Introduction

Years of selection for rapid growth rate and increased lean has resulted in modern pig genotypes that have larger mature body weight and reduced voluntary feed intake at any given weight. If modern genotypes are mated at the same weight or age as they were some years ago they will begin their reproductive life at a lower proportion of their mature body size and they will be physiologically younger. These animals will be still actively growing during pregnancy so there will be competition for nutrients between lean growth and the developing fetus.

A further consequence of leaner, heavier sows is an increase in energy requirements for maintenance. Although modern genotypes have reduced appetites, maximum milk yield has increased by up to 30% and the energy content of sows milk has increased by 14%. The length of lactation has been reduced to 21 to 26 days or less which means that there is very little opportunity for the gilt or sow to achieve a positive energy balance before weaning.

So, modern sows are larger, more productive and have smaller appetites than sows of 10 to 15 years ago and they are being bred at a younger age. The consequence of all this is that many sows do not consume sufficient energy or nutrients to meet their nutrient demands and therefore they break down their own body tissues to meet their requirements.

Many studies have shown that sows, especially primiparous sows, losing excessive amounts of live weight or body condition (both protein and fat) will have extended remating intervals, a lower percentage in estrus within 10 days of weaning, reduced pregnancy rate and reduced embryo survival. Feed intake in lactation does not appear to reduce ovulation rate. It is obvious that sow feed intake during lactation can very significantly affect subsequent reproductive efficiency.

For a thorough review of this subject see the reviews by Aherne, 1997; Aherne and Williams, 1992; Close and Mullan, 1996; Einarsson and Rojkittikhun, 1993, Whittemore, 1996.

Energy, protein and lysine requirements of lactating sows.

Energy requirements
The energy and lysine requirements of a lactating sow depend on the weight of the sow, her milk yield and its composition, and the change in body weight and body composition. Estimates of the energy cost of milk production are 2 Mcal DE/kg milk. Over 80% of the energy requirement of a lactating sow is for milk production. It requires 4 kg milk to produce 1 kg litter gain.

Therefore, we can estimate milk production and the energy required by the sow from an estimate of litter weight gain. NRC (1988) suggested that sows of 145 to 185 kg live weight at farrowing, housed at 18 to 20 C and with daily feed intakes of 4.4 to 6.1 kg of a corn-soybean diet containing 3.34 Mcal DE per kg, 13% protein and 0.6% lysine would produce 5 to 7 kg milk per day and lose approximately 6.5 kg live weight in a 28 day lactation.

However, recent estimates of milk production have shown that modern sows produce 10 to 12 kg milk/day and therefore, would require much higher lysine and energy intakes than those suggested by NRC (1988). An energy balance for a 150 kg sow producing 9.4 kg milk per day is shown in Table 1. As can be seen, this sow would require an average of 7 kg feed per day of a diet containing 3.34 Mcal DE/kg.

The sow used in this example is suckling 10 piglets and the average piglet weight at 7, 14, 21, 28 days of age is 2.5, 4.0, 6.0, and 8.0 kg. To produce this weight gain will require 6.4, 8.8, 11.2, and 11.2 kg milk per day in weeks 1,2,3 and 4 of lactation, respectively. The feed intake required for sow maintenance and milk production and the expected daily feed intake are also presented in Table 1. The feed intake shown for this sow throughout a four week lactation is typical of that reported in many surveys. As can be seen in Table 1, approximately 75% of the energy required goes to milk production and 25% for maintenance. This sow does not consume enough food to meet her needs. Therefore some of the energy and nutrients required by the sow must be derived from mobilization of body tissues during lactation. In this example the sow would lose nearly 22 kg body weight during a four week lactation.

In a recent experiment, it was shown that increasing the energy density of the diet from 2.98 to 3.80 Mcal DE per kg diet did not increase piglet growth. However, average daily feed intake of the sows was not influenced by energy content of the diet. Therefore DE intake increased with an increase in energy density of the diets up to 3.34 Mcal DE/kg. This energy level would be typical of a corn-soybean diet without added fat. Weaning-to-service interval and number of pigs born alive in a subsequent litter were not affected by energy density of the diet.

Protein and lysine requirements
A protein and lysine balance for the 150 kg sow used in the energy balance is shown in Table 2. For a 150 kg sow weaning 10 pigs of 8.0 kg, average daily protein and lysine intakes of 1080 and 63 grams / day are required. At the expected feed intakes shown the diet should contain 19.67% protein and 1.05% lysine. These protein and lysine levels are considerably higher than the values recommended by NRC (1988).

For a 150 kg sow lysine requirement for maintenance would be about 2 g/d and would be more than adequately met by the lysine supplied by lean tissue loss. By far the largest requirement for amino acids is for milk production. A requirement of 26 g/d of lysine per kg of litter gain has been suggested, which is in close agreement with our estimate of lysine requirement. There is a strong interaction between the energy and lysine level in the diet. As energy intake increases the response to increased levels of lysine also increases. Thus, milk yield is dependent on both lysine and energy intake.

