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A 21-d experiment was conducted to test the hypothesis that the Ca requirement to maximize growth performance expressed as the standardized total tract digestible (STTD) Ca to STTD P ratio is less than 1.40:1. The second hypothesis was that increasing dietary Ca increases plasma Ca and downregulates abundance of genes related to Ca absorption (TRPV6 and S100G) and tight junction proteins (OCLN, CLDN1, and ZO1). Twenty corn-soybean meal diets were formulated using a 4 5 factorial design (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), and 640 pigs (11.1 ± 1.4 kg) were allotted to the 20 diets in a randomized complete block design. On d 21, weights of pigs and feed left in feeders were recorded and blood, duodenal tissue, ileal mucosa, and the right femur were collected from 1 pig per pen. Abundance of mRNA was determined in duodenum and ileum via quantitative RT-PCR. Data were analyzed using a response surface model by removing the terms in the model that were not significant. Interactions (P < 0.01) between dietary Ca and P for average daily gain (ADG), gain to feed ratio (G:F), and bone ash were observed (Table 1). The predicted maximum ADG (614 g), G:F (0.65), and bone ash (11.68 g) 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. There was a linear negative effect (P < 0.05) of Ca on the abundance of S100G, TRPV6, OCLN, and Z01 in the duodenum and on CLDN and ZO1 in the ileum. 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 reduces transcellular but increases paracellular absorption of Ca. |
