早籼稻谷替代玉米的肉牛饲料粮粗脂肪在瘤胃中降解特性的研究

以3头带有永久瘤胃瘘管的湖北本地阉黄牛用尼龙法对早籼稻谷替代玉米的肉牛饲粮粗脂肪在瘤胃中的降解特性进行了比较研究。试验处理为早籼稻谷分别替代精料中0％,50％,75％,和100％的玉米4个替代水平的肉牛饲粮。结果表明：随早籼稻谷替代水平的提高粗脂肪胃动态降解率呈下降趋势,分别为76．48％,75．22％,74．48％和66．71％,经单因子方差分析,除100％替代水平组与其它各组间差异显著外（P＜0．05）,其它各组间差异不显著（P＞0．05）;动态消化参数各组饲粮间差异不显著（P＞0．05）;能较好地符合模拟方程P＝a＋b（1－e^-ct）,拟合度高（R^2＝0．960－0．998）。     

甜菜颗粒替代玉米对奶牛消化及瘤胃消化动力的影响

试验使用4头安装有瘤胃瘘管和十二指肠套管的处于泌乳后期的中国荷斯坦奶牛,研究了甜菜颗粒替代玉米对荷斯坦奶牛消化和瘤胃消化动力的影响。试验日粮中粗饲料(玉米青贮+紫花苜蓿)占40%,精饲料占60%,精饲料中的玉米分别用0%、6%、12%和24%的甜菜颗粒进行替代(干物质基础)。4个处理组日粮中的中性洗涤纤维和淀粉的质量分数分别为(24.3%,34.5%)、(26.2%,30.6%)、(28.0%,26.5%)和(31.6%,18.5%)。结果表明,随着甜菜颗粒替代玉米比例的提高,瘤胃干物质周转率下降,瘤胃中性洗涤纤维的周转率提高;潜在降解中性洗涤纤维在瘤胃中的降解率提高,而且降解的速率也提高了,最终日粮总消化道的中性洗涤纤维的消化率提高了。不可消化中性洗涤纤维的瘤胃外排速率并没有受到不同处理的影响。瘤胃淀粉的消化率随着甜菜颗粒替代玉米比例的提高而显著下降,但是,总肠道淀粉的消化率并没有受到影响。随着甜菜替代玉米比例的提高,日粮纤维的消化率显著提高,淀粉的消化率没有受到影响。因此,处于泌乳后期的奶牛在低粗饲料饲喂条件下,日粮中15%~20%的玉米可以用甜菜颗粒进行替代。     

油茶籽粕反刍动物饲料化利用价值初探

在分析油茶籽粕常规营养成分基础上,本试验采用体外产气和尼龙袋技术对油茶籽粕瘤胃发酵和降解特性进行了评定。结果显示:(1)油茶籽粕的粗蛋白、粗脂肪、中性洗涤纤维、酸性洗涤纤维、酸性洗涤木质素分别为13.1%、8.6%、66.2%、44.5%和25.4%(绝干物质基础);油茶籽粕的代谢能为4.30 MJ/kg DM;(2)油茶籽粕在体外发酵前期(2~12 h)的累积产气量显著高于羊草(P0.05),但36 h起油茶籽粕的累积产气量显著低于羊草(P0.05)。这与尼龙袋法测定的干物质降解率的变化趋势一致,油茶籽粕的干物质降解率在24~72 h显著低于羊草(P0.05),但在前期(3~12 h)二者的干物质降解率没有显著性差异(P0.05);(3)油茶籽粕25%替代羊草时不影响潜在产气量(P0.05),随替代比例的进一步增加(≥50%)慢速产气量显著降低(P0.05),但产气速度显著增加(P0.05),油茶籽粕25%替代羊草时不影响丙酸浓度(P0.05)。结果表明油茶籽粕含有较高的粗蛋白和粗脂肪,油茶籽粕替代羊草可提升全混合饲粮的发酵速度,提高丙酸所占挥发性脂肪酸比例;较高含量的木质素降低了其营养价值。     

