between GP and SP in two large commercial production systems using the most common genetic lines in Australia (PrimegroTM Genetics and PIC Australia). In both studies, GP, relative to SP, were born (60 to 90 g), weaned (730 to 920 g) and sold (4.2 to 5.2 kg) lighter and had a 2% higher mortality rate in the immediate post-weaning period (from weaning to 10 weeks of age). These data clearly showed that a large proportion of the difference in performance between GP and SP in the weaner and grower-finisher stages was a consequence of GP being weaned lighter. These results highlighted the critical importance of the pre-weaning period for determining the growth performance of GP up until sale, as well as the importance of the management of primiparous sows in late gestation and lactation. In these experiments, segregation of GP and SP, both from birth to slaughter (Experiment 2) and in the grower-finisher period only (Experiment 3), were unsuccessful as management strategies for improving the growth performance of GP (and SP). Experiments 4 and 5 (Chapters 6 and 7) were conducted to further understand some of the anatomical and physiological factors that may affect GP performance in early life, in order to identify targeted management strategies (e.g., nutritional) most likely to improve the growth, health status and survival rates of GP. Literature on this topic suggests that the composition of colostrum and milk from primiparous sows may differ to that from multiparous sows across different stages of lactation in terms of concentrations of different nutrients, immunoglobulins and overall energetic value, all of which are important for the piglet’s early physical and immunological development. Furthermore, the immunological naivety of primiparous sows in their first lactation compromises the transfer of maternal immunity (i.e. of immunoglobulins such as IgG) to colostrum and milk and subsequently to their piglets. Experiment 4 investigated differences in the concentrations of macronutrients, energy, and IgG of colostrum and milk between primiparous and multiparous sows throughout lactation (from birth to day 21). This experiment showed that the composition and overall net energy (NE) content of colostrum and milk of primiparous and multiparous sows was largely similar, with lower lactose concentrations in primiparous sow milk in comparison to multiparous sows in late lactation only (days 14 to 21). Experiment 5 was conducted to examine differences in absolute and relative organ and tissue development in early life around birth and weaning between GP and SP, as well as the transfer of IgG to the piglet’s circulation, and aspects of the development and function of the gastrointestinal tract (GIT). Various tissues of GP (such as the brain, spleen, liver, quadriceps muscle and small intestine) grew at a different rate to those of SP, particularly around birth, and similar to that of intra-uterine growth restricted (IUGR) piglets. These results indicate that the postnatal performance of GP may be restricted by their delayed anatomical and functional development in late gestation, having impacts on transfer of maternal immunity, and early GIT and musculoskeletal development. Therefore, strategies to improve birth weight and growth rates in GP may be limited by these factors. Experiment 6 (Chapter 8) was conducted under commercial research conditions and examined feeding CLA and (or) MCFA to primiparous and multiparous sows in late gestation and lactation. It was hypothesised that feeding these ingredients would improve energy levels in neonatal GP and increase IgG concentrations in colostrum, enabling them to obtain higher quantities of this colostrum and subsequently improve their immunity and pre-weaning survival. These nutritional strategies were mostly unsuccessful in improving piglet growth performance, colostrum and (or) milk composition, or piglet energetic metabolite concentrations. However, feeding CLA improved overall liveborn pre-weaning survival (although increased stillbirth rate), and piglets on primiparous and multiparous sows fed CLA had (numerically) the highest serum IgG concentrations at day 3 post-partum. These findings indicated that attempts to increase GP energy levels, growth performance and IgG absorption through feeding CLA and (or) MCFA were largely not effective. Therefore, nutritional or other management strategies to improve GP performance may be more successful if targeted elsewhere, but feeding CLA may be effective at different inclusion rates, which requires further investigation. In conclusion, the research conducted in this thesis has shown that GP represent a substantial loss of production in commercial pig production systems, and focusing on improving their performance in the pre-weaning period is crucial to improving their lifetime productivity. Thus, management strategies to improve lifetime performance of GP should target foetal growth in late gestation to improve their physiological development, particularly of the GIT and skeletal muscle. Management strategies should target growth of GP in the pre-weaning period to increase weaning weights, as this determines their lifetime performance. Efforts in this area may be more beneficial if focused on increasing colostrum and milk production in primiparous sows, or improving the efficiency of IgG absorption in GP, rather than improving colostrum or milk composition in these animals.