饱和脂肪酸对泌乳中期热应激奶牛产奶性能、采食量和能量代谢的影响

研究饱和脂肪酸对泌乳中期热应激奶牛产奶性能、采食量和能量代谢的影响。选择产后150~210 d的中国荷斯坦奶牛48头,采用完全随机区组试验设计,根据产奶量、分娩时间和胎次分为对照组、1.5%和3%饱和脂肪酸试验组。试验期间牛舍最小湿热指数(THI)在72以上。结果显示,日粮添加饱和脂肪酸显著提高奶牛产奶量(P0.05),1.5%与3%脂肪组之间差异不显著;乳脂和总固体产量,3%脂肪组显著高于1.5%脂肪组和对照组(P0.05),1.5%脂肪组显著高于对照组(P0.05)。饱和脂肪酸对DMI无影响,净能和粗脂肪采食量随脂肪添加量而呈线性增加(P0.01)。3%和1.5%脂肪组分别比对照组净能转化率提高5.69%和6.04%;粗蛋白转化率分别提高8.48%和6.20%,体重和体况三组间无差异。血液中NEFA含量,3%脂肪组显著低于对照组(P0.05);血糖、胆固醇、BHBA、胰岛素三组间无差异。饱和脂肪酸可以提高奶牛产奶性能和维持能量平衡,对能量代谢、采食量、体重和体况无明显影响。     

枯草芽孢杆菌对奶牛生产及乳脂脂肪酸组成的影响

研究不同枯草芽孢杆菌对奶牛生产性能和乳脂脂肪酸成分的影响。与对照组相比,试验组A和B日产奶量呈增加趋势,乳脂产量、乳脂率显著增加(P<0.05);除B组乳中体细胞数显著增加外,各试验组乳蛋白、乳糖、非脂固形物等指标无显著变化。脂肪酸的组成比例与对照组相比无显著差异(P>0.05)。但在A和B组中,对长链及中短链脂肪酸的合成均可起促进作用。     

脂肪酸钙与苜蓿干草对热应激奶牛产奶性能和血液指标的影响

选择2～4胎次泌乳中期的荷斯坦牛48头,配对分为4组,每组12头:CK(基础日粮)、Ⅰ组(基础日粮+400 g脂肪酸钙)、Ⅱ组(基础日粮+2.5 kg苜蓿干草)和Ⅲ组(基础日粮+400 g脂肪酸钙+2.5 kg苜蓿干草),于7～9月进行50 d的饲养试验,以研究添加脂肪酸钙或(和)苜蓿干草对热应激奶牛生产性能和血液指标的影响。结果表明:(1)添加脂肪酸钙或(和)苜蓿干草可以减缓热应激条件下奶牛产奶量的下降,提高产奶量,其中,Ⅲ组产奶量极显著高于CK(P0.01);(2)添加脂肪酸钙或(和)苜蓿干草有提高乳脂率的趋势(P0.05);(3)有提高血清T_4和COR水平的趋势(P0.05),且Ⅲ组血清T_3含量显著高于CK(P0.05)。     

中草药抗热应激剂对高温环境奶牛生产性能的影响

选用32头健康无病,体重、年龄、胎次、泌乳量、乳脂率、乳蛋白和乳固形物等相近的中国黑白花奶牛,对照组饲喂基础日粮,试验组在饲喂基础日粮的基础上分别添加50、100和150 g/(头·d)中草药抗热应激剂,进行中草药抗热应激剂缓解高温天气对奶牛的影响试验,试验期为40 d.结果表明,B、C和D组每头奶牛平均日产奶量比A组分别增加1.89、2.62和2.83 kg,提高幅度分别为11.91%、16.51%和17.83%,差异均极显著(P＜0.01).在乳脂率方面,B、C和D组比A组分别提高0.10、0.23和0.15个百分点,提高幅度分别为3.47%、7.99%和5.21%,其中C组差异显著(P＜0.05);在乳蛋白方面,B、C和D组比A组分别提高0.06、0.10和0.08个百分点,差异不显著(P＞0.05);B、C和D组乳固形物比A组分别提高0.39、0.42和0.38个百分点.B、C和D组比A组每头每日分别多收入2.64、3.08和2.53元.     