In Sacco法评定玉米的不同加工处理方式对泌乳奶牛瘤胃干物质和淀粉降解规律的影响

以3头安装有永久性瘤胃瘘管的泌乳前期荷斯坦奶牛为试验动物,在精粗比为55：45的日粮条件下,采用In sacco法测定了不同加工处理的玉米在奶牛瘤胃内干物质和淀粉的降解规律.试验结果表明,不同加工处理方法可以程度不同的影响干物质和淀粉在瘤胃中的降解,与未处理玉米相比,挤压膨化和制粒加工处理能明显的提高玉米中淀粉在瘤胃中的降解率（P＜0.05）,压扁处理对干物质降解率无明显影响（P＞0.05）,但使淀粉瘤胃降解率明显提高（P＜0.05）,烘烤与焙炒加工处理能明显的降低玉米中干物质和淀粉在瘤胃中的降解率（P＜0.01）.淀粉瘤胃降解率由高到低顺序依次为膨化玉米＞颗粒玉米＞压扁玉米＞未处理玉米＞焙炒玉米＞140℃/35min烘烤玉米.淀粉在瘤胃中的降解的影响因素最主要是淀粉的快速可溶部分和降解速率.     

日粮补充过瘤胃硫酸锌对肉牛瘤胃营养物质降解率的影响

为研究过瘤胃硫酸锌对肉牛瘤胃营养物质瘤胃降解率的影响,试验选用4头,24月龄,体重(513±5.7)kg的装有永久性瘤胃瘘管的晋南牛阉牛,采用4×4拉丁方试验设计,对照组日粮中添加300 mg锌(ZnSO47H2O),LRPZn、MRPZn和HRPZn组分别在日粮中添加200、300和400 mg的锌(过瘤胃硫酸锌,RPZn)。结果表明,玉米青贮干物质和有机物的有效降解率从高到低依次为HRPZn、MRPZn、LRPZn和对照组差异显著(P0.05);中性洗涤纤维有效降解率HRPZn组显著高于其余各组(P0.05)。混合精料干物质和有机物有效降解率HRPZn组显著高于对照组(P0.05);粗蛋白质有效降解率HRPZn组显著高于对照组和LRPZn(P0.05)。肉牛日粮中以过瘤胃硫酸锌形式添加400mg锌对提高日粮养分瘤胃降解率效果最佳。     

可降解蛋白及非蛋白氮对模拟瘤胃体外营养物质降解率的影响

试验以11.85%和8.38%两个水平的可降解蛋白(RDP)及非蛋白氮(尿素)作为试验因子,进行2×2试验设计.应用体外短期发酵技术,研究了RDP、RDP与非蛋白氮组合对模拟瘤胃中营养物质降解率的影响.结果表明,8.38%RDP日粮的干物质(DM)、中性洗涤纤维(NDF)降解率分别为51.32%和31.17%,11.85%RDP日粮的分别为60.01%和46.11%,两者差异显著(P＜0.05);说明RDP水平的提高对模拟瘤胃内营养物质降解率具有明显的促进作用.日粮中RDP水平与适量添加的非蛋白氮使模拟瘤胃中营养物质降解率提高,添加尿素日粮与不添加尿素日粮营养物质的消化率相比,差异显著(P＜0.05);说明RDP与非蛋白氮的添加对瘤胃营养物质降解率有明显的互作作用.     

丙酸钙对西门塔尔牛精饲料瘤胃降解率的影响

以混合精料和玉米秸秆为基础日粮,研究丙酸钙对西门塔尔牛精饲料瘤胃降解率的影响.结果表明:添加丙酸钙200 g/d组的精饲料干物质、有机物和粗蛋白质瘤胃有效降解率较对照组均显著提高(P<0.05);丙酸钙的适宜添加水平为200 g/d.     

木薯代替玉米对鹅饲养效果的研究

选择28日龄四川白鹅96只,随机分为4个组,每组3个重复,每个重复8只,进行为期42 d的饲养试验.结果表明:玉米组、木薯替代35%玉米饲粮组、木薯替代55%玉米饲粮组、木薯替代75%玉米饲粮组,平均日增重分别为42.30、39.19、39.38和39.82 g,但组间差异不显著(P＞0.05);各组鹅的胴体肉用性能指标无明显差异(P＞0.05);用木薯饲粮喂鹅会造成经济效益不同程度的下降.     

