|HEINRICHS, ARLYN - Pennsylvania State University|
Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/23/2016
Publication Date: 4/13/2017
Citation: Zanton, G.I., Heinrichs, A.J. 2017. Short communication: Glucose kinetics in dairy heifers limit-fed a low- or high-forage ration at four levels of nitrogen intake. Journal of Dairy Science. 100:3718-3724.
Interpretive Summary: Limit-feeding dairy heifers has been shown to increase feed efficiency, reduce manure output, and reduce heifer feed costs. In an effort to improve nitrogen use efficiency when limit-feeding dairy heifers, dairy producers and nutritionists can formulate heifer rations for lower nitrogen intake by replacing high-nitrogen feedstuffs with high-starch feedstuffs. However, it is not well known how a diet lower in nitrogen affects the metabolism of glucose, an important energy source for dairy heifers. This study was conducted to determine if dietary modification of the level of forage or nitrogen impacts glucose kinetics in growing dairy heifers. This was accomplished by the use of an intravenous glucose tolerance test to determine the glucose concentration changes that occur over time after a large dose of glucose. Glucose kinetics were affected by dietary nitrogen intake. This finding shows the need for additional research to help dairy nutritionists formulate rations for low-nitrogen intake by replacing high-nitrogen feedstuffs with high-starch feedstuffs when limit-feeding dairy heifers.
Technical Abstract: The objective of this research was to evaluate the effect of level of forage and nitrogen (N) intake on glucose kinetics in growing dairy heifers. Eight Holstein heifers (beginning at 362 ± 7 kg body weight (BW) and 12.3 ± 0.4 months of age) were fed eight rations according to a split-plot, 4 x 4 Latin square design with rations containing either high (HF) or low (LF) levels of forage and 4 levels of N intake [0.94 (Low), 1.62 (MLow), 2.30 (MHigh), 2.96 (High) g N/kg BW0.75 per d]. Diets were limit-fed to maintain equal predicted ME intake over the four 28-day periods; dietary N was increased through the exchange of high-nitrogen ingredients for high-starch ingredients. Blood samples were collected from all heifers at times throughout days 19-20 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) 4 hours prior to feeding. Data were modeled kinetically and statistically evaluated with evidence for significance declared at P < 0.05. Glucose concentration transiently declined after feeding for all dietary groups, but there was 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. Kinetics of glucose disposal after an intravenous glucose bolus were not affected by dietary treatment. Area under the curve through 120 minutes tended to linearly decrease with decreasing dietary N intake. This corresponded to the kinetic analysis in which heifers consuming higher N had an attenuated return to baseline glucose levels, and heifers consuming lower N maintained a period of glucose concentration below baseline before returning to baseline. It was concluded that glucose kinetics differ in dairy heifers fed diets differing in N intake; this should be considered when formulating rations for low nitrogen intake by replacing high-nitrogen feedstuffs for high-starch feedstuffs when limit-feeding dairy heifers.