Boutton T W, Tyrrell H F, Patterson B W, Varga G A, Klein P D
Dept. of Pediatrics, Baylor College of Medicine, Houston.
J Anim Sci. 1988 Oct;66(10):2636-45. doi: 10.2527/jas1988.66102636x.
Carbon transfer to milk in Holstein cows in late lactation was measured by introducing changes in the natural stable carbon isotope composition of the feed. Six Holstein cows in mid-lactation were placed on a diet naturally low in 13C (-25.0% vs Pee Dee belemnite [PDB] an international carbon isotope standard), based on alfalfa-barley, and six others were placed on a diet naturally enriched in 13C (-11.5% vs PDB), based on corn. After a 7-wk equilibration period on these diets, three cows were switched from alfalfa-barley to corn, and three were switched from corn to alfalfa-barley. The three other cows in each group served as controls. 13C/12C ratios were measured in daily morning milk samples during the week before and for 6 wk after the changes in diet. After the diets had been switched, milk isotope ratios rapidly approached the isotopic composition of the new diet, indicating rapid transfer of dietary carbon into milk. The data were consistent with a model whereby milk was synthesized from a single precursor pool that responded rapidly to dietary perturbation. The milk precursor pool had a half-life of approximately .9 d and had a mass of approximately 7 kg of carbon, which was renewed daily by the entry of 5 kg of digestible dietary carbon.
通过改变饲料中天然稳定碳同位素组成来测定荷斯坦奶牛泌乳后期碳向牛奶中的转移。六头处于泌乳中期的荷斯坦奶牛被置于以苜蓿 - 大麦为基础、天然13C含量低（相对于国际碳同位素标准皮迪菊石[PDB]为-25.0%）的日粮中，另外六头被置于以玉米为基础、天然13C含量高（相对于PDB为-11.5%）的日粮中。在这些日粮上经过7周的平衡期后，三头奶牛从苜蓿 - 大麦日粮转换为玉米日粮，三头从玉米日粮转换为苜蓿 - 大麦日粮。每组中的另外三头奶牛作为对照。在日粮改变前一周及改变后6周期间，每天早晨采集牛奶样本测量13C/12C比率。日粮转换后，牛奶同位素比率迅速接近新日粮的同位素组成，表明日粮碳快速转移到牛奶中。数据与一个模型相符，即牛奶由一个对日粮扰动反应迅速的单一前体池合成。牛奶前体池的半衰期约为0.9天，质量约为7千克碳，每天通过5千克可消化日粮碳的进入而更新。
