Energy partitioning and reproduction in primiparous sows : effects of dietary energy source

<strong>Introduction</strong>As a result of extensive changes in pig husbandry in the last 50 years, nutritional requirements of especially lactating sows nowadays differ strongly from those of sows in the past. Although diets are optimized to meet the requirements, sows still loose body reserves during lactation. Especially in primiparous sows, losses of body reserves during lactation are severe. These sows have not reached their mature body weight and therefore need nutrients for body development. However, due to a limited feed intake capacity, lactating primiparous sows mobilize body fat and protein to meet their requirements for maintenance and milk production.In general, these losses of body reserves result in impaired reproductive performance after weaning. The number of sows that remain anestrous after weaning is considerable and in sows that show estrus, weaning-to-estrus interval (WEI) is prolonged, ovulation rate seems decreased and embryonal mortality is increased.Therefore, nutrition is an important factor for optimal reproductive performance, especially in primiparous sows. An intermediate between nutrition and reproduction is the hormone insulin. Insulin is associated with the pulse frequency of luteinizing hormone (LH), WEI, ovulation rate, and farrowing rate. Feeding level, exogenous insulin injections, and diet composition can affect plasma insulin concentration. Effects of dietary energy source on plasma insulin concentration and its relationships with reproductive performance are hardly investigated.This study was conducted in primiparous sows to investigate: 1) effects of two specific dietary energy sources (fat: less insulin-stimulating vs starch: more insulin- stimulating) fed during different parts of the reproduction cycle, on reproductive traits, and 2) whether effects of these dietary energy sources on reproduction are dependent on the metabolic status during lactation (induced by feeding level).<strong>Dietary energy sources and plasma insulin concentration (Experiment 1)</strong>To investigate effects of dietary manipulated plasma insulin concentration on reproductive performance, firstly diets had to be composed that differ in insulin stimulation. Therefore, an experiment was conducted in cyclic gilts in which three diets, differing in major dietary energy sources, were investigated in their ability to stimulate plasma insulin secretion (Chapter 1). The studied major dietary energy sources were tallow, maize starch, and maize starch plus dextrose. The diet with tallow gave only a slight postprandial increase in plasma insulin level. The maize starch diet resulted in a higher postprandial insulin concentration, but two hours after feeding the plasma insulin concentrations were not different between the diet with tallow and with maize starch. The diet with maize starch plus dextrose resulted in the highest postprandial insulin levels and remained at a higher level during approximately 4.5 h after feeding. Preprandial insulin concentrations did not differ between diets.<strong>Design Experiment 2</strong>Because the diet with tallow (fat) and the diet with maize starch plus dextrose (starch) gave the largest contrast in plasma insulin concentration, these two diets were used in a large experiment with lactating primiparous sows. Sows received either the fat- or starch-rich diet from farrowing up to d 35 of the subsequent pregnancy. During lactation, sows were fed either a high (44 MJ NE/d; 1050 g CP/d) or a low (33 MJ NE/d; 790 g CP/d) feeding level. This was done to study, in addition to the main effects, the interactions between dietary energy source and feeding level on energy partitioning and reproduction. Within each feeding level, diets were fed to be isocaloric and isonitrogenous. After weaning, all sows remained at the same experimental diet, but all received the same amount of energy and protein (from weaning to estrus: 31 MJ NE/d; 740 g CP/d; from breeding to slaughter: 17.5 MJ NE/d; 420 g CP/d).During lactation, energy and nitrogen balance were determined and blood samples were collected to analyze insulin and LH profiles. After weaning (day 22 after farrowing), additional blood samples were taken to analyze periovulatory profiles of estradiol, LH, and progesterone. Sows were inseminated each day of standing estrus and slaughtered on d 35 of pregnancy. Results obtained from this experiment are described in Chapter 2 to 5.<strong>Dietary energy source and energy partitioning (Experiment 2)</strong>Effects of dietary energy source and feeding level on milk production, milk composition, piglet body composition, and energy partitioning of lactating primiparous sows are described in Chapter 2. At the low feeding level no differences between the two dietary energy sources were observed for milk composition, body composition of the piglets, or energy and nitrogen balance of the sow. At the high feeding level, however, the fat-rich diet resulted in milk with a higher fat concentration and in piglets with a higher body fat concentration compared with the starch-rich diet. This resulted in an interaction between dietary energy source and feeding level for energy balance of the sows. The energy balance of sows fed the low feeding level and of sows fed the fat-rich diet at the high feeding level were all similar, but the energy balance of sows fed the starch-rich diet at the high feeding level was less negative. These results suggest that extra fat in the diet enhances milk fat output, whereas extra starch is used for both milk production and prevention of severe body reserve losses of the sow.