All phases of the reproductive cycle are related, and therefore the feeding program in one phase may have significant effects on performance in another phase. Therefore, there is some risk in discussing nutrition of the gestating sow and the lactating sow separately. Also, the effects of any feeding strategy adopted will also be influenced by factors such as genotype, parity, season, health status and length of lactation. Therefore, feeding programs have to be farm specific, indeed sow specific, and thus will vary from farm-to-farm. There is however, general agreement that targeting a moderate sow weight gain during gestation and a minimum weight loss in lactation is advisable. However, there is still a distinct lack of consensus as to the best feeding strategy of gestating and lactating sows. Modern sows are younger and leaner at time of mating, have poorer appetites, are more fertile, and produce more milk than sows of even 5 to 10 years ago. It is not surprising, therefore, that it is a challenge to develop feeding programs that support these highly productive modern sows. For excellent reviews of the topic of sow nutrition, see: (Aherne 1997; Aherne and Williams, 1992; Close and Mullan, 1996; Einarsson and Rojkittikun, 1993; Whittemore 1996).

Feeding the Sow During Gestation

Feed intake during gestation and body condition (fatness) at the time of farrowing are two of the more important factors influencing feed intake during lactation. There is some evidence that in certain genotypes, backfat levels of greater than 15 mm P2 will result in some reduction in feed intake. However, the effect of backfat on feed intake is most severe when backfat exceeds 25 mm P2. The effect of sow fatness on feed intake is greatest in the first two weeks of lactation and in multiparous sows. The reduced feed intake in lactation may also be associated with the effects of gestation feed intake on insulin secretion or insulin resistance in sows after farrowing.

Because of the negative relationship between feed intake during gestation and in the subsequent lactation it is important not to overfeed sows during gestation. Therefore, we have to base feeding levels on something other than guess work .

The energy and protein requirement of a sow gestation depends on her size (maintenance), how much weight she puts on during gestation and the requirements of the developing litter. During pregnancy, maintenance represents about 80% of the sow's total energy requirements and, therefore, some estimate of the sow weight is helpful. About 15% of the sows energy requirement during gestation goes to maternal weight gain and the remaining 5% of the energy goes to litter growth. The energy requirement for maintenance can be calculated as:

13 + 0.2 Mid-gestation wt (kg) x 0.11 Mcal DE/kg.

Therefore, a 150 kg sow targeted to gain 30 kg weight during gestation would have a mid-gestation weight of 165 kg (150 + 15). Her maintenance energy requirement would be:

13 + 0.2 (165) x .11 Mcal DE/kg = 5.06 Mcal DE/day.

If the diet contains 3 Mcal DE/kg, the energy requirement to meet the maintenance needs of the sow would be 1.69 kg/day (5.06 ) 3). More simply, estimated daily feed requirements for maintenance is 1% of mid-gestation sow weight i.e. 1% of 165 kg or 1.65 kg feed/day.

Maternal weight gain represents approximately 15% of the energy requirements of the gestating sow. Depending on parity and backfat levels at the time of breeding, target maternal weight gains during gestation of 10 to 40 kg may be appropriate. Each 5 kg maternal gain should increase sow backfat at day 100 of gestation by 0.75 mm P2. Therefore, a maternal gain of 30 kg will increase backfat by 4.5 mm P2. A backfat level of 18 mm P2 at time of farrowing may be appropriate for all parity sows. Measuring backfat at time of breeding will indicate how much backfat the sow must gain to achieve the target level of 18 mm at time of farrowing. Each kg of weight gain will require about 5 Mcal DE/kg. Therefore, a maternal gain of 30 kg would require a total energy intake of 150 Mcal DE or 1.5 Mcal DE/day. The products of conception require only 0.2 Mcal DE/day. Using these estimates of the energy required for maintenance, maternal gain and litter development a rough guide to the feeding of a gilt or sow during gestation can be developed (Table 1.)

Most producers do not weigh sows or measure backfat but rather use condition scoring as their guide to an appropriate feeding program. However, there is a great deal of variation among people in their estimates of body condition. Also, people are not consistent over time and a condition score of 3 or 3.5 on a gilt vs a sow may not reflect the same level of backfat. There is evidence also, that there is wide differences in the backfat levels of different genotypes with the same score. In practice, use of condition scoring to set feeding levels is resulting in a wide variation in backfat levels at time of farrowing. A preferred program is to weigh sows and measure backfat (P2) at the time of breeding. Backfat can be checked at monthly intervals throughout gestation to insure reaching a target backfat of 18 mm P2 at parturition. Backfat is taken at a point at the last rib about 65 mm from the mid-back - this is called a P2 measurement. Feed allowances to each sow can be adjusted at the time the backfat is being measured. There are now available robust machines that are relatively inexpensive and can measure backfat thickness reasonably accurately and quickly (1 min or less/sow).