The concept of an ideal protein or ideal amino acid balance is widely used. It is assumed that the dietary amino acids should be supplied in the diet in the same proportions as they are in sows milk. For example, methionine + cystine would be supplied at 60% of the lysine level, threonine at 72% and tryptophan at 18%. However, in one experiment feeding an ideal protein based diet versus a more conventional diet to lactating sows did not influence sow performance and increased sow weight loss.

In a recent experiment gilts fed 1.3% dietary lysine weaned heavier pigs and had 1.2 more pigs in their subsequent litter than gilts fed lower levels of lysine. Piglet growth rate was also highest for sows fed 1.31% lysine. However, this data clearly demonstrates the danger of using percentages without relating them to total feed intake. Using the recommendation of 26 g lysine per kilogram litter gain suggested previously the lysine requirement of the sows fed 1.06% lysine could be calculated to be 50 g per day (9.4 pigs, 206 g piglet growth per day at 26 g lysine per kg piglet gain). But they were fed only 44 g lysine per day. Clearly lysine requirements are animal specific and should be determined on the basis of expected litter gain. Because the appetite of first litter sows is usually low it has been suggested that it may be advisable to formulate two lactation diets, one containing 1.2% lysine for parity one sows and one with lower (0.90% lysine) for later parity sows.

Pattern of feed intake in lactation

The feed intake levels shown in Table 1 are for sows fed ad libitum on the day after farrowing. Adoption of a commonly practised feeding program that gradually increases feed intake over at least the first 3 to 5 days of lactation could significantly increase the nutrient deficit in week one of lactation.

There is evidence that pattern of feed intake during lactation can significantly affect post-weaning reproductive function. In a recent experiment restricting gilts to 40% of the energy intake of ad libitum fed gilts (L) for any week of a 3 week lactation delayed post-weaning return to estrus compared to ad libitum fed sows (H) (Table 3).

We have also shown that a 50% restriction of feed intake in the first two weeks of lactation completely prevented the development of follicles. But ad libitum fed gilts had substantial follicle development two weeks after farrowing. We have also confirmed that restricting feed intake at any stage of lactation will increase a weaning-to-service interval and decrease ovulation rate compared to ad libitum fed sows (Table 4).

Embryo survival was reduced in sows restrict-fed during the last week of lactation. Note how increasing feed intake of the restricted sows during the last week of lactation increased embryo survival.

Feed intake of lactating sows

As mentioned previously there is considerable evidence that the voluntary feed intake of modern day lactating sows is low, especially for the first and second parity sows, and often does not provide sufficient energy or other nutrients to meet the sows requirements. Recent survey data suggests that nearly 60% of sows show a dip of 1 to 2 kg in feed intake for two or more days at the end of the second week of lactation. However, we have not observed such dips in feed intake. The authors of the survey suggest that to avoid a dip in feed intake in lactation, feed intake in early lactation should be restricted and increased gradually over the first week of lactation. However, many herds can feed sows ad libitum immediately after farrowing with no detrimental effects on the sow or litter. For such herds feed intake during lactation is likely to be 10 to 15% larger than that of herds using a restricted - feeding system. Some of the factors that influence the voluntary feed intake of sows during lactation are:

• Genotype, parity, litter size, length and stage of lactation, body composition (prior nutrition)
• Barn temperature, humidity, floor type, water flow rate, access to drinker, feeder design
• Digestibility, palatability of diet, level of feeding, frequency of feeding, nutrient balance in the diet, physical form of diet (particle size, mash, pellet).

Of these factors, genotype, body composition at farrowing and farrowing barn temperature are probably the most important factors influencing feed intake. To maximize daily feed intake in lactation target 20 mm backfat at farrowing with a barn temperature of 18 degrees C. Addition of fat (5 to 10%) to the diet will increase energy intake only slightly. The main effect of fat supplementation of lactation diets is to increase the fat content and gross energy of the sow's milk. Therefore, the effect of fat supplementation on sow weight loss will be small. Fat supplementation of the sow's diet will be most effective during hot weather and can maintain energy intake under high temperature conditions.

References

Aherne, F.X., 1997. Nutrition of the early weaned sow. Proc. 18th Western Nutr. Conf., pp43-61. Winnipeg Man
Aherne, F.X. and I.H. Williams, 1992. Nutrition of optimising breeding herd performance. Vet. Clinics of N. America: Food-Anim. Proc. 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. Elsevier, NY. pp 169-202.
Einarsson, 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, D.C. National Academy Press.
Whittemore, C.T., 1996. Nutrition-reproduction interactions in pumparious sows. A review. Livest. Prod. Sci. 46:65-83.