Effects of lipid and propionic acid infusions on feed intake of lactating dairy cows

Propionic acid is more hypophagic for cows with elevated hepatic acetyl coenzyme A (CoA) concentration in the postpartum period. The objective of this experiment was to evaluate the interaction of hepatic acetyl CoA concentration, which is elevated by intravenous lipid infusion, and intraruminal propionic acid infusion on feed intake and feeding behavior responses of lactating cows. Eight multiparous, ruminally cannulated, Holstein dairy cows past peak lactation were used in a replicated 4 × 4 Latin square experiment with a 2 × 2 factorial arrangement of treatments. Treatments were propionic acid (PI) infused intraruminally at 0.5 mol/h for 18 h starting 6 h before feeding and behavior monitoring or sham control (CO), and intravenous jugular infusion of lipid (LI, Intralipid 20%; Baxter US, Deerfield, IL) or saline (SI, 0.9% NaCl; Baxter US) infused at 250 mL/h for 12 h before feeding and behavior monitoring, and then 500 mL/h for 12 h after feeding. Changes in plasma concentrations of metabolites and hormones and hepatic acetyl CoA from before infusion until the end of infusion were evaluated. We observed a tendency for an interaction between PI and LI for the change in plasma nonesterified fatty acid (NEFA) concentration from the preliminary day to the end of the infusion period. Infusion of propionic acid decreased dry matter intake (DMI) 15% compared with CO, but lipid infusion did not affect DMI over the 12 h following feeding. Infusion of propionic acid tended to decrease hepatic acetyl CoA concentration from the preliminary day to the end of the infusion compared with CO, consistent with PI decreasing DMI by stimulating oxidation of acetyl CoA. Contrary to our expectations, LI did not increase concentration of NEFA or β-hydroxybutyrate in plasma, concentration of acetyl CoA in the liver, or milk fat yield, suggesting that the infused lipid was stored or oxidized by extra-hepatic tissues. As a result, we detected no interaction between PI and LI for DMI. Although the effect of PI on DMI was consistent with our previous results, this lipid infusion model using cows past peak lactation was not useful to simulate the lipolytic state of cows in the postpartum period in this experiment.     

Patterns of nutrient uptake by the mammary glands of lactating dairy cows.

Twenty-one multiparous lactating dairy cows with previous 305-d milk production records varying from 5900 to 13,600 kg were used to examine patterns of nutrient uptake by the mammary glands. On d 71 and continuing until d 126 of lactation, animals were injected daily with 40 mg of sometribove (bST group) or bicarbonate buffer (control group). Arterial and venous blood plasma samples were collected over a 12-h period on d 35, 70, 105, and 126 of lactation. Regression equations developed to evaluate linear effects of plasma arterial concentrations on net arterial-venous difference across the mammary glands demonstrated that, for acetate, NEFA, and D-beta-hydroxybutyrate, plasma arterial concentration accounted for over 50% of variation in uptake by the mammary glands. Additionally, a sigmoidal equation fitted the relationship between D-beta-hydroxybutyrate plasma arterial concentration and mammary gland uptake (r2 = .70). Triacylglyceride concentration was less effective in predicting uptake (r2 = .25). Administration of bST did not alter patterns of nutrient uptake, but a fourfold increase in NEFA uptake was predicted for bST-treated cows from this study, using NEFA concentrations from the literature. These observations indicate that plasma concentrations of acetate, NEFA, D-beta-hydroxybutyrate, and triacylglyceride are major determinants of uptake by the mammary glands. Factors other than plasma concentration, such as mammary gland biosynthetic capacity, availability of other nutrients, and blood flow, determine uptakes of glucose, lactate, and total and free cholesterol (r2 less than or equal to .03).     

Predicting manure volatile solid output of lactating dairy cows

Organic matter (OM) in livestock manure consisting of biodegradable and nonbiodegradable fractions is known as volatile solids (VS). According to the Intergovernmental Panel on Climate Change (IPCC) Tier 2 guidelines, methane produced by stored manure is determined based on VS. However, only biodegradable OM generates methane production. Therefore, estimates of biodegradable VS (dVS; dVS = VS ? lignin) would yield better estimates of methane emissions from manure. The objective of the study was to develop mathematical models for estimating VS and dVS outputs of lactating dairy cows. Dry matter intake, dietary nutrient contents, milk yield and composition, body weight, and days in milk were used as potential predictor variables. Multicollinearity, model simplicity, and random study effects were taken into account during model development that used 857 VS and dVS measurements made on individual cows (kg/cow per day) from 43 metabolic trials conducted at the USDA Energy and Metabolism laboratory in Beltsville, Maryland. The new models and the IPCC Tier 2 model were evaluated with an independent data set including 209 VS and dVS measurements (kg/cow per day) from 2 metabolic trials conducted at the University of California, Davis. Organic matter intake (kg/d) and dietary crude protein and neutral detergent fiber contents (% of dry matter) were significantly associated with VS. A new model including these variables fitted best to data. When evaluated with independent data, the new model had a root mean squared prediction error as a percentage of average observed value (RMSPE) of 12.5%. Mean and slope biases were negligible at <1% of total prediction bias. When energy digestibility of the diet was assumed to be 67%, the IPCC Tier 2 model had a RMSPE of 13.7% and a notable mean bias for VS to be overpredicted by 0.4 kg/cow per day. A separate model including OM intake as well as dietary crude protein and neutral detergent fiber contents as predictor variables fitted best to dVS data and performed well on independent data (RMSPE = 12.7%). The Cornell Net Carbohydrate and Protein System model relying on fat-corrected milk yield and body weight more successfully predicted dry matter intake (DMI; RMSPE = 14.1%) than the simplified (RMSPE = 16.9%) and comprehensive (RMSPE = 23.4%) models to predict DMI in IPCC Tier 2 methodology. New models and the IPCC Tier 2 model using DMI from the Cornell Net Carbohydrate and Protein System model predicted VS (RMSPE = 17.7–19.4%) and dVS (RMSPE = 20%) well with small systematic bias (<10% of total bias). The present study offers empirical models that can accurately predict VS and dVS of dairy cows using routinely available data in dairy farms and thereby assist in efficiently determining methane emissions from stored manure.     

