Phytase Supplementation Impacts Dairy Cow Nutrient Utilization

The sample size calculation was based on a two-sided test with a confidence level of 95% and a power of 0.80 to detect a statistically significant difference in ATTD of P in the phytase supplemented treatments compared to CON. The expected effect size was based on published data concerning the variance in P digestibility in lactating dairy cows (Valk et al., 2002; Wu et al., 2003 (link); Kincaid et al., 2005 (link); Knowlton et al. 2007 (link)). The chemical composition of the total rations was calculated based on the chemical composition and intakes of both forages and concentrates. The fecal excretion of DM was calculated for each cow from the daily TiO₂ administration (g/animal) divided by the TiO₂ concentration (g/kg DM) in feces. For this, a fecal recovery of TiO₂ of 100% (Glindemann et al., 2009 (link)) was assumed. The fecal excretion of CP, starch, NDF, P, Ca, and PP was calculated as DM fecal excretion multiplied by the concentration of the respective chemical component in the feces. The ATTD of DM, CP, starch, NDF, P, Ca and PP was computed as ATTD (%) = [(intake – feces excretion)/intake]. For this calculation, the intake and feces excretion of each chemical component was in kilogram per day on a DM basis. The yield of FPCM (kg/d) was calculated on a 4% fat and 3.3% protein basis. The feed efficiency was calculated as FPCM divided by DMI both expressed in kg. Data on somatic cell count (SCC) were log transformed to obtain a normal distribution before statistical analysis. All data were averaged per cow and week for the statistical analysis.
All statistical analyses were performed using Genstat 18th edition (VSN International, Hemel Hempstead, UK). Data were analyzed by ANOVA to identify treatment effects. Treatment means comparisons were carried out using the Tukey test. Data are presented as least squares means and associated pooled SEM values. The statistical analyses for all variables (except P in milk and blood, and ATTD) were carried out using the data of the preperiod as a covariate, using the following model:
where Y_ijk is the response variable, μ is the overall mean, Block_i is the effect of block (i = 1−10), Cov_j is the covariate (response during preperiod), T_rtk is the effect of dietary treatment (k = 1−3), and ε_ijk is the residual error.
For the statistical analysis of P content in milk and blood, and ATTD, the same model was used but without the preperiod as a covariate. In addition, the effect of phytase dose level on nutrient intake ATTD and fecal excretion of nutrients during the fecal collection period was analyzed by polynomial contrasts to determine the linear and quadratic response to increasing phytase dose, with consideration of uneven distribution between phytase dose levels. Statistical significance was declared at P < 0.05. 0.05 ≤ P < 0.1 was considered a tendency.