高粱替代玉米对肉牛瘤胃发酵和养分消化的影响

试验选用4头年龄和体重近似、装有永久性瘤胃瘘管的晋南牛阉牛,采用4×4拉丁方设计,研究不同比例高粱(对照组和处理1~3组分别用高粱替代日粮中0、33%、66%和100%的玉米)对晋南牛瘤胃发酵和养分消化的影响。结果表明,处理2组和3组瘤胃pH值、乙酸与丁酸摩尔分数、乙酸与丙酸的比例以及氨态氮质量浓度均高于对照组(P0.05);处理3组瘤胃总挥发性脂肪酸质量浓度、丙酸摩尔分数、日粮干物质和有机物的消化率、尿嘌呤衍生物质量浓度和甲烷能排放均低于对照组(P0.05);处理2组日粮粗蛋白质、中性洗涤纤维和酸性洗涤纤维的消化率高于对照组(P0.05)。综合考虑各项指标,建议肉牛日粮中高粱替代玉米的比例为66%。     

瘤胃尼龙袋法评定木薯柠檬酸渣对山羊的营养价值

利用瘤胃尼龙袋法测定了木薯柠檬酸渣对山羊的营养价值,样品在瘤胃中分别培养0、2、6、12、24、36和48 h后干物质、中性洗涤纤维和粗蛋白质的降解率来评定其对山羊的营养价值.结果表明,木薯柠檬酸渣的消化能为9.061 MJ/kg,干物质有效降解率为45.32%,中性洗涤纤维有效降解率为23.37%,粗蛋白质有效降解率为48.86%.     

In situ dry matter and crude protein degradation of fresh forages during the spring growth.

We compared ruminal dry matter (DM) and crude protein (CP) degradation kinetics of fresh forages of alfalfa (vegetative, early bud, early flowering, and late flowering stages) and bromegrass with endophyte-free and endophyte-infected tall fescue (tillering, stem elongation, heading, and flowering stages) by using nonlinear models. Duplicate Dacron bags were incubated for 0, 3, 6, 9, 12, 16, 24, 36, 48, and 60 h in 2 Simmental x Angus steers fitted with ruminal cannulas. The effects of animal, forage species, and maturity within forage species were evaluated. A first-order kinetics model was determined to be suitable for estimation of degradation profiles relative to models with variable rate of degradations. Alfalfa had higher soluble DM (36.6%), lower insoluble potentially digestible DM (43.0%), higher rate (13.8%/h), and higher extent of DM degradation (66.3%) than grasses (27.2, 53.5, 6.7%/h, and 54.6% for soluble DM, insoluble potentially digestible DM, rate, and extent of DM degradation, respectively). The extent of CP degradation was similar among forages (74.7%), but alfalfa had a higher CP degradation rate than grasses (16.1 vs. 12.5%/h). Extents of degradation of DM and CP decreased with maturity in alfalfa and in grasses. Ruminally undegradable CP (RUP) was higher in alfalfa (40.0 g/kg of DM) than in grasses (34.9 g/kg of DM), and decreased with maturity in grasses (40.4 to 28.3 g/kg of DM) but not in alfalfa. The amount of RUP that was potentially degradable in the rumen was not different among forage species (22.1 g/kg). As forage CP concentration decreased, the RUP (as a percentage of CP) increased but, as a percentage of forage DM, decreased. Species of forage had important effects on ruminal DM and CP degradation when incubated in fresh form.     

Short communication: effects of 2-hydroxy-4-(methylthio) butanoic acid isopropyl ester on milk production and composition of lactating Holstein dairy cows

Sixteen multiparous Holstein cows were used to determine the effects of 2-hydroxy-4-(methylthio) butanoic acid isopropyl ester (HMBi: 0 vs. 1.26 g/kg of total ration dry matter (DM) and dietary crude protein (CP) concentration [14.7% (low) vs. 16.9% (standard), DM basis] on milk yield and composition using a replicated 4 × 4 Latin square design experiment with 4-wk periods. Cows were fed ad libitum a total mixed ration with a 1:1 forage-to-concentrate ratio (DM basis), and diets provided an estimated 6.71 and 1.86% lysine and methionine, respectively, in metabolizable protein for the low-protein diet and 6.74 and 1.82% in the standard protein diet. Dry matter intake, milk yield, and composition were measured during wk 4 of each period. There were no effects on DM intake, which averaged 24.7 kg/d. There was an interaction between dietary CP and HMBi for milk yield and 3.5% fat-corrected milk (FCM). Feeding HMBi decreased milk and FCM yield when fed with the low-CP diet but did not affect milk or FCM yield when fed with the standard CP diet. Feeding HMBi increased milk protein concentration regardless of diet CP concentration and increased milk protein yield when added to the standard CP diet but not the low-CP diet. The positive effect of HMBi on milk protein yield was only observed at the standard level of dietary CP, suggesting other factors limited the response to HMBi when dietary protein supply was restricted.     

