For example, metabolizable (ME) energy needed for maintenance (basic body functions) is dependent on sow body weight, and can be calculated from the following equation: ME (kcal) = 106 * (body weight in kg)^0.75. Energy for uterine growth (fetuses, fluids and membranes and empty uterus) and maternal growth (mammary gland and sow body weight gain) can be estimated based on some assumptions on parity, litter size, weight at birth and sow body weight. Most of the energy required by sows is due to maintenance as shown in Table 1. However, during the later stage of gestation a large portion of the energy requirements will shift towards fetal and maternal development (Figure 2). Typically, requirement estimates are made for the overall gestation phase and attempts are made to provide proper levels of nutrition through changes in feed allowance. Estimates for the nutritional needs of sows are shown in Table 2. These estimates were based on the assumption that a sow should reach a certain body protein mass and that a slow development of this protein mass is desired to maximize protein utilization. Assuming a standard corn-soybean meal ration containing 1480 kcal of ME/lb. of feed and 0.60% total lysine one can calculate the amounts of feed needed to meet these requirement estimates. When considering energy, 4.4 to 4.6 lbs. of feed is required to meet the average energy requirement for gestating sows. To meet the energy requirement of a sow at mating, 3.9 lbs. of feed is needed, whereas 5.3 lbs. is required for a sow at the end of gestation. If we do the calculation based on lysine, the average lysine requirement during gestation can be met with 3.6 to 4.0 lbs. of feed. The requirement at mating can be met with just 1.6 lbs. of feed, however to meet the requirement at the end of gestation 5.9 lbs. of feed is needed. To meet the requirement for both nutrients, it appears that 4 to 6 lbs. of feed would cover the range. This would only hold true if sows are not excessively thin at mating and are managed properly, under thermoneutral (proper temperature for sows) conditions.

Function	Kcal ME/day	% of Total
Maintenance	5160	72
Uterine Growth	356	5
Maternal Growth	1690	23
Total	7206	100
Source: Noblet and Etienne, 1987

Figure 2. Development of fetuses, fluids, membranes, and the empty uterus during gestation (from: Walker and Young, 1992).

Table 2. Estimated nutrient requirements for gestating sows

			ME Requirement, kcal/d			Total Lysine Requirement, g/d
Parity	Weight at Mating, lb	Weight gain, lb	Day 0	Day 115	Overall	Day 0	Day 115	Overall
1	260	120	5260	7650	6455	5.8	16.0	10.9
2	300	120	5500	7890	6695	5.4	16.0	10.7
3	335	110	5740	7890	6815	4.4	15.3	9.9
4	360	100	5740	7890	6815	4.4	15.3	9.9
5	380	100	5740	7890	6815	4.4	15.3	9.9
Adapted from Everts, 1994

The amount of feed supplied during specific phases of gestation can positively or negatively affect sow performance. There are a number of specific phases to consider, each of which will bebriefly described below.

Weaning to breeding

The rate of ovulation, particularly in first litter sows and sows that are very thin, appears to be influenced by feed intake following weaning. King and Williams (1984) showed that first parity sows fed 9 lbs. per day between weaning and breeding had greater ovulation rates and litter sizes (10.0 vs. 8.8) than control sows fed 3.5 lbs. per day. Additionally, some studies suggest that feeding ad libitum after weaning shortens the rebreeding interval (Aherne and Kirkwood, 1985; King and Williams, 1984). However, results are not completely clear because other studies have shown no effect and the response to high levels of feeding may be related in part to body weight at weaning. Allowing sows to consume feed ad libitum between weaning and breeding may be a prudent practice, however, to help sows recover from weight loss during lactation and take advantage of a possible increase in ovulation rate and shorter weaning to breeding interval.

Early Gestation

High levels of feed intake during the first 3 weeks of gestation may have a negative impact on embryo survival, particularly in gilts. For example, Dyck et al. (1980) reported a reduction in embryo survival from 82.8% to 71.9% when feeding levels were increased from 3.3 lbs. to 6.6 lbs daily. The critical time period appears to be 24 to 48 hours after mating, when embryos migrate to the uterus. Limiting feed intake to 4 to 4.5 lbs. per day during the first 21 days post-breeding may be a safe practice.

Early to Mid Gestation

This period encompasses day 21 to day 75 of gestation. It is the most appropriate time period to get sows back into the correct body condition. A body condition score of 3 is desirable (Figure 3). Body condition scoring should done on a regular basis by monitoring and scoring individual sows according to the guidelines in Figure 3. Feeding level should then be adjusted for each sow to target a body condition score of 3. Everts (1994) suggested a moderate net body weight gain of sows of approximately 75 lbs. for first parity sows, 65 lbs. for 2^nd parity sows, 55 lbs. for 3^rd parity sows and 45 lbs. for older sows. Boyd and Touchette (1997) estimated the amounts of feed needed to replenish body reserves based on body condition scores (Table 3). The level of nutrition from day 21 to 50 may also be important to maximize the number of muscle fibers in piglets born. Primary muscle fibers set the stage for how fast and lean the baby pig can grow after birth and may therefore affect subsequent nursery and growing-finishing pig performance. These muscle fibers develop between day 21 and 50 of gestation and their number can be affected by nutrition. In studies where feed intake of sows was severely restricted during gestation, growth of piglets after birth was reduced (Pond et al. 1985, Pond and Mersmann, 1988). The extent to which this occurs under more practical circumstances is not clear at this point. Therefore, the current recommended feeding level during this phase of gestation is 4.0 to 4.5 lbs. Of feed for sows with a body condition of 3. Feed level should be increased by approximately 1.5 lbs (ie. 6 lbs. total) for sows with a body condition of 2.