Metaanalysis of input/output kinetics in lactating dairy cows

The kinetics of dry matter intake, raw milk yield, milk fat yield, and body weight of 140 treatment groups of cows reported in 37 articles published between 1959 and 1999 were collected in a database. The majority of these experiments were performed with Holstein-Friesian cows (primiparous: n = 51, multiparous: n = 89). Animals were fed with forages and concentrates, offered separately (n = 98) or as a total mixed ration (n = 42). Empty body weight was calculated assuming that an increase of 1 kg in dry matter intake corresponded to an increase of 4 kg in gastrointestinal tract content. Each kinetic, denoted Y, was fitted with the model proposed by Grossman (1999) for raw milk yield. This model allowed the partitioning of the kinetic into three stages (early, middle, and late stages of lactation), each characterized by its own linear tendency. Kinetics were therefore summarized with parameter estimates of the model: the plateau value of Y in middle stage, the slopes of Y in early and late stages, and the weeks of start and end of the middle stage. The following additional variables were also calculated: the total increase of Y in early stage, the extrapolated value of Y at calving, and the persistency of Y defined as the duration of the middle stage. Variability of these parameters for each kinetic across treatment groups was examined through mixed models including the year of publication as covariate, parity as a fixed between-group factor, and trial as a random factor. Correlations between parameters within a Y-kinetic and between different Y-kinetics were calculated by parity group and feed allocation system. Finally we propose predictive models which emphasized (1) the effect of empty body weight at calving on the depletion of empty body weight in early lactation, (2) the incidence of the kinetic of dry matter intake on the kinetic of raw milk yield in middle and late stages of lactation, and (3) the relationships between the kinetics of raw milk yield and milk fat yield in middle and late stages of lactation.     

Effects of bovine somatotropin on beta-casein mRNA levels in mammary tissue of lactating cows

Bovine somatotropin (bST) increases milk production in lactating cows through its effect on nutrient partition and maintenance of mammary cell function. A positive relationship between bST treatment and abundance of β-casein mRNA in mammary tissues from lactating cows was hypothesized. In mammary tissue isolated from 14 midlactation Holstein cows, β-casein mRNA was 35.4% higher among 7 cows receiving continuous bST infusions at 29 mg/d for 63 d compared with tissue from 7 untreated control cows. To investigate whether increased β-casein mRNA resulted from a direct effect of bST on the mammary gland, explants of mammary tissue from other lactating cows that had not received bST were incubated with bST and prolactin in 2 experiments. Mammary explant cultures taken from 2 lactating cows that had not been milked for 48 h were supplemented with either prolactin or bST. Both prolactin and bST stimulated higher levels of β-casein mRNA in the mammary explants compared with their non-supplemented counterparts. Explant cultures from 4 additional lactating cows were prepared from rear quarter mammary tissue subjected to milking intervals of 6 h for right rear quarters or 20 h for left rear quarters. Both bST- and prolactin-mediated increases in β-casein mRNA were dependent on milking intervals. That is, levels of β-casein mRNA were increased by bST or prolactin supplementation in explants isolated from the mammary quarters biopsied 20 h after milking but not for those biopsied at 6 h after milking. Results are consistent with a potential role for bST in up-regulating or sparing β-casein mRNA levels in lactating bovine mammary tissue in a manner similar to prolactin.     

