Requirements for digestible calcium by growing pigs

Two experiments were conducted to determine the digestible Ca requirements by pigs from 11 to 25 kg and from 50 to 85 kg as indicated by growth performance and bone ash. In Exp. 1, the hypothesis was that the requirement to maximize growth performance of 50- to 85-kg pigs expressed as the STTD Ca:STTD P ratio, is less than 1.35:1. Fifteen corn-soybean meal based diets were formulated to contain 0.14, 0.27, or 0.41% STTD P and 0.13, 0.25, 0.38, 0.50, or 0.63% STTD Ca. Ninety barrows (50.2 ± 2.1 kg) were individually housed and randomly allotted to the 15 diets. On d 30, the amount of feed left in the feeders and the weight of the pigs were recorded. Pigs were euthanized on d 31, the right femur was removed, and ash, Ca, and P were determined in dried defatted femurs. Data were analyzed using the response surface model in NLREG. Results indicated that there were interactions (P < 0.10) between Ca and P for final BW, ADG, G:F, and bone ash. The predicted maximum final BW, ADG and bone ash at 0.27% STTD P was obtained at STTD Ca:STTD P ratios of 1.20:1, 1.25:1, and 2.03:1, respectively, and the STTD Ca to STTD P ratio needed to assure adequate bone mineralization without affecting growth performance was about 1.23:1 if the concentration of P was at the requirement. In Exp. 2 the hypothesis was that the requirement for dietary Ca to maximize growth performance of 11- to 25-kg pigs expressed as the STTD Ca:STTD P ratio, is less than 1.40:1. A second objective was to test the hypothesis that increasing dietary Ca increases plasma Ca concentration, and downregulates expression of genes related to intestinal Ca absorption (i.e., TRPV6 and S100G) and tight junction proteins (i.e., OCLN and ZO1) in the duodenum. Twenty corn-soybean meal based diets were formulated to contain 0.16, 0.33, 0.42, or 0.50% STTD P and 0.14, 0.29, 0.44, 0.59, or 0.74% STTD Ca. A total of 640 pigs (11.1 ± 1.4 kg) were allotted to 20 diets in a randomized complete block design. On day 21, weights of pigs and feed left in feeders were recorded and blood, duodenal tissue, and the right femur were collected from one pig per pen. Gene expression was determined in duodenal tissue via quantitative RT-PCR. Data were analyzed using a response surface model in NLREG. Results indicated that there were interactions (P < 0.01) between Ca and P for ADG, ADFI, G:F, and bone ash. The predicted maximum ADG, G:F, and bone ash at 0.33% STTD P was obtained at STTD Ca:STTD P ratios of 1.39:1, 1.25:1 and 1.66:1, respectively. Plasma Ca concentration was positively affected by increasing dietary Ca (quadratic, P < 0.01) and negatively affected by increasing dietary P (linear, P < 0.01). There was a linear negative effect (P < 0.05) of dietary Ca on the expression of S100G, TRPV6, OCLN, and Z01. In conclusion, the STTD Ca:STTD P ratio needed to maximize growth performance of 11- to 25-kg pigs is less than 1.40:1, if P is at the requirement. Increasing dietary Ca decreases expression of genes related to transcellular Ca absorption but stimulates paracellular absorption of Ca by decreasing the expression of tight junction proteins in the duodenum