Can you maximize pigs/sow/year, feed:gain, lean growth rate, and carcass merit premiums? New management challenges are being offered to the US swine breeding herd with the introduction of increasingly leaner genotypes. The circumstances contributing to this change are improved sow productivity levels due to more intensive management combined with marketing swine on carcass value pricing systems. Simultaneous advances have been made in pigs/sow/year, predominantly due to management, and lean (growth rate and percentage) predominantly due to genetics. Today we are managing a more prolific mature sow that may be both leaner and larger in mature body size; in addition, gilts now grow faster, reach puberty at a heavier weight and are mated both younger and leaner. Young breeding females of lean genotypes must maintain fatness throughout breeding life. This can be accomplished by minimizing lactation fat losses and encouraging gestation fat recovery. Several studies have clearly demonstrated that a P2 backfat depths of less than .5 in. (12 mm) are likely to be associated with reproductive inefficiencies. However, there is also a likely negative effect in the sow achieving more than 1 - 1.2 in. (25 - 30 mm) of P2 fat. At lactation milk yield can reach 26 lb. per day. For the gestating sow all of this implies that there is a substantially increased maintenance requirement and during lactation the pressure is on management actions to encourage feed intake. The appropriate management program for the breeding female as defined by Whittemore (1993) is one that will:

• maximize number of pigs per litter
• optimize pig birth weight
• maximize litters per year
• maximize lactation yield
• optimize longevity and lifetime productivity.

Appropriate age and weight at first mating is very dependent on genotype. In many cases a mating weight of approximately 275 lb. live weight and an age of 210 days appears most appropriate for optimum subsequent fertility and longevity. It is also important at first mating that adequate fat stores are available for good lactation and a short weaning to estrus interval. This adequate body composition may be represented by a P2 fat measure in excess of .7 in. (18 mm). However, age, P2 fat and live weight are not themselves the targets for the right time for first mating but are indicators of age pattern of puberty, the weight pattern of fatty tissue growth, and the relationship between ultimate mature size and the proportion of mature size that is required before reproduction should be initiated.

Achieving the desired combination of age, weight and P2 fat depth in lean genotypes may necessitate a gilt development program that is very different from conventional finishing. Replacement gilts raised as finishing pigs will lead to increases in mature body weight and maintenance requirement of the breeding herd. Breeding females should most likely be reared on a conservative diet of adequate protein and relatively low energy. Some restriction of feeding may also be required to achieve the target fat, weight and age endpoint. There is new evidence (Foxcroft, et al.; 1996) to support the concept of short-term "flushing" before first mating. Flush feeding by allowing gilts to eat to appetite is simply a corrective process to ensure the potential ovulation rate of the female. Restricted feeding of gilts nearing puberty will inhibit the secretion of luteinizing hormone (LH) which regulates ovulation. When the animals are put back on ad libitum feeding LH releases immediately begin again.

The flush feeding should end immediately after mating in the gilt. It has been demonstrated (Foxcroft et al., 1996; Ashworth, 1991) that high feed intakes immediately after mating in the gilt result in a significant decrease in embryonic survival. Therefore, flush feeding should only continue until mating and then feed intake of gilts and sows should be restricted to normal gestation levels of 1.5 times maintenance. For group housed females this poses an additional management problem because unmated gilts should be fed ad libitum and gilts that are mated should be restricted in their intake. Forming new groups is also undesirable during the first 3 weeks of pregnancy as fighting during the establishment of the social hierarchy may lead to embryo losses and the subsequent reduction in litter size.

For sows post weaning the objective is for them to conceive in 3 to 5 days with as large a litter as possible. There is no advantage to "drying off" sows by restricting feed in this 3 to 4 day period. Instead restricting feed is more likely to delay heat. Sows should be feed to appetite postweaning (6 to 8 lb.) and for the first 3 weeks of pregnancy fed to achieve large litter size. Both high and low feeding levels for the first 3 weeks may compromise the number of fetuses. Feeding levels for this period will range from 4 to 6 lb. daily.

Management during gestation should provide for a planned increase of 80-100 lb. for parity 1, 80-90 lb. for parity 2-5, and 55 lb. for sows greater than fifth parity (Johnston, 1996) P2 fat depth should also increase by about .2 in. (5 mm). These targets will vary according to sow maturity, body weight at conception and body condition. Overfeeding in gestation has a well proven negative impact on feed intake in lactation and results in fat and lean tissue loss to provide for milk production.

Measurement of P2 fat depth, condition scoring and weighing sows are all effective ways to feed the gestating sow to target weight and body condition at farrowing. Backfat measurement although relatively simple is not always convenient. In this case, sow condition scores are very good indicators of body fatness. Backfat or condition score combined with body weight gives an even greater level of accuracy in assessing the sows nutritional needs. Feeding to condition demands that when a sow is seen to be in lower condition than she need be, the feed allowance should be increased; if the sow is in higher condition feed allowance should be reduced. The condition score demands that the animal be fed what it requires, not a previously stipulated amount. Condition scores and their related backfat depths may also vary by the genotype that they are applied to. Johnston (1996) describes a 5 point condition score and Whittemore (1993) provides a ten point system. Table 1 presents the recommended fat depths for condition scores in the 5 point system. Whittemore (1993) states that in general the P2 fat depth is 3 times the numerical condition score in the 10 point system.

A six step program for feeding gestating sows according to body condition (Whittemore, 1993) is as follows: