Pigs per sow per year is highly and positively correlated to both litters per sow per year ( r=0.69, P<0.001, (Duffy and Stein, 1988); r=0.65, P<0.01 , (Wilson et al., 1986) ) and pigs weaned per litter ( r=0.73, P<0.001, (Duffy and Stein, 1988); r=0.59, P<0.01 , (Wilson et al., 1986)). However, improvements in the biological performance may best be made through efforts to increase litters per sow per year rather than through pigs weaned per litter. Being highly related to litters per sow per year (r = -0 .79 , P < 0.01, (Wilson et al., 1986); r= -0 .91 , P < 0.001, (Duffy and Stein, 1988) ), non-productive days (NPD) are one of the best biological predictors of litters per sow per year. A non-productive day has been defined as 'any day a sow or gilt of breeding age is present in the breeding herd and is not either gestating or lactating' (Duffy and Stein, 1988 ). The three factors influencing litters per sow per year are gestation length, lactation length, and NPD. Since gestation and lactation lengths are typically set for each herd, the best means to improve litters per sow per year would appear to be through decreased NPD. While lactation length influences litters per sow per year in a fashion similar to NPD (Wilson et al., 1986), facility and management constraints often do not allow reductions in lactation length. Non-productive days affect both pigs per rate per year and pigs per sow per year through their influence on litters per sow per year. Thus, NPD are useful for improving both sow and facility utilization. When broken down into its components, NPD have the additional advantage that, unlike other measures of performance, they yield a considerable amount of diagnostic information useful for determining why breeding herd productivity deviates from target. A survey of 30 Ontario swine farms revealed a NPD of 50.8_+25.1 days (mean _+ standard deviation ) with a range of 14.1-116.3 days (Wilson et al., 1986). Duffy and Stein (1988) reported a mean NPD of 72.5 with an upper quartile value of 57.4 and a bot tom quartile value of 92.3 for 68 North Amer-