大孔树脂联合Sephadex LH-20葡聚糖凝胶分离纯化芦荟大黄素

目的 对芦荟药材中芦荟大黄素进行分离纯化及含量测定。方法 以DM301、X5、DM130 3种大孔树脂的静态吸附率, 动态吸附率为指标, 分离芦荟大黄素, 用Sephadex LH-20葡聚糖凝胶纯化芦荟大黄素, 采用高效液相色谱法(high performance liquid chromatography, HPLC)测定芦荟大黄素的含量。结果 芦荟大黄素线性回归方程为Y=41131X?44.34(r2=0.9993), DM301大孔树脂联合Sephadex LH-20葡聚糖凝胶分离纯化芦荟大黄素效果好。结论 该方法快速、简便, 为芦荟中有效成分大黄素分离和测定提供了依据,为大黄素的纯化提供了参考。     

Milk production response to varying protein supply is independent of forage digestibility in dairy cows

The aim of this experiment was to examine whether the positive response in milk production to increased crude protein (CP) supply in dairy cows was dependent on the digestibility of the forage. Forty-eight lactating Danish Holstein cows were used in a 4 × 4 Latin square design experiment with 4 rations: (1) high digestibility and high CP concentration (HdHp), (2) high digestibility and low CP concentration (HdLp), (3) low digestibility and high CP concentration (LdHp), and (4) low digestibility and low CP concentration (LdLp). All rations contained 30% corn silage, 25% grass-clover silage, and 45% concentrate on a dry matter (DM) basis. Different digestibilities were obtained by replacing a high-digestible grass-clover silage combined with a high-digestible corn silage with a low-digestible grass-clover silage combined with a low-digestible corn silage. Organic matter digestibilities were 79.8 and 74.7% in the high- and low-digestibility rations, respectively. Dietary CP concentration in the ration was increased by substituting barley and sugar beet pulp with rapeseed meal and soybean meal, whereby CP increased from 13.9 to 14.0% (Lp) to 15.7 to 16.0% (Hp). All cows were offered 3 kg of the same concentrate per day in the automatic milking system in addition to the mixed ration. Every feeding period lasted 3 wk, and DM intake and milk yield were measured in the last week in each period, and milk samples for determining milk composition, including fatty acid content, and blood samples were taken during the last 3 d of each period. Dry matter intake increased by 2.2 kg/d on Hd compared with Ld and by 0.7 kg/d on Hp compared with Lp. The positive effect on DM intake was reflected in the energy-corrected milk (ECM) yield, as a higher ration digestibility increased the ECM yield by 1.7 kg/d and a higher CP concentration increased it by 1.2 kg/d. We detected no interaction between forage digestibility and CP concentration on milk production. Reduced digestibility was accompanied by an increase in the plasma level of glucose, suggesting that other nutrients were limiting to milk production. In conclusion, milk production responses to dietary CP supply appeared independent of forage digestibility.     

Evaluation of models to describe ruminal degradation kinetics from in situ ruminal incubation of whole soybeans

Different mathematical models were evaluated as candidates to describe ruminal dry matter (DM) and crude protein (CP) degradation kinetics of raw and roasted whole soybeans from data obtained using the in situ polyester bag technique. Three models were used: segmented with up to 3 straight lines (model I), negative exponential (model II), and rational function or inverse polynomial (linear over linear; model III). A fourth, a generalized sigmoidal model, was also considered but the data did not exhibit sigmoidicity, so it was dropped from the analysis. Lagged and nonlagged versions of each model were fitted to the DM and CP disappearance curves of 6 different feeds (2 cultivars of raw or differently heat-processed whole soybean). The comparison between lagged and nonlagged versions of each model, based on statistical and behavior characteristics, showed for all models that the discrete lag parameter did not significantly improve the fit to ruminal DM and CP disappearance curves. The comparison between models (using nonlagged equations) showed that models I and II gave better goodness-of-fit than model III. Based on biological characteristics, models II and III underestimated the undegradable DM and CP fractions, but there was no significant difference between models for extent of degradation.     

Diets deficient in rumen undegraded protein did not depress milk production

The objective of this study was to evaluate the National Research Council's recommendations for feeding levels of rumen undegraded protein (RUP) for cows fed a one-group total mixed ration. Sixty Holstein cows were paired by parity (1 to 6) and DIM (23 to 315) and were randomly assigned to one of two treatment sequences. Diets contained alfalfa silage (30% diet DM) and corn silage (26% diet DM), and were isonitrogenous (16% CP) and isocaloric (1.71 Mcal/kg). Soybean meal, protected soybean meal (Soy Best), and urea were used to make ration protein fractions that were predicted to be 35 or 29% RUP. The 35% RUP diet was formulated to provide 98 and 105% of the average requirement for RUP and rumen degraded protein (RDP), respectively. The ration containing 29% RUP provided 79 and 117% of average required RUP and RDP, respectively. All cows were group-fed the high RUP diet during a 2-wk pretreatment period, and then were fed one ration for 4 wk followed by the other for 4 wk according to their assigned treatment sequence. Data were collected in the last wk of each period. Mean milk production, milk fat, and milk protein were 32.6 kg/d, 4.35%, and 3.36%, respectively, with no treatment differences. Treatment response was not affected by degree of predicted RUP deficiency. National Research Council requirements for RUP may be too high for cows fed diets similar in energy to a one-group total mixed ration. Alternatively, estimates of RUP content of feedstuffs may be low.     

Replacing alfalfa hay with triticale hay has minimal effects on lactation performance and nitrogen utilization of dairy cows in a semi-arid region of Mexico

In the semi-arid highlands of central Mexico, triticale (× Triticosecale L.) is emerging as an alternative, less water-demanding forage crop than alfalfa for dairy cattle. Studies reported here were aimed at evaluating triticale hay (TH) relative to alfalfa hay (AH) for lactating cow performance, apparent digestibility, N partition, and ruminal degradation kinetics of dry matter (DM), crude protein (CP), and neutral detergent fiber (NDF). Study 1 was conducted on a privately owned farm. Four barns were used to conduct 4 replicated 3 × 3 Latin squares (1 barn = 1 square), where each barn included 3 pens (experimental units) receiving 1 of 3 dietary treatments. Each pen had 62 Holstein dairy cows. All diets included a forage-to-concentrate ratio of 42:58 (DM basis), which is typical for intensive dairy farms of the region. Dietary treatments were formulated to replace AH with TH on a CP basis, and included (DM basis) 15.1% AH and 0% TH, 9.0% AH and 7.4% TH, and 0% AH and 16.4% TH. Diets were iso-energetic (1.64 Mcal of net energy for lactation/kg of DM) and iso-nitrogenous (17.9% CP). Pen-level DM intake and milk production were from all cows in the pen, but pen-level milk composition, apparent digestibility, and N partitioning were from 8 cows (observational units) randomly selected in each pen. Orthogonal contrasts were used to determine linear and quadratic effects of increasing TH from 0 to 7.4, and 16.4% of dietary DM. Although DM intake was not affected, there was a tendency for CP intake to decline linearly and for NDF intake to increased linearly as TH replaced AH in the diet. Milk production declined linearly by 0.077 kg/d for each additional percentage unit of TH in the diet, which amounted to a 3.5% decline when TH replaced AH entirely. However, no effect was observed on energy-corrected milk production because of a compensatory linear effect of increasing milk fat concentration with the incorporation of TH in the diet. Total-tract NDF digestibility tended to increase linearly by 18.5%, but no differences were detected for urinary urea-N excretion and for N utilization estimated as milk N/(fecal N + urinary N + milk N). Study 2 was an in situ trial conducted to determine the degradation kinetics of AH and TH used in study 1. In spite of differences in degradation kinetics parameters for DM, CP, and NDF, only NDF effective ruminal degradation tended to be greater for TH than AH. Replacing AH with TH at the level typically found in intensive dairy farms of the semi-arid regions of Mexico had minimal effects on milk production and N utilization.     