精料和饱和脂肪酸对奶牛生产性能和乳中脂肪酸组成的影响

为研究高精料和饱和脂肪酸对泌乳早期奶牛生产性能和乳中脂肪酸组成的影响。选择产后55 d左右的泌乳高峰期奶牛30头,根据随机区组设计原则,按照产奶量、胎次和泌乳天数将奶牛分为对照组(C)、高精料组(HC)和饱和脂肪酸组(FA),分别饲喂以干物质为基础含精料40%,60%和40%添加3%饱和脂肪酸的等蛋白日粮。结果 HC组产奶量(26.53±3.72)kg/d和FA组产奶量(26.98±3.36)kg/d显著高于C组(24.43±2.63)kg/d。FA组乳脂含量显著高于HC组(P≤0.05),HC组有低于C组的趋势(P=0.15)。FA组短链脂肪酸的含量显著低于C组(P≤0.05),HC组与C组差异不显著;HC组长链脂肪酸的含量显著低于FA组(P≤0.05),与C组差异不显著。HC组饱和脂肪酸含量显著高于FA组(P≤0.05)、FA组单不饱和脂肪酸含量显著高于C组和HC组(P≤0.05)、HC组多不饱和脂肪酸高于C组(P≤0.05)、各组间共轭亚油酸(10c12t CLA)含量差异不显著。结论为饲喂高精料日粮能够提高产奶量、降低乳脂率、增加乳脂中中链饱和脂肪酸和多不饱和脂肪酸含量。日粮添加饱和脂肪酸能够提高产奶量、增加乳脂率、增加单不饱和脂肪酸含量。     

日粮中不同比例小麦替代玉米对奶牛乳脂合成和乳脂脂肪酸组成的影响

选择8头泌乳时间为(84±17)d,体重为(569±47)kg的经产中国荷斯坦奶牛作为试验动物,研究了日粮中不同比例小麦替代玉米对奶牛乳脂合成和乳脂脂肪酸(FA)组成的影响。采用重复4×4拉丁方设计,分别饲喂含不同比例粉碎小麦(GW)和粉碎玉米(GC)的日粮,4个处理组分别为(DM基础):W0组(0%GW+27.9%GC),W9.6组(9.6%GW+19.2%GC),W19.2组(19.2%GW+9.6%GC)和W28.8组(28.8%GW+0%GC)。结果表明:随着日粮中小麦比例的增加,瘤胃p H值线性降低(P=0.01);产奶量未受显著影响,平均为27.1 kg/d;乳脂率(P=0.02)、乳脂产量(P=0.02)和3.5%乳脂校正乳(FCM)(P=0.05)产量呈线性降低趋势;乳脂FA(g/100 g总FA)中C16∶0呈线性(P0.01)和二次曲线增加(P0.01),而C18∶0和FAC16呈线性(P0.01)和二次(P=0.03和0.01)曲线降低;C8∶0(P=0.01)、C15∶0(P=0.06)、C16∶1(P=0.01)和C18∶1n9t(P=0.10)呈线性增加趋势,而C14∶0(P=0.09)和C18∶2n6c(P=0.08)呈线性降低趋势;与W0和W9.6组相比,W19.2和W28.8组乳脂C16∶0、C16∶1及C16∶0与C16∶1之和显著增加(P0.01),而C18∶0显著降低(P0.01)。结果提示:奶牛日粮中可用适当比例的粉碎小麦来替代玉米,替代比例不宜超过日粮DM的19.2%;高比例小麦日粮饲喂奶牛(W28.8组)可引起瘤胃p H值和乳脂率下降,进一步引起乳脂FA组成发生变化。     

油料籽实对奶牛生产性能及乳脂共轭亚油酸的影响

研究日粮中添加几种油料籽实对中国荷斯坦奶牛生产性能及乳脂脂肪酸组成的影响.选用48头高产中国荷斯坦奶牛,采用完全随机区组设计(对照组、大豆组、胡麻籽组、葵花籽组、花生组和棉籽组),试验组用7.5%不同油料籽实替换基础日粮中的部分豆粕.试验期6周,预试期2周,正式试验期4周.与对照组相比,各试验组奶牛的平均日采食量、产奶量、乳蛋白、乳糖及非脂固形物无显著差异,而乳脂率及日乳脂产量降低.其中,胡麻籽组降低最多,乳脂率和乳脂产量较对照组下降了0.34%和0.18 kg/d.大豆组的C18:1、tllC18:1及C18:2显著高于对照组和其它试验组,胡麻籽组的C18:3含量最高.各试验组CLA含量高于对照组,且差异极显著(P＜0.01).大豆组CLA含量最高,比对照组提高了60.38%.     

