Macromineral requirements for maintenance, body weight gain, and pregnancy of dairy cows

We aimed to predict macromineral requirements for maintenance, weight gain, and pregnancy in dairy cows. In total, 62 nonlactating cows (initial BW of 522 ± 10.1 kg [mean ± SD], initial age of 5 ± 0.5 yr, and 3 lactations) were enrolled and assigned to 3 groups: pregnant (n = 44), nonpregnant (n = 12), and baseline (n = 6). Baseline cows, which were not inseminated, were harvested at the beginning of the trial to determine the initial body composition. Both pregnant and nonpregnant groups were then divided into 2 feeding treatments: ad libitum or restricted intake at 1.15% of BW (approximating maintenance). Pregnant cows were slaughtered at 140, 200, 240, and 270 d of gestation, and nonpregnant cows were slaughtered at corresponding intervals to compare mineral accretion due to pregnancy. Total-tract digestibility was measured in six 28-d periods (d 122, 150, 178, 206, 234, and 262 of gestation) by collecting DMI, feces, and urine. The net requirements of maintenance (mg/kg of empty BW) for calcium (Ca), phosphorus (P), magnesium (Mg), sodium (Na), potassium (K), and sulfur (S) were 13.48, 8.35, 4.06, 10.08, 45.89, and 7.82, respectively. For BW gain in pregnant cows, the models for Ca, P, Mg, K, Na, and S were set as Net Ca_gain = 0.4168 × EBW^0.7115 × EBG_c; Net P_gain = 0.8441 × EBW^0.4762 × EBG_c; Net Mg_gain = 0.0492 × EBW^0.4391 × EBG_c; Net K_gain = 0.1738 × EBW^0.5169 × EBG_c; Net Na_gain = 0.0284 × EBW^0.7880 × EBG_c; Net S_gain = 0.2530 × EBW^0.7982 × EBG_c, respectively; EBW = empty BW, EBG_c = empty body gain, carcass and noncarcass. Estimates of net requirements of pregnancy were adjusted as follows: Net Ca_preg = 0.0042e^0.0286×GD; Net P_preg = 0.0059e^0.0253×GD; Net Mg_preg = 0.0006e^0.0219×GD; Net Na_preg = 0.0197e^0.0166×GD; Net K_preg = 0.0111e^0.0176×GD; Net S_preg = 0.0106e^0.0181×GD, where GD = gestation days. Finally, we propose an innovative method to estimate the efficiency of macromineral utilization by gestational tissues. The macromineral efficiency for pregnancy (k_preg) for each mineral was modeled as: Ca k_preg = 0.0004e^0.0263×GD; P k_preg = 0.2974e^0.0048×GD; Mg k_preg = 0.00006e^0.0233×GD; K k_preg = 0.0003e^0.0234×GD; Na k_preg = 0.0038e^0.0200×GD; S k_preg = 0.0004e^0.0199×GD. These results provide valuable insights into macromineral requirements in dairy cows and offer innovative approaches to evaluating nutrient efficiency during pregnancy.