Calcium and phosphorus requirement of swine from weaning to market weight: II. Development of response curves for bone criteria and comparison of bending and shear bone testing.

Three trials involving 251 crossbred pigs were conducted to establish response surfaces for effects of Ca/P levels (70, 85, 100, 115 and 130% of NRC [1979] estimated requirements) and time (weaning to market) on bone criteria and to compare bending and shear bone testing. Nine replicates, each consisting of five pigs, were used over the three trials. One pig per replicate was slaughtered every 4 wk following the start of the trials. Third metacarpals and metatarsals (3M) and fourth metacarpals and metatarsals (4M) were collected at slaughter; the bone length, bending or shear stress, and dry, fat-free ash percentage (DFF%) were determined. Most bone criteria increased (P less than .01) at a decreasing rate (P less than .05) in response to increasing dietary Ca/P intake and increased linearly (P less than .01) with time on test, although bone length and stress increased at a decreasing rate with time on test. Asymptotic models relating continuous effects of total Ca + P intake ratio (CAP; expressed as a percentage of NRC) and number of days on test before slaughter to bone measurements were fit to least squares slaughter time by diet means. Bone length and DFF% reached 98% maximum at or near 100% the NRC estimated CAP level; however, to reach 98% of maximum bone shear and breaking stress higher CAP levels were required. Shear testing of bones was more desirable than bend testing as a method of determining bone strength due to reduced variability, better discrimination of Ca/P and time effects and ease of calculation.