NPD = 365 – [(litters/female/year) x (gestation days + lactation days)]
For convenience, NPD can be subdivided into several component intervals which are influenced by different management factors. By addressing each component of NPD separately, producers will be able to more effectively implement needed management changes for specific problem areas. The intervals which have the greatest potential to alter NPD and realistic targets for these intervals are listed below:
Industry Avg. Target Interval (days) (days)
Wean to first service 8 <7 Wean to cull 16 <15 Service to reservice 36 <30
Overall NPD 54 <45
Several other intervals can be calculated and monitored if herd reproductive performance is sufficient to justify changes that result in small gains. The intervals listed above represent the greatest opportunity to improve reproductive efficiency and should, therefore, receive greater emphasis in herds experiencing reproductive problems. Results from the Swine Graphics database has identified characteristics of herds which fall in the top and bottom 10% of all herds based on profitability (cost/weaned pig). Top herds average about 36 NPD (10%) while the herds at the bottom average about 72 NPD (20%). Figures for pigs/sow/year and farrowing rate in top herds were 23 and 87%, respectively. Herds in the bottom 10% averaged less than 17 pigs/sow/year and a farrowing rate of about 70%.
Weaning to service interval is primarily influenced by parity, lactation length, nutrition and environment. Lower parity, especially primiparous, females have longer weaning to estrus intervals than later parity females. When combined with the greater pigs per litter achieved with parity 3-5 females, this is incentive to minimize forced culling of parity 1-2 females. Lactation length is inversely related to weaning to service interval. A general rule is that for each 10-day decrease in lactation length, weaning to service interval will increase by 1 day. However, improvements in ration formulation, feed intake, environmental conditions and sow condition at farrowing can at least partially offset the negative effect of early weaning. Proper nutrition is crucial for timely return to estrus following lactation. Females experiencing severe negative energy balance will have prolonged intervals from weaning to service regardless of other factors. Environment is a component that includes many factors. Two environmental factors of primary concern are season and temperature. Summer infertility is compounded by heat stress but is also a function of seasonal reproductive activity inherent to the pig. Use of proper cooling techniques during the summer is effective, but a decrease in reproductive efficiency (including extended weaning to service interval) should be anticipated during the summer
months. Heritability of weaning to conception interval is low (h2=.24) which means gains through genetic selection will be slow and heterosis will be highly variable.
The weaning to cull interval is a function of herd culling pressure and the willingness of managers to implement and maintain strict culling practices. This interval is best minimized by setting the target cull date and enforcing it rigorously. However, strict culling practices can only be maintained if forced culling throughout the herd is minimized. Commercial swine farms typically cull at rate of 25-40% annually. Farms on which forced culling predominates will have difficulty controlling NPD until the causes for excessive forced culling are corrected.
Farrowing rate and pregnancy detection intensity are the predominate factors influencing the interval between services. Farrowing rates below 85% suggest a problem with the breeding program or maintenance of pregnancy. Differentiating between these problem areas can be simplified with application of sound pregnancy detection programs. Estrus detection using a boar is the most accurate (>95%) method and currently offers the earliest opportunity to define pregnant and nonpregnant females post-service. Amplitude (A-mode) or doppler ultrasound devices are not reliable for detecting pregnancy until after the fourth week of gestation and have an accuracy of around 90%. When ultrasound is used, examinations should be performed just prior to the second post-service estrus (29-35 days post-service). A second examination prior to the third post-service estrus (56-63 days) is recommended to avoid retaining sows which may have aborted or were misdiagnosed during previous exams. Again, forced culling pressure and age of the herd are factors in this decision process. Young herds with a high proportion of parity 1-2
females (many expensive F1 replacements) may need to allow 2 services before culling. Mature herds with low forced culling rates should cull after any repeat breeding.
For producers who choose to enter gilts into the breeding pool prior to first service, the entry to service interval is also an important component of NPD. The target for this interval is less than 21 days. Management procedures available to minimize this interval include boar exposure, relocation, hormone therapy and age at entry. These factors may affect age at onset of puberty. The optimum time to initiate these procedures is around 150-165 da ys of age depending on the particular genetic line. It is important to remember that techniques used to hasten or synchronize onset of puberty will only be profitable if breeding management and estrus detection are adequate.