荷斯坦奶牛抗热应激环境调控技术研究

研究了通风、喷雾、通风+喷雾三种方法在高温季节里不同的日时间段对牛舍内温湿环境调控的效果.结果表明,通风+喷雾处理调控效果最佳,在上午1000、下午1400和下午1800分别降低舍内温度2.16℃、4.08℃、4.0℃,降低THI为2、3.6和3.1,因而在一定程度上缓减热应激对奶牛的影响.     

添加脂肪酸钙对高产荷斯坦奶牛生产性能的影响

选择6头经产、胎次相同、泌乳天数相近的中国荷斯坦奶牛，采用配对设计，研究脂肪酸钙对处于泌乳中期的中国荷斯坦奶牛生产性能的影响。试验处理分别为：试验组（A）为基础日粮＋450g／d脂肪酸钙；处理组（B）为基础日粮。结果表明，在满足能量需要基础上添加脂肪酸钙使产奶量显著增加，对乳脂率和乳蛋白质量分数无显著影响。     

大米蛋白肽对热应激时蛋鸡生产性能和相关生理生化指标的影响

研究在热应激的情况下,大米蛋白肽对蛋鸡生产性能和血液生理生化指标的影响.选择健康无病,体重相近的罗曼蛋鸡320只,随机分为4组,每组80只,下设4个重复,每个重复20只鸡.1组为对照组,2组、3组、4组为试验组,1组喂基础日粮,2、3、4组分别饲喂含1％、1.5％和2％的试验日粮.结果:在热应激条件下,添加1.5％～2％的大米蛋白肽能明显提高鸡的产蛋率、采食量、血液中的总蛋白、总脂和甲状腺素浓度(P＜0.05),显著降低产蛋鸡的料蛋比、血液中的血糖、醛固酮和皮质醇(P＜0.05),还能一定程度地降低鸡的死亡率.表明:在夏季,使用1.5％～2％的大米蛋白肽有利于提高蛋鸡的生产性能及改善相关的生理生化指标.     

Climate sensitivity of milk production traits and milk fatty acids in genotyped Holstein dairy cows

The aim of this study was the evaluation of climate sensitivity via genomic reaction norm models [i.e., to infer cow milk production and milk fatty acid (FA) responses on temperature-humidity index (THI) alterations]. Test-day milk traits were recorded between 2010 and 2016 from 5,257 first-lactation genotyped Holstein dairy cows. The cows were kept in 16 large-scale cooperator herds, being daughters of 344 genotyped sires. The longitudinal data consisted of 47,789 test-day records for the production traits milk yield (MY), fat yield (FY), and protein yield (PY), and of 20,742 test-day records for 6 FA including C16:0, C18:0, saturated fatty acids (SFA), unsaturated fatty acids (UFA), monounsaturated fatty acids (MUFA), and polyunsaturated fatty acids (PUFA). After quality control of the genotypic data, 41,057 SNP markers remained for genomic analyses. Meteorological data from the weather station in closest herd distance were used for the calculation of maximum hourly daily THI. Genomic reaction norm models were applied to estimate genetic parameters in a single-step approach for production traits and FA in dependency of THI at different lactation stages, and to evaluate the model stability. In a first evaluation strategy (New_sire), all phenotypic records from daughters of genotyped sires born after 2010 were masked, to mimic a validation population. In the second strategy (New_env), only daughter records of the new sires recorded in the most extreme THI classes were masked, aiming at predicting sire genomic estimated breeding values (GEBV) under heat stress conditions. Model stability was the correlation between GEBV of the new sires in the reduced data set with respective GEBV estimated from all phenotypic data. Among all test-day production traits, PY responded as the most sensitive to heat stress. As observed for the remaining production traits, genetic variances were quite stable across THI, but genetic correlations between PY from temperate climates with PY from extreme THI classes dropped to 0.68. Genetic variances in dependency of THI were very similar for C16:0 and SFA, indicating marginal climatic sensitivity. In the early lactation stage, genetic variances for C18:0, MUFA, PUFA, and UFA were significantly larger in the extreme THI classes compared with the estimates under thermoneutral conditions. For C18:0 and MUFA, PUFA, and UFA in the middle THI classes, genetic correlations in same traits from the early and the later lactation stages were lower than 0.50, indicating strong days in milk influence. Interestingly, within lactation stages, genetic correlations for C18:0 and UFA recorded at low and high THI were quite large, indicating similar genetic mechanisms under stress conditions. The model stability was improved when applying the New_env instead of New_sire strategy, especially for FA in the first stage of lactation. Results indicate moderately accurate genomic predictions for milk traits in extreme THI classes when considering phenotypic data from a broad range of remaining THI. Phenotypically, thermal stress conditions contributed to an increase of UFA, suggesting value as a heat stress biomarker. Furthermore, the quite large genetic variances for UFA at high THI suggest the consideration of UFA in selection strategies for improved heat stress resistance.     