Utility of near-infrared reflectance spectroscopy to predict nutrient composition and in vitro digestibility of total mixed rations

Total mixed ration (TMR) samples (n = 110) were analyzed for dry matter (DM), crude protein (CP), soluble CP, neutral detergent fiber (NDF), NDF CP, starch, ash, fat, total ethanol-soluble carbohydrate, and nonfiber carbohydrate (NFC). Rapidly and slowly degraded and undegraded in situ CP fractions and in vitro DM, organic matter, and NDF digestibility were determined on each TMR. The TMR were scanned using near-infrared reflectance spectroscopy (NIRS); spectra were retained with NIRS calibration and cross-validation statistics were determined using partial least squares regression methods. The CP, NDF, starch, in vitro DM, and in vitro indigestible NDF contents of TMR were predicted by NIRS with good degrees (R² >0.85) of accuracy with proportionally low standard errors of prediction. Moderate utility of NIRS to predict the NFC (R² = 0.83) and fat content (R² = 0.81) of TMR was observed. Rapidly, slowly, and undegraded in situ CP fractions in TMR were not well predicted by NIRS. Similarly, soluble CP, NDF CP, total ethanol-soluble carbohydrate, and in vitro NDF digestibility (% of NDF) were not well predicted by NIRS. Ratios of nutrient range to reference laboratory method error were calculated and found to be positively related (R² = 0.84) to NIRS predictability of a given TMR nutrient, suggesting some laboratory procedures were not precise enough to yield suitable NIRS predictions. Data suggest that NIRS has utility to predict basic nutrients such as CP, NDF, starch, NFC, and fat in TMR. However, difficulty was observed using NIRS in predicting key biological nutrients in TMR such as in situ CP fractions and in vitro NDF digestibility. Difficulty of NIRS in predicting these nutrients is related to the level of reference method error in relationship to the range of nutrient values in TMR, but other sources of prediction error may exist.     

Ruminal degradability of dry matter,crude protein,and amino acids in soybean meal,canola meal,corn, and wheat dried distillers grains

Different protein sources, such as canola meal (CM) or dried distillers grains (DDG), are currently used in dairy rations to replace soybean meal (SBM). However, little data exists comparing their rumen degradation in a single study. Therefore, the objective of this study was to compare the ruminal degradation of dry matter (DM), crude protein (CP), and AA of SBM, CM, high-protein corn DDG (HPDDG), and wheat DDG plus solubles (WDDGS). In situ studies were conducted with 4 rumen-fistulated lactating Holstein cows fed a diet containing 38% grass hay and 62% corn-based concentrate. Each protein source was incubated in the rumen of each cow in nylon bags for 0, 2, 4, 8, 16, 24, and 48 h to determine DM and CP rumen degradation kinetics, whereas additional bags were also incubated for 16 h to evaluate AA ruminal disappearance. Rumen DM and CP degradability was calculated from rumen-undegraded residues corrected or not for small particle loss. Data were fitted to an exponential model to estimate degradation parameters and effective degradability (ED) was calculated with a passage rate of 0.074 h⁻¹. The WDDGS and SBM had higher uncorrected ED (DM = 75.0 and 72.6%; CP = 84.8 and 66.0%, respectively) than CM and HPDDG (DM = 57.2 and 55.5%; CP = 59.3 and 48.2%, respectively), due to higher soluble fraction in WDDGS and a combination of higher potentially degradable fraction and rate of degradation in SBM. Correction for small particle loss from bags, higher for WDDGS than for the other protein sources, decreased estimated ED but did not alter feed ranking. The ruminal disappearance of AA after 16 h of incubation reflected the overall pattern of CP degradation between protein supplements, but the ruminal disappearance of individual AA differed between protein supplements. Overall, these results indicate that, in the current study, (1) SBM and WDDGS were more degradable in the rumen than CM and HPDDG, and (2) that small particle loss correction is relevant but does not alter this ranking.     

Protein synthesis and degradation in the mammary gland of lactating goats

Lactating goats were given a close arterial infusion of [1-14C]leucine and [4,5-3H]4-methyl-2-oxopentanoic acid into one half of the mammary gland at 2-3 weeks and 34-39 weeks after kidding. Rates of protein synthesis, degradation and net output were determined from measurements of arteriovenous difference and blood flow using a model of leucine metabolism previously developed for muscle (Oddy & Lindsay, 1986). Protein leucine output in milk (Y mumol/min) correlated well with the difference between synthesis and degradation (X mumol/min) derived from the model: Y = 1.30 + 1.24X (r2 = 0.9; n = 9, P less than 0.01). There was substantial synthesis and degradation of protein within the mammary gland. Although only an approximate value could be obtained for the partitioning of protein synthesis and degradation between tissue and milk proteins, there was evidence of appreciable turnover of both. There was no significant difference between mammary leucine and protein metabolism in early and late lactation other than that imparted by a greater mass of mammary tissue in early lactation, although there was a tendency for greater oxidation of leucine in late lactation.