The hypothesis of this dissertation was that deriving statistical models using advanced statistical techniques and a database collected in recent years would yield predictions that better fit the current dairy production system than the models of the NRC (2001). In chapter three, we determined the effects of dry matter intake (DMI), body weight (BW), and diet characteristics on total tract digestibilities of dry matter, neutral detergent fiber, and starch (DMD, NDFD, and StarchD, respectively) in high-producing dairy cows using a database composed of 1,942 observations from 662 cows in 54 studies from Michigan, Ohio, and Georgia. Our results suggest that DMD decreases as intake increases but at a lower rate than the model of NRC (2001). Both NDFD and StarchD were best estimated with diet characteristics as well as intake; starch content has a much greater impact on NDFD than does intake. In chapter four, we modeled DMI in Holstein dairy cows based on milk energy (MilkE), BW, change in body weight (ΔBW), body condition score (BCS), height (Ht), days in milk (DIM), and parity (primiparous and multiparous) using a database containing 47,253 weekly observations on 3,607 cows enrolled in 57 studies from 8 states across the US. The proposed model was validated against the NRC (2001) prediction equation for DMI using an independent dataset. The proposed model outperformed the NRC (2001) model to predict DMI. Whereas both models were similar at predicting DMI during early-lactation (1 – 75 DIM), the proposed model outperformed the NRC (2001) during mid- and late-lactation (76 – 368 DIM). In chapter five, we performed a meta-regression using data from 5 experiments conducted at Michigan State University to determine the effect of dietary fatty acid (FA) composition on digestibilities of DM, NDF, FA, 16-carbon FA, 18-carbon FA, and digestible energy (DMD, NDFD, FAD, 16-CD, 18-CD, and EnergyD, respectively) and on DE intake (DEI). The final database was composed of 423 individual observations collected on 183 lactations from 124 Holstein mid-lactation cows receiving diets that varied on FA composition. Palmitic and stearic were the FAs with greatest impacts on digestibilities and DEI; palmitic increased DEI when included in the diet up to 1.2% of DM, and stearic linearly decreased DEI. In chapter six, we conducted a meta-regression analysis on 129 treatment means from 26 peer-reviewed publications to predict body composition (EBFat, EBProtein, EBAsh, and EBWater) as a percentage of empty body weight (EBW) in Holsteins. The statistical models contained the random effect of study and fixed effects of method (direct, carcass, and dilution), stage (heifer and cow), and one of the three possible ways to express EBW (4th order polynomial of EBW - polEBW, natural logarithm of EBW - lnEBW, or EBW to the power of 0.75 – EBW^0.75), average daily gain (ADG) for heifers, and BCS for cows. Additionally, the models were weighted by the inverse of the standard deviations of the studies. Compared to NRC 2001, the proposed model suggests that fat content of the gain, and thus the energy content of gain, is greater for young heifers and less for older heifers than the values predicted by NRC 2001.
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