Protein requirements of the gestating sow are based on 1 g per kg sow weight and 2.0 g per kg total weight gain, (i.e. targeted maternal weight gain plus 20 kg for the products of conception (litter and placenta). Thus, for a 150 kg sow with a target weight gain of 30 kg, the estimated daily protein requirement would be:

150 x 1 + 50 x 2.0 = 250 g protein/day.

The lysine requirements can be calculated by allowing 2, 1.75 1.5 and 1.3 g lysine per Mcal DE intake for sows with weight gains during gestation of 25 to 30 kg, 20 to 25 kg, 15 to 20 kg and 10 to 15 kg respectively. Thus a sow eating 2 kg of feed a day would have a daily energy intake of 6 Mcal DE and should be fed 12 g lysine/day (2 g lysine Mcal DE).

This would require that the diet contain 0.6% lysine. However, if this sow had a target weight gain of 20 kg, her daily feed intake might be 1.9 kg/day or 5.8 Mcal DE/day. Her lysine requirement would be based on 1.75 g lys/Mcal DE for a daily lysine requirement of 10.2 g lysine. However, if she is fed a diet containing 0.6% lysine and receives 1.9 kg feed/day, her actual lysine intake would be 11.4 g lysine / day. We cannot meet the lysine requirement of every parity using one diet, so we have to compromise. My suggestion is to use a diet containing 13.5% protein and 0.55% lysine and follow the feed intake suggestions shown in Table 1.

Because gestating sows are usually fed once daily, it is not recommended that synthetic lysine be used to meet the dietary lysine levels. It should be noted that the feeding levels suggested in Table 1 assume that sows are individually fed and that the environmental temperature is kept at 18 to 20^oC. For each ^o C below the temperature range a daily increase of 60 g feed is required.

Pattern of Feeding During Gestation

Under nutrition in early gestation has to be very severe to reduce embryo survival. In contrast, high levels (>2.5 kg feed/day) of feed intake in early gestation will reduce embryo survival of gilts by approximately 5%. However, recent evidence suggests that this increase in embryo survival is not reflected in a significant decrease in number of pigs born per litter. For sows,in poor condition at weaning or for sows that will farrow in the hot months of the year, high levels (3 to 3.5 kg feed/day) for the first three weeks of gestation may improve subsequent reproductive performance.

In recent studies it was shown that doubling feed intake (2.5 vs 5.0 kg/ day) from day 25 to 80 of gestation, the period of muscle hyperplasia in the fetus, increased the number of secondary muscle fibres in the progeny. This increase in muscle fibre numbers resulted in a 10% improvement in growth rate and a 9% reduction in feed required per unit gain in pigs from day 70 to day 130 of the growing-finished period.

As mentioned previously, the nutrient requirements of sows increases with an advance in pregnancy, following the pattern of fetal development and increase in sow weight. Fetal weight doubles over the last month of pregnancy, with fetal growth being especially rapid in the last 10 days of gestation. Therefore, it is highly recommended that sow feed intake be increased by 1.5 to 2 kg per day from day 100 of gestation until farrowing. This increased feed intake will reduce backfat loss, increase nitrogen retention by sow and litter and does not necessarily increase the incidence of mastitis, metritis, agalactia or reduce sow feed intake during lactation. In a recent experiment we fed sows 2.3 or 3.9 kg feed per day during the last two weeks of gestation and ad libitum during a 25 day lactation. There was no significant difference in lactation feed intake, piglet growth rate, weaning-to-service interval or subsequent litter size.

Because the energy, protein and lysine requirement of gestating sows increase with an increase in sow weight and fetal growth it would probably be cheaper and better to change the composition of the sows diet for each trimester of gestation. The calculated daily protein requirement of a 120 kg sow targeted to gain 30 kg maternal body weight during gestation will vary from 210 g/day in the first one third of gestation to nearly 300 g/day in the last third of gestation. On a standard feeding level of 2 kg/day, the diet fed should vary from 10.5% protein to 15% protein to meet these requirements. In future, in large units in which sows are stalled by week of breeding and using blend feeding, it may be economical and more appropriate to phase feed sows throughout gestation according to their protein needs.

References

Aherne, F.X. 1997. Nutrition of the early weaned sow. Proc. 18th Western Nutr. Conf., pp.43-61 Winnipeg Man.

Aherne, F.X. and I.H. Williams. 1992 Nutrition for optimising breeding herd performance. Vet Clinics of N. America: Food Anim. Prac. 8(3):589-608.

Close, W.H. and B.P. Mullan. 1996. Nutrition and feeding of breeding stock. In: Pig Production. Ed. M.R. Taverner and A.C. Dunkin. Elservier, N.Y. pp. 169-202.

Einarasson, S. and T. Rojkittikhun, 1993. Effects of nutrition on pregnant and lactating sows. J. Reprod. Fert. Suppl. 48:229-239.

NRC (1988). Nutrient requirements of swine. Washington, DC. National Academy Press.

Whittemore, C.T. 1996. Nutrition-reproduction interactions in primiparous sows. A review. Livest. Prod. Sci. 46:65-83

* Diet to contain 3 Mcal DE/kg, 13.5% protein and 0.55% lysine.