To determine if increased glucose demand affects feed intake or hepatic metabolism, 12 lactating Holstein cows were treated with phlorizin or control in a crossover experiment. Phlorizin injected subcutaneously at 4 g/d caused 474 g/d of glucose excretion in urine. While treatment caused a decrease in lactose synthesis and milk production, feed intake and 3.5% fat-corrected milk production were not altered by treatment. The lost glucose was likely accounted for by increased gluconeogenic fl ux. Hepatic pyruvate carboxylase mRNA concentration increased with phlorizin treatment, suggesting that retention of glucose precursors may be involved in regulation of gluconeogenesis.

Lactating dairy cows were used to determine effects of feeding glyphosate-tolerant or insect-protected corn hybrids on feed intake, milk production, milk composition, and ruminal digestibility. Corn resistant to European corn borer (Ostrinia nubilalis) infestation (Bt-MON810), or its nontransgenic control (Bt-CON), were planted in alternating fields during two successive years. One-half of each strip was harvested for whole plant corn silage and the remainder was allowed to mature and harvested as grain. Effects of feeding diets containing either Bt-MON810 or Bt-CON grain and silage were determined in two experiments (1 and 2) conducted during successive years. In experiment 3, glyphosate-tolerant Roundup Ready corn (RR-GA21) or its nontransgenic control (RR-CON) corn were grown in alternating fields during one cropping season. Diets contained 42 to 60% corn silage and 20 to 34% corn grain from Bt-MON810, RR-GA21, or the appropriate nontransgenic counterpart; treatments were applied using a switchback design. Cows were fed ad libitum and milked twice daily. There were no differences for nutrient composition between silage sources or between grain sources within an experiment. Data for experiments 1 and 2 indicated similar dry matter intake (DMI), 4% fat-corrected milk (FCM) production, and milk composition between Bt-MON810 and Bt-CON diets. There were no differences for DMI, 4% FCM production, and milk composition between RR-GA21 and RR-CON diets. There was no difference in ruminal degradability, determined separately for corn silage and corn grain, for RR-GA21 or Bt-MON810-hybrids compared with their respective controls. These data demonstrate equivalence of nutritional value and production efficiency for corn containing Bt-MON810 compared with its control and for RR-GA21 corn compared with its control.

Exogenous glucagon increases hepatic glucose synthesis in part by increasing hepatic extraction of amino acids from blood for conversion to glucose. To examine the role of glucagon in orchestrating gene expression of gluconeogenic and ureagenic enzymes, we determined the mRNA concentrations of key hepatic ureagenic and gluconeogenic enzymes at d 11, 15, and 22 postpartum in multiparous Holstein cows that received 0 or 5 mg of glucagon in 60 mL of saline by subcutaneous injection every 8 h for 14 d starting on d 8 postpartum. On d 11 postpartum, glucagon increased the hepatic mRNA concentrations for all measured ureagenic enzymes (carbamoylphosphate synthetase I, ornithine transcarbamylase, and argininosuccinate synthetase) and gluconeogenic enzymes (pyruvate carboxylase and cytosolic and mitochondrial forms of phosphoenolpyruvate carboxykinase) and increased or tended to increase mRNA concentrations of gluconeogenic enzymes on d 15 postpartum but not on d 22. The effect of glucagon to increase mRNA concentrations of ureagenic and gluconeogenic enzymes was limited to times when concentrations of plasma insulin were not increased. Our results suggest that hepatic gene expression of key ureagenic and gluconeogenic enzymes in early-lactation dairy cows is responsive to hormonal regulation by glucagon.

Somatotropin (ST) increases milk production and through coordinated changes in hepatic glucose synthesis and amino acid metabolism in dairy cows. The objective of this study was to determine the effects of ST on hepatic mRNA expression for phosphoenolpyruvate carboxykinase (PEPCK) and pyruvate carboxylase (PC), enzymes that are critical to the synthesis of glucose in liver and hepatic mRNA expression for carbamylphosphate synthetase I (CPS-I), argininosuccinate synthetase (AS), and ornithine transcarbamylase (OTC), critical enzymes of the urea cycle. Eighteen cows were randomly allocated to 2 treatment groups and received either recombinant bovine ST (Posilac; Monsanto, St. Louis, MO) or saline injections at 14-d intervals during a 42-d period. Expression of mRNA was determined using Northern blot analysis. Nuclei, isolated from liver biopsy samples, were used to determine effects of ST on transcription rate of PEPCK. Milk production was increased with ST (37.3 vs. 35.1+/-0.6 kg/ d). Plasma NEFA was increased with ST (299 vs. 156+/-34 microM). There were no differences in the expression of CPS-I, AS, and OTC mRNA with ST. Expression of PEPCK and IGF-I mRNA were increased with ST but PC mRNA was unchanged. The data indicate increased PEPCK mRNA in cows given ST and indicates a greater capacity for gluconeogenesis from gluconeogenic precursors that form oxaloacetate. The effects of ST to elevate PEPCK mRNA expression require chronic administration and involve increased transcription of the PEPCK gene.

Different feeding systems for beef cattle production were studied at the Corozal and Isabela Substations of the Agricultural Experiment Station, University of Puerto Rico. The data for each location were analyzed separately. Observations at Isabela indicate that Pangola-grass soilage is not suitable for rearing beef cattle. Ad libitum feeding of a bulky ration showed no comparative advantage over grazing alone in weight gains of the heifers. Similarly, a finishing period on ad libitum bulky feed was no better than grazing alone. The dressing percentage for the ad libitum bulky-fed group was 64.26 percent compared to 62.30 percent for the group on grazing alone and to 62.23 percent for the group on grazing plus a finishing period on ad libitum bulky feed. There was no significant difference between these two latter groups. The soilage group was not slaughtered. Data from the Corozal Substation indicate that corn supplementation was superior to the other treatments in regard to weight gains of the heifers. However, when economical aspects are considered, corn supplementation may prove unjustifiable. The molasses supplementation was no better than grazing alone. The dressing percentage of the corn supplemented heifers was significantly different from grazing alone (60.70 vs. 59.03 percent) and from grazing plus concentrate supplementation when pastures were poor (60.70 vs. 59.10 percent). Dressing data for the molasses-fed group (60.40 percent) were not statistically analyzed because individual liveweights of heifers were not obtained for reference against the corresponding "hot carcass" weights.