Genetic effects of heat stress on milk fatty acids in Brazilian Holstein cattle

The present study aimed to estimate covariance components of milk fatty acids (FA) and to compare the genomic estimated breeding values under general and heat-stress effects. Data consisted of 38,762 test-day records from 6,344 Holstein cows obtained from May 2012 through January 2018 on 4 dairy herds from Brazil. Single-trait repeatability test-day models with random regressions as a function of temperature-humidity index values were used for genetic analyses. The models included contemporary groups, parity order (1–6), and days in milk classes as fixed effects, and general and thermotolerance additive genetic and permanent environmental as random effects. Notably, differences in heritability estimates between environments (general and heat stress) increased (0.03 to 0.06) for unsaturated FA traits, such as unsaturated, monounsaturated, and polyunsaturated, at higher heat-stress levels. In contrast, heritability estimated between environments for saturated FA traits, including saturated FA, palmitic acid (C16:0), and stearic acid (C18:0) did not observe significant differences between environments. In addition, our study revealed negative genetic correlations between general and heat-stress additive genetic effects (antagonistic effect) for the saturated FA, C16:0, C18:0, and C18:1, which ranged from ?0.007 to ?0.32. Spearman's ranking correlation between genomic estimated breeding values ranged from ?0.27 to 0.99. Results indicated a moderate to strong interaction of genotype by the environment for most FA traits comparing a heat-stress environment with thermoneutral conditions. Our findings point out novel opportunities to explore the use of FA milk profile and heat-stress models.     

Effects of fat source and copper on unsaturation of blood and milk triacylglycerol fatty acids in Holstein and Jersey cows

Fatty acid composition of plasma triacylglycerides and milk fat was analyzed from Holstein and Jersey cows with control or depleted copper status and fed roasted whole soybeans or tallow. Conjugated linoleic acid in plasma was higher in Jersey cows. Dietary fat source influenced the proportions of all fatty acids in plasma and in milk, except for conjugated linoleic acid in milk. Feeding soybeans increased plasma C14:1, C18:0, C18:2, and conjugated linoleic acid, and decreased C14:0, C16:0, C16:1, and cis- and trans-C18:1 compared with feeding tallow. Low copper diets decreased C18:0 and increased cis- and trans-C18:1, and conjugated linoleic acid in plasma. A fat source x copper status interaction occurred for cis-C18:1 in plasma. Proportions of C4:0 to C14:0 were higher, and cis16:1, cis- and trans-C18:1, and conjugated linoleic acid were lower in milk fat of Jersey compared with Holstein cows. Generally, the effects of copper depletion were less apparent in milk than in plasma. Copper depletion increased C4:0, trans-C18:1, and conjugated linoleic acid, and decreased C16:1 in milk. Feeding whole soybeans increased C4:0 to C14:0, C18:0, C18:2, and C18:3, and decreased C14:1, C16:0, C16:1, and cis- and trans-C18:1 in milk. Breed x fat interactions occurred for C4:0, C14:1, C16:1, and conjugated linoleic acid in milk. Copper status x fat source interaction occurred for trans-C18:1. The breed x copper status interaction was apparent in milk fat for C16:1 and C18:0 and conjugated linoleic acid in milk. Both C18:0 and trans-C18:1 were desaturated by mammary tissue; however, whereas desaturation of C18:0 was linear, desaturation of trans-C18:1 reached a plateau that could have been caused by presence of the trans-10 isomer, which is not desaturated and was not separated from trans-11 C18:1 in our analysis. Comparison of the plasma triacylglycerol fatty acid profile with the milk fat profile was useful to interpret separate events of biohydrogenation in the rumen and desaturation by the mammary gland.     

Effects of rumen-protected choline supplementation in Holstein dairy cows during electric heat blanket-induced heat stress

Dairy cows experiencing heat stress (HS) attempt to thermoregulate through multiple mechanisms, such as reducing feed intake and milk production and altering blood flow to increase heat dissipation. Effects of choline on energy metabolism and immune function may yield it a viable nutritional intervention to mitigate negative effects of HS. The primary objective of this experiment was to determine if supplementation of rumen-protected choline during, or before and during, an increased heat load would ameliorate the negative effects of HS on production and immune status. Heat stress was induced via an electric heat blanket model with a 3-d baseline period and 7-d HS period for all cows. Multiparous mid-lactation (208 ± 31 days in milk) Holstein cows were fed the same basal herd diet, blocked by pre-experiment milk yield, and randomly assigned to receive one of the following: (1) no rumen-protected (RP) choline (n = 7); (2) RP choline (60 g/d) via top-dress during the HS period (n = 8); or (3) RP choline (60 g/d) via top-dress during the baseline and HS periods (n = 8). Imposing HS via electric heat blanket raised respiration rate with all cows surpassing the HS threshold of 60 breaths/min. The increase in respiration rate tended to be ameliorated with either schedule of RP choline supplementation. Milk yield tended to increase when RP choline was supplemented in both the baseline period and during HS. Supplementation of RP choline tended to reduce blood fatty acid and triglyceride and tended to increase the revised quantitative insulin sensitivity check index. The role of RP choline supplementation to partially ameliorate the effects of HS should be further explored as a potential nutritional strategy to mitigate the negative consequences of HS on health and production.     

Within-milking variation in milk composition and fatty acid profile of Holstein dairy cows

Changes in milk composition during a milking are well characterized, but variation in milk fatty acid (FA) profile is not well described and may affect the accuracy of in-line milk composition analyzers and could potentially be used for selective segregation of milk. Within-milking samples were collected from 8 multiparous high-producing Holstein cows (54.86 ± 6.8 kg of milk/d; mean ± standard deviation). A milk-sampling device was designed to allow collection of multiple samples during a milking without loss of vacuum or interruption of milk subsampling. Milk was collected during consecutive morning and afternoon milkings (12-h intervals) and was replicated 1 wk later. Each sample represented approximately 20% of the milking and was analyzed for fat, true protein, and lactose concentration and FA profile. Milk fat concentration markedly increased over the course of milk let down (4.4 and 4.2 percentage units at the a.m. and p.m. milking, respectively), whereas milk fat globule size did not change. Milk protein and lactose concentration decreased slightly during milking. Modest changes in milk FA profile were also observed, as milk de novo and 16-C FA concentrations increased approximately 10 and 8%, respectively, whereas the concentration of preformed FA decreased about 7% during the milking. In agreement, mean milk FA chain length and unsaturation modestly decreased during milking (0.59 and 0.014 U, respectively). The observed changes in milk fat concentration during a milking are consistent with previous reports and reflect the dynamic nature of milk fat secretion from the mammary gland. Changes in milk FA profile are not expected to practically affect the accuracy of spectroscopy methods for determination of milk fat concentration. Furthermore, the small variation in FA profile during a milking limits the use of within-milking milk segregation to tailor milk FA profile.     

Effects of increased supplementation of n-3 fatty acids to transition dairy cows on performance and fatty acid profile in plasma, adipose tissue, and milk fat