<strong>Dietary energy source, feeding level, and reproductive traits (Experiment 2)</strong>Whether the treatments affected plasma insulin profiles and reproductive traits during and after lactation is described in Chapter 3 and 4. Plasma insulin concentration was higher in sows fed the starch-rich diet than in sows fed the fat-rich diet during lactation. LH pulse frequency during lactation tended to be lower in sows fed the fat-rich diet compared to those fed the starch-rich diet. At the high feeding level, no effect of dietary energy source on WEI was found. However, at the low feeding level, the fat-diet resulted in a 22 h longer WEI compared with the starch-rich diet. Dietary energy source did not affect other reproductive traits.Feeding level did not affect plasma insulin profiles during lactation. Sows fed the high feeding level during lactation had a higher LH pulse frequency during and after lactation, a lower risk to remain anestrous after weaning, and a higher ovulation rate compared with sows fed the low feeding level. Periovulatory profiles of estradiol, LH, and progesterone hardly differed between treatments. No differences between treatments were observed for the number of embryos, uterine-, and placental development.Plasma insulin concentrations during lactation were never related with the determined reproductive traits. Furthermore, no relationships were found between energy- or nitrogen balance and reproductive characteristics.Based on the results obtained from experiment 2, it can be concluded that dietary energy source during lactation affects plasma insulin concentration but has minor effects on reproductive performance of primiparous sows. A reduction of 25% in lactational feed intake seems to have more impact on reproductive traits.Overall analyses, regardless of treatments, showed that the relationship between the number of LH pulses on the day of weaning and the WEI was best explained by a linear-plateau model. An increase from two to seven LH pulses per 12 h resulted in a linear decrease in WEI, whereas more than 7 pulses did not shorten the WEI further. Furthermore, it was demonstrated that the mean plasma progesterone concentration in early pregnancy was positively related with the percentage of embryo survival, especially in progressing pregnancy (from 170 h after the LH surge onwards).<strong>Dietary energy source and reproductive performance (Experiment 3).</strong>In the experiment described above (Chapter 2 to 4), sows were fed either the fat- or starch-rich diet from parturition until d 35 of subsequent pregnancy It is therefore, impossible to distinguish effects of dietary energy source fed during the lactation period, the WEI, or the early pregnancy, on reproduction. Effects found during pregnancy may be a result of changes during WEI or even lactation. Therefore, another experiment was conducted in which primiparous sows were fed either the fat- or starch-rich diet during the weaning-to-ovulation interval or during early pregnancy (until d 35). The results obtained from this experiment are described in Chapter 6. Sows fed the fat-rich diet before ovulation had a higher risk to maintain anestrous after weaning than sows fed the starch-rich diet. No effect of dietary energy source fed either before ovulation or during early pregnancy was found on uterine, placental, and embryonic traits.<strong>IGF-1 and reproductive performance (Experiment 2 and 3)</strong>Because plasma insulin concentration did not show relationships with reproductive traits (Chapter 3 and 4), additional analyses were performed to determine plasma IGF-1 (insulin-like growth factor-I) concentration during and after lactation, and its relationships with reproductive characteristics. In literature, IGF-1, together with insulin, is often posited as an intermediate between nutrition and reproduction in the pig. Results of these analyses are described in Chapter 5. Plasma IGF-1 concentration was higher in sows fed the high feeding level during lactation and also in sows fed the starch-rich diet. Furthermore, IGF-1 concentrations on d 21 of lactation and on d 22 (weaning) were positively related with the LH pulse frequency on d 22 and the height of the preovulatory LH surge. Finally, IGF-1 concentration in sows with a low body weight at farrowing and severe lactational body weight loss was decreased compared to heavier sows at farrowing or sows with less lactational body weight loss.<strong>Conclusions and implications</strong>The major question to be answered in the present study was whether dietary energy source affects plasma insulin concentrations and reproductive performance of primiparous sows. Based on the results obtained from the three experiments, it can be concluded that fat-rich diets fed during lactation depress the secretion of insulin compared to starch-rich diets. The LH secretion by the pituitary during lactation tended to be decreased in sows fed the fat-rich diet, possibly explaining the finding that the return to estrus was delayed in these sows. However, this lower LH pulsatility and delayed return to estrus were not related to plasma insulin concentration. Dietary energy source does not seem to affect reproductive traits on the uterus level.The second question to be answered was whether effects of dietary energy source on reproductive performance are dependent on the metabolic status of sows during lactation. In the present study, no interaction between dietary energy source and metabolic status on reproductive traits was found, even though the energy balance of the lactating sows was affected by dietary energy source.Based on these results, the use of fat-rich diets for primiparous sows, and probably also for multiparous sows, needs to be critically evaluated. Based on the metabolic and reproductive point of view, starch-rich diets seem to be more beneficial for sows than fat-rich diets. Effects of fat-rich diets on litter performance after weaning need to be investigated further.