Short communication: Glucose kinetics in dairy heifers limit-fed a low- or high-forage ration at 4 levels of nitrogen intake.

The objective of this research was to evaluate the effect of level of forage and N intake on glucose kinetics in growing dairy heifers. Eight Holstein heifers (beginning at 362 ± 7 kg of body weight and 12.3 ± 0.4 mo of age) were fed 8 rations according to a split-plot, 4 × 4 Latin square design with rations containing either high (75% forage dry matter) or low (25% forage dry matter) levels of forage and 4 levels of N intake (0.94, 1.62, 2.30, 2.96 g of N/kg of metabolic body weight per day). Diets were limit-fed to maintain equal predicted metabolizable energy intake over the four 28-d periods; dietary N was increased through the substitution of high-N ingredients for corn. Blood samples were collected from all heifers at times throughout d 18 to 19 to characterize glucose concentration over the course of a day, and glucose tolerance tests were conducted over the last 8 d of each period (1 heifer/d) at 4 h before feeding. Glucose concentration transiently declined after feeding for all dietary groups, but we found no evidence of a differential response over time that could be attributed to diet. When averaged over a day, glucose concentration was affected by an interaction between level of forage and N intake; however, this response appeared related more to the level of starch in the diet than to the effects of either forage or N intake per se. Early-phase kinetic response of glucose disposal after an intravenous glucose bolus was not affected by dietary treatment, which is consistent with no difference in area under the curve through 30 min. Area under the curve through 120 min tended to linearly decrease with decreasing dietary N intake. This response corresponded to the kinetic analysis, in which heifers consuming higher N intake had an attenuated return to baseline glucose levels. Additionally, heifers consuming lower N intake maintained a period of glucose concentration below baseline before returning to baseline. We concluded that the response to an intravenous glucose bolus differs in dairy heifers fed diets differing in N intake, whereas forage level did not affect this response. This should be considered when formulating rations for low N intake by replacing high-N for high-starch feedstuffs when limit-feeding dairy heifers.