The objective of this study was to determine the effects of feeding an increased amount of extruded flaxseed with high proportions of n-3 fatty acids (FA) to transition dairy cows on performance, energy balance, and FA composition in plasma, adipose tissue, and milk fat. Multiparous Israeli-Holstein dry cows (n = 44) at 256 d of pregnancy were assigned to 2 treatments: (1) control cows were fed prepartum a dry-cow diet and postpartum a lactating-cow diet that consisted of 5.8% ether extracts; and (2) extruded flaxseed (EF) cows were supplemented prepartum with 1 kg of extruded flaxseed (7.9% dry matter)/cow per d, and postpartum were fed a diet containing 9.2% of the same supplement. The EF supplement was fed until 100 d in milk. On average, each pre- and postpartum EF cow consumed 160.9 and 376.2 g of C18:3n-3/d, respectively. Postpartum dry matter intake was 3.8% higher in the EF cows. Milk production was 6.4% higher and fat content was 0.4% U lower in the EF group than in the controls, with no differences in fat and protein yields. Energy balance in the EF cows was more positive than in the controls; however, no differences were observed in concentrations of nonesterified fatty acids and glucose in plasma. Compared with controls, EF cows had greater proportions of C18:3n-3 in plasma and adipose tissue. The proportion of n-3 FA in milk fat was 3.7-fold higher in the EF cows, and the n-6:n-3 ratio was decreased from 8.3 in controls to 2.3 in the EF cows. Within-group tests revealed that the C18:3n-3 content in milk fat in the EF cows was negatively correlated with milk fat percentage (r = –0.91) and yield (r = –0.89). However, no decrease in de novo synthesis of less than 16-carbon FA was found in the EF group, whereas C16:0 yields were markedly decreased. It appears that the enrichment of C18:3n-3 in milk fat was limited to approximately 2%, and the potential for increasing this n-3 FA in milk is higher for cows with lower milk fat contents. In conclusion, feeding increased amounts of C18:3n-3 during the transition period enhanced dry matter intake postpartum, increased milk production, decreased milk fat content, and improved energy balance. Increased amounts of EF considerably influenced the FA profile of plasma, adipose tissue, and milk fat. However, the extent of C18:3n-3 enrichment in milk fat was limited and was negatively correlated with milk fat content and yield.     

Effects of dietary deoiled soy lecithin supplementation on milk production and fatty acid digestibility in Holstein dairy cows

Deoiled soy lecithin is a feed additive enriched in phospholipids. Our study evaluated the effects of dietary deoiled soy lecithin supplementation on (1) milk production and composition, (2) plasma and milk fatty acid (FA) content and yield, and (3) apparent FA digestibility and absorption in lactating dairy cows fed fractionated palm fat. In a split-plot Latin square design, 16 Holstein cows (160 ± 7 days in milk; 3.6 ± 1.2 parity) were randomly allocated to a main plot receiving a corn silage and alfalfa haylage-based diet with palm fat containing either moderate (MPA) or high palmitic acid (HPA) content at 1.75% of ration dry matter (72 or 99% palmitic acid, respectively; n = 8/palm fat diet). On each palm fat diet, deoiled soy lecithin was top-dressed at 0, 0.12, 0.24, or 0.36% of ration dry matter in a replicated 4 × 4 Latin square design. Following a 14-d covariate period, lecithin supplementation spanned 14 d, with milk and blood collected during the final 3 d. Milk composition and pooled plasma markers were measured. The statistical model included the fixed effects of palm fat type, lecithin dose, period, and the interaction between palm fat type and lecithin dose. The random effect of cow nested within palm fat group was also included. Lecithin linearly decreased dry matter intake. In cows fed HPA, lecithin feeding reduced milk fat content and tended to decrease milk fat yield. Although no changes in milk yield were observed, a quadratic reduction in 3.5% fat-corrected milk was observed with increasing lecithin dose. Lecithin linearly increased energy-corrected milk efficiency in cows fed MPA. Lecithin supplementation also decreased milk urea nitrogen, relative to unsupplemented cows. The proportion of 16-carbon FA in milk fat decreased linearly with lecithin dose, whereas 18-carbon FA increased linearly. Lecithin reduced de novo FA (<16-carbon) content and tended to increase preformed FA (>16-carbon) content in a linear manner. Compared with MPA, HPA diets reduced apparent total and 16-carbon FA digestibility and absorption. Deoiled soy lecithin feeding did not modify FA digestibility or absorption. Our observations suggest that soy lecithin feeding modifies rumen digestion to reduce dry matter intake and change milk composition.     

Effects of monensin and dietary soybean oil on milk fat percentage and milk fatty acid profile in lactating dairy cows