Mid to Late Gestation

The development of the mammary gland reaches the critical stage between day 75 and 90. This is the period that secretory cell proliferation occurs, which will determine cell number and ultimately milk synthesis capacity. Excess energy intake (10.5 Mcal/d vs. 5.7 Mcal/d) reduced secretory cell numbers (Weldon et al., 1994) and could, therefore, reduce milk production. An energy intake of 10.5 Mcal/d would be equivalent to approximately 7 lbs of feed (corn-soy type diet; 1480 kcal/lb of feed). Therefore, high levels of feed should be avoided during this time-frame. Sows should be in the correct body condition at this time if they were properly fed in the early to mid gestation period.

Late Gestation

Fetal weight gain will increase substantially during days 90 to 115 of gestation (Figure 2). Consequently, nutrient requirements of the sows will increased (Table 2). According to this table, 5.3 to 5.9 lbs. of feed is required for sows at day 115 of gestation (depending if calculations are based on energy or lysine needs). However, care should be taken not to over feed sows during this period either, because it appears to decrease feed intake and reproductive performance during lactation (Dourmad, 1991; Weldon et al., 1994; Xue et al., 1997). Doumad (1991) calculated that every 1 lb increase in gestation feed intake was associated with a 0.64 lb decrease in lactation feed intake (Figure 4). The reduction in feed intake during lactation was more pronounced during the first week of lactation and at the highest level of gestation feed intake. An even greater feed intake in gestation of 7.5 to 8.2 lbs./day has been observed to reduce lactation feed intake by 0.95 to 1.30 lbs/day (Weldon et al., 1994; Xue et al., 1997). Therefore, gestation feed intake needs to be controlled (less than 6 lbs) during the late gestation period to maximize feed intake and milk production in lactation.

Summary

Proper nutritional management of gestating sows includes feeding the correct nutrient levels to meet the sow's requirement, but also correct management of feed amounts given to the sows at critical times in their development. Recommended feeding levels for sows during the different stages of gestation are summarized in Table 4.

Literature Cited

Aherne, F. X., and R. N. Kirkwood. 1985. Nutrition and sow prolificacy. J. Reprod. Fertil. Suppl. 33:169.

Baker, D. H., D. E. Becker, H. W. Norton, C. E. Sasse, A. H. Jensen, and B. G. Harmon. 1969. Reproductive performance and progeny development in swine as influenced by feed intake during pregnancy. J. Nutr. 97: 489.

Boyd, R. D., and K. J. Touchette. 1997. Current concepts in feeding prolific sows. Proc. 13^th Annual Carolina Swine Nutr. Conf., Nov. 11, Raleigh, NC.

Dourmad, J. Y. 1991. Effect of feeding level in the gilt during pregnancy on voluntary feed intake during lactation and changes in body composition during gestation and lactation. Livest. Prod. Sci. 27: 309.

Dyck, G. W., W. M. Palmer, and S. Simarks. 1980. Progesterone and luteinizing hormone concentration in serum of pregnant gilts on different levels of feed consumption. Can. J. Anim. Sci. 63:579.

Everts, H. 1994. Nitrogen and energy metabolism of sows during several reproductive cycles in relation to nitrogen intake. Ph.D. dissertation, Wageningen Agricultural University, The Netherlands.

King, R. H., and I. H. Williams. 1984. The effect of nutrition on the reproductive performance of first-litter sows. 1. Feeding level during lactation and between weaning and mating. Anim. Prod. 38:241.

Miller, H. M. 1996. Nutrition of the periparturient sow. Ph.D. dissertation. University of Alberta, Canada.

Noblet, J. and M. Etienne. 1987. Metabolic utilization of energy and maintenance requirements in pregnant sows. Livestock Prod. Sci. 16:243.

Patience, J. F., and P. A. Thacker. 1989. Swine nutrition guide. Prairie Swine Centre, University of Saskatchewan, Saskatoon, pp. 149-171.

Pond, W. G., and H. J. Mermann. 1988. Comparative responses of lean or genetically obese swine and their progeny to severe feed restriction during gestation. J. Nutr. 118:1223.

Pond, W. G., H. J. Mersmann, and J. T. Yen. 1985. Severe feed restriction of pregnant swine and rats: Effect on postweaning growth and body composition of progeny. J. Nutr. 115:179.

Walker, B., and B. A. Young. 1992. Modelling the development of uterine components and sow body composition in response to nutrient intake during pregnancy. Livestock Prod. Sci. 30:251.

Weldon, W. C., A. J. Lewis, G. F. Louis, J. L. Kovar, M. A. Giesemann, and P. S. Miller, 1994. Postpartum hypophagia in primiparous sows: I. Effects of gestation feeding level on feed intake, feeding behavior, and plasma metabolite concentrations during lactation. J. Anim. Sci. 72:387.

Xue, J., Y. Koketsu, G. D. Dial, J. Pettigrew, and A. Sower. 1997. Glucose tolerance, luteinizing hormone release, and reproductive performance of first litter sows fed two levels of energy during gestation. J. Anim. Sci. 75: 1845.

Reviewed by:
Allen Harper, Ph.D., Tidewater Agricultural Research and Extension Center, Virginia Polytechnic Institute and State University.
Kevin Rozeboom, Ph.D., Department of Animal Science, North Carolina State University

Since June 1, 2000