Phosphorus requirements of reproducing sows were estimated using 24-h urinary P excretion. Thirty-six multiparous sows were fed one of six maize–soybean meal-based diets with total P ranging from 0·40 to 0·80 % in 0·08 % increments with a constant Ca:total P ratio (1·25:1). Diets were fed from day 7·5 ± 1 after breeding until the end of lactation (day 26 ± 1). Urine samples were collected in mid and late gestation (days 77·1 ± 2 and 112·4 ± 1) and early and late lactation (days 4·5 ± 1 and 18·2 ± 1). Phosphorus requirements were estimated using linear and nonlinear regression models. Based on a single 24-h urinary P excretion, estimated daily dietary total P requirements in mid and late gestation were 10·3 g (6·0 g standardised total tract digestible P, STTD P), and estimates for early and late lactation were 31·1 g (16·6 g STTD P) and 40·3 g (22·1 g STTD P), respectively. Plasma P and Ca concentrations were maintained within normal ranges at the estimated levels of P requirements. No differences among treatments were observed for plasma parathyroid hormone (P ≥ 0·06) and bone formation marker (P ≥ 0·16). In lactation, bone resorption marker decreased (P ≤ 0·001) as sows consumed more P. Among the analysed variables, urinary P was the most sensitive response to changes in dietary P intake. Urinary P excretion offers a practical method to estimate P requirements in sows. Our recommended daily total P requirements are 10·3 g for gestation and 35·7 g for lactation.

Piglet survival is a major problem, especially during the first 3 days after birth. Piglets are born deficient of energy, but at the same time they have a very high energy requirement because of high physical activity, high need for thermoregulation (because of their lean body with low insulation) and high heat production in muscle tissues. To be able to survive, newborn piglets may rely upon three different sources of energy, namely, glycogen, colostrum and transient milk, which orchestrate to cover their energy requirements. Piglets are born with limited amounts of energy in glycogen depots in the liver and muscle tissues and these depots are sufficient for normal activity for ∼16 h. Intake and oxidation of fat and lactose from colostrum must supply sufficient amount of energy to cover at least another 18 h until transient milk becomes available in the sow udder ∼34 h after the first piglet is born. Selection for large litters during the last two decades has challenged piglets even further during the critical neonatal phase because the selection programs indirectly decreased birth weight of piglets and because increased litter size has increased the competition between littermates. Different attempts have been made to increase the short-term survival of piglets, that is, survival until day 3 of lactation, by focusing on improving transfer of vital maternal energy to the offspring, either in utero or via mammary secretions. Thus, the present review addresses how sow nutrition in late gestation may favor survival of newborn piglets by increasing glycogen depots, improving colostrum yield or colostrum composition, or by increasing production of transient milk.

The increasing demand for efficiency in pork production requires great specialization of all sectors involved in this activity. In this context, the development of strategies that could reduce undesirable traits related with negative effects on piglet survival and postnatal growth and development are essential for the pig industry. Currently, special attention is given to variation in birth weight, as some evidences suggest an increased within-litter birth weight variation in modern sows. This variation has been shown to be associated with preweaning mortality, variable weights at weaning and deteriorated growth performance, which results in economic losses and lower efficiency. Therefore, understanding the factors that can influence the events that occur during gestation and that have an impact on the fetal growth and development are important to achieve better efficiency and also to develop strategies that can be used to achieve increased within-litter uniformity of piglet birth weight. This study concludes that even at a given placental size, fetal growth may vary because of differences in placental vascularization and efficiency. Feeding extra feed or energy during late gestation only marginally improves birth weight, and positive effects are not consistent between different studies. The detrimental effects of protein restriction on fetal growth during early gestation may be due to altered placental and endometrial angiogenesis and growth, which leads to a reduction in placental-fetal blood flow, nutrient supply from mother to the fetuses and ultimately to fetal growth retardation. The number of studies that attempted to influence within-litter birth weight variation by means of sow nutrition during gestation is limited. Therefore, more research concerning sow nutrition during gestation associated with the provision of balanced diets to meet requirements of the sows and fetuses are still required. This knowledge may subsequently provide starting points for the design of nutritional strategies that can influence within-litter birth variation.