Energy and nitrogen metabolism and insulin response to glucose challenge in lactating German Holstein and Charolais heifers

Energy and protein metabolism were investigated in lactating Holstein (n = 6) and Charolais (n = 8) heifers, thus in cattle with different potential for milk production and nutrient accretion, but comparable body size. Insulin responses to glucose challenge during lactation were also investigated. Feed intake, milk production and nutrient loss through faeces and urine were measured for 5 days to calculate energy and nitrogen balance. 15N-glycine was infused to measure 15N turnover and to calculate protein synthesis and degradation. Blood samples were taken 3 h after feed intake on day 12 to measure postprandial glucose and insulin concentrations. On day 16, heifers received intravenous glucose infusions (1 g/kg BW0.75) and blood samples were taken before and 7, 14, 21 and 28 min after glucose challenge. Blood glucose and insulin concentrations were measured, and areas under the curve were calculated. Dry matter intake was higher (P < 0.01) in Holstein than in Charolais heifers. Milk yield was higher (P < 0.01) and protein content in milk was lower (P < 0.01) in Holstein than in Charolais heifers. Charolais heifers showed a higher energy and nitrogen balance (P < 0.01) than Holstein heifers, whereas net protein synthesis and efficiency of protein synthesis were greater in Holstein than in Charolais heifers. Plasma insulin concentrations after feed intake and before glucose challenge, as well as glucose increase after glucose challenge were higher (P < 0.05) in Charolais heifers when compared to Holstein heifers. In conclusion, lower energy and nitrogen balances in Holstein than in Charolais heifers were particularly caused by differences in their potential for milk production and were accompanied by lower plasma insulin levels in Holstein heifers. Charolais heifers showed a greater potential for energy and protein accretion combined with higher insulin levels and lower glucose half-life. As Charolais and Holstein represent important bovine breeds for nutrient accretion and secretion type, respectively, females of these two breeds are appropriate to investigate the genetic and physiological background for differences in regulation of nutrient partitioning in cattle.