Prepartum nutrition and solar radiation in beef cattle. I. Relationships of body fluid compartments, packed cell volume, plasma urea nitrogen, and estrogens to prenatal development

Adaptations in body fluid pools during pregnancy were monitored in cows (n = 19) assigned to either low (LO, 70% NRC total feed intake) or high (HI, 110% NRC total feed intake) nutritional level (sudangrass hay) and to either shade (S) or no shade (NS) treatments in a 2 X 2 factorial experiment. Body water distribution (empty body water [EBW] by urea dilution; extracellular water [ECW] by thiosulfate dilution; intracellular water [ICW] by difference; plasma volume by Evans Blue dilution; interstitial water [ISW] by difference) was measured at 4-wk intervals beginning at 3 mo of pregnancy until birth and then immediately after birth. Both EBW and ICW in LO cows showed a steady decline (P less than or equal to 0.05), whereas HI cows tended to maintain these body pools throughout gestation. Shading did not affect the pattern of change in EBW; however, it did produce a greater (P less than or equal to 0.05) ICW in the S than in the NS cows throughout gestation. Generally, other body fluid pools (ECW, ISW, and plasma) were either not affected, or were just slightly affected, by shading or nutrition. Most of the body fluid pools (EBW, ECW, ICW, and ISW) inversely followed the seasonal changes in solar radiation. Calf birth weights were not affected by treatments but were moderately correlated to EBW (r = 0.49; P less than or equal to 0.05) and ICW (r = 0.50; P less than or equal to 0.05). Plasma urea nitrogen change, although not affected by nutrition, was affected ( P less than or equal to 0.05) by shading. All treatment groups showed a similar pattern of change in estradiol-17beta, whereas estrone sulfate tended to be lower in the NS-HI cows. Results indicate that monitoring body fluid compartments may provide insight into maternal adaptation and thus the development of the fetus