The objective of this study was to investigate the effect of monensin (MN) and dietary soybean oil (SBO) on milk fat percentage and milk fatty acid (FA) profile. The study was conducted as a randomized complete block design with a 2 × 3 factorial treatment arrangement using 72 lactating multiparous Holstein dairy cows (138 ± 24 d in milk). Treatments were [dry matter (DM) basis] as follows: 1) control total mixed ration (TMR, no MN) with no supplemental SBO; 2) MN-treated TMR (22 g of MN/kg of DM) with no supplemental SBO; 3) control TMR including 1.7% SBO; 4) MN-treated TMR including 1.7% SBO; 5) control TMR including 3.4% SBO; and 6) MN-treated TMR including 3.4% SBO. The TMR (% of DM; corn silage, 31.6%; haylage, 21.2%; hay, 4.2%; high-moisture corn, 18.8%; soy hulls, 3.3%; and protein supplement, 20.9%) was offered ad libitum. The experiment consisted of a 2-wk baseline, a 3-wk adaptation, and a 2-wk collection period. Monensin, SBO, and their interaction linearly reduced milk fat percentage. Cows receiving SBO with no added MN (treatments 3 and 5) had 4.5 and 14.2% decreases in milk fat percentage, respectively. Cows receiving SBO with added MN (treatments 4 and 6) had 16.5 and 35.1% decreases in milk fat percentage, respectively. However, the interaction effect of MN and SBO on fat yield was not significant. Monensin reduced milk fat yield by 6.6%. Soybean oil linearly reduced milk fat yield and protein percentage and linearly increased milk yield and milk protein yield. Monensin and SBO reduced 4% fat-corrected milk and had no effect on DM intake. Monensin interacted with SBO to linearly increase milk fat concentration (g/100 g of FA) of total trans-18:1 in milk fat including trans-6 to 8, trans-9, trans-10, trans-11, trans-12 18:1 and the concentration of total conjugated linoleic acid isomers including cis-9, trans-11 18:2; trans-9, cis-11 18:2; and trans-10, cis-12 18:2. Also, the interaction increased milk concentration of polyunsaturated fatty acids. Monensin and SBO linearly reduced, with no significant interaction, milk concentration (g/100 g of FA) of short- and medium-chain fatty acids (<C16). Soybean oil reduced total saturated FA and increased total monounsaturated FA. These results suggest that monensin reduces milk fat percentage and this effect is accentuated when SBO is added to the ration.     

Effects of heat stress on energetic metabolism in lactating Holstein cows

Heat stress has an enormous economic impact on the global dairy industry, but the mechanisms by which hyperthermia negatively affect systemic physiology and milk synthesis are not clear. Study objectives were to evaluate production parameters and metabolic variables in lactating dairy cows during short-term heat stress or pair-fed conditions coupled with bST administration. Twenty-two multiparous Holstein cows were subjected to 3 experimental periods: 1) thermoneutral conditions with ad libitum intake for 7 d (P1); 2) heat stress (HS) with ad libitum intake (n = 10) or pair-fed (PF) in thermoneutral conditions (n = 12) for 7 d (P2), and 3) 7 d of HS or PF in conditions as described in P2 with recombinant bovine somatotropin administered on d 1 (P3). All cows received an intravenous glucose tolerance test (GTT) on d 5 of each period. Heat stress conditions were cyclical and temperatures ranged from 29.4 to 38.9°C. Rectal temperatures and respiration rates increased during heat stress (38.6-40.4°C and 44-89 breaths/min, respectively). Heat stress reduced dry matter intake by 30% and by design PF cows had similar intake reductions (28%). During heat stress and pair-feeding, milk yield decreased by 27.6% (9.6 kg) and 13.9% (4.8 kg), respectively, indicating that reduced feed intake accounted for only 50% of the decreased milk production. Milk yield increased with recombinant bovine somatotropin in both HS (9.7%) and PF (16.1%) cows. Cows in both groups were in positive energy balance (3.95 Mcal/d) during P1 but entered negative energy balance during P2 and P3 (−5.65 Mcal/d). Heat stress and pair-feeding treatments decreased (9.3%) basal glucose concentrations. Heat stress conditions had no effect on basal NEFA levels during P2; however, PF cows (despite a similar calculated energy balance) had a 2-fold increase in basal NEFA concentrations. Both groups had increased plasma urea nitrogen levels during P2 and P3 compared with P1. Basal insulin levels increased (37%) during P2 and P3 in HS cows but did not differ between periods in PF cows. During P2 and compared with P1, PF cows had a decreased rate of glucose disposal, whereas HS cows had a similar disposal rate following the GTT. During P2 and compared with P1, PF cows had a reduced insulin response whereas HS cows had a similar insulin response to the GTT. In summary, reduced nutrient intake accounted for only 50% of heat stress-induced decreases in milk yield, and feed intake-independent shifts in postabsorptive glucose and lipid homeostasis may contribute to the additional reduction in milk yield.