Whole-body glucose metabolism and mammary energetic nutrient metabolism in lactating dairy cows receiving digestive infusions of casein and propionic acid

This study analyzed the effect of propionate (C3) and casein (CN) on whole-body and mammary metabolism of energetic nutrients. Three multiparous Holstein cows fitted with both duodenal and ruminal cannulas were used in 2 replicated Youden squares with 14-d periods. Effects of CN (743 g/d in the duodenum) and C3 (1,042 g/d in the rumen) infusions, either separately or in combination as supplements to a grass silage diet, were tested in a factorial arrangement. The control diet provided 97% of energy and protein requirements. Within each period, blood samples were taken (d 11) from the carotid artery and the right mammary vein to determine net uptake of energetic nutrients. Plasma blood flow was calculated using the Fick principle (based on Phe and Tyr). On d 13, [6,6-²H₂]glucose was infused in the jugular vein to determine whole-body glucose rate of appearance (Ra) based on enrichments in arterial plasma. Both C3 and CN treatments increased whole-body Ra (17% and 13%, respectively) but only CN increased milk (18%) and lactose (14%) yields, suggesting no direct link between whole-body Ra and milk yield. When CN was infused alone, the apparent ratio of conversion of CN carbon into glucose carbon was 0.31 but, when allowance was made for the CN required to support the extra milk protein output, the ratio increased to 0.40, closer to the theoretical ratio (0.48). This may relate to the observed increases in arterial glucagon concentrations for CN alone. Conversely, the apparent conversion of infused C3 carbon alone to glucose was low (0.31). With C3, mammary plasma flow increased as did uptakes of lactate, Ala, and Glu whereas the uptake for β-hydroxybutyrate (BHBA) decreased. Mammary net carbon balance suggested an increase with C3 treatment in glucose, lactate, Ala, and Glu oxidation within the mammary gland. Mammary glucose uptake did not increase with CN treatment, despite an increase in glucose arteriovenous difference and extraction rate, because plasma flow decreased (−17%). Whereas CN, alone or in combination with C3, increased both lactose and protein yields, only mammary AA (and BHBA in CN alone) uptake increased because plasma flow decreased (−17%). These data suggest that the observed variations of milk lactose yield (and other milk components) are linked to metabolic interchanges between several energetic nutrients at both the whole-body and mammary levels and are not explained by increases in whole-body glucose availability.     

Assessing amino acid uptake and metabolism in mammary glands of lactating dairy cows intravenously infused with methionine,lysine, and histidine or with leucine and isoleucine

The objective of this study was to evaluate the effect of jugular infusions of 2 groups of AA on essential AA (EAA) transport and metabolism by mammary glands. Four Holstein cows in second lactation (66 ± 10 d in milk) were used in 4 × 4 Latin square design with a 2 × 2 factorial arrangement of treatments. Treatments were jugular infusions of saline; Met, Lys, and His (MKH); Ile and Leu (IL); or MKH plus IL (MKH+IL). Each period consisted of 8 d of no infusion followed by 8 d of jugular vein infusion of the treatment solutions. Amino acids were infused at rates of 21 g of Met, 38 g of Lys, 20 g of His, 50 g of Leu, and 22 g of Ile per day. Cows were fed a basal diet consisting of 15.2% crude protein with adequate rumen degradable protein but 15% deficient in MP based on estimates by Cornell Net Carbohydrate and Protein System (v6.5). On the last day of each period, ¹³C-AA derived from algae was infused into the jugular vein over 6 h, and blood and milk samples were collected before, during, and after infusion. Plasma and milk samples were analyzed for AA isotopic enrichment, and a mammary compartmental model was fitted to the data to derive bidirectional transport and metabolism rates for individual EAA. Influx of Leu increased with IL, whereas influx of other EAA was not different among treatments. Cellular efflux of Met and Lys to venous plasma represented 12 to 34% of influx, whereas cellular efflux of Phe and BCAA represented 29 to 59% of influx. Increased efflux/influx ratios of Ile and Leu with IL but not Met and Lys with MKH demonstrated that increased Ile and Leu influx was mostly returned to plasma resulting in no change in net uptake or efficiency. The isotope results showed that mammary net uptake of Lys and Ile increased during MKH infusion. Net uptake of Met increased with MKH but only in the absence of IL. Catabolism of Lys and Met only increased with MKH alone, resulting in decreased efficiency for milk protein, which demonstrated that Ile and Leu infusion can spare Lys and Met for milk protein synthesis. Total AA uptake to milk output was not different from 1, implying the catabolized Met and Lys contributed nitrogen to nonessential AA. Overall, EAA uptake and metabolism in mammary glands of dairy cows varied across individual EAA and responded differently to respective AA supplements. In addition, uptake, retention, and end use of AA by mammary tissue is variable and dependent on the mix of AA provided. This variability, depending on the mix of AA absorbed, will change the efficiency of utilization of individual AA at the mammary gland level and consequently the whole-body level. Thus, it is inaccurate to use a fixed, constant efficiency within and across AA to represent tissue activity.     

Effects of rumen or duodenal glucose infusions on intake in dairy cows fed fresh perennial ryegrass indoors

The aim of this study was to determine whether the intake of fresh highly digestible ryegrass could be limited by the total amount of energy absorbed. Moreover, it investigated whether the limitation was more specific to energy absorbed as volatile fatty acids in the rumen compared with energy absorbed in the lower gastrointestinal tract. Four treatments were compared: infusion of 1.25 kg of glucose into the rumen (R1.25), infusion of 2.5 kg of glucose into the rumen (R2.5), infusion of 1.5 kg of glucose into the duodenum (D1.5), and a control treatment consisting of water and salts. Treatments R2.5 and D1.5 were assumed to supply about 16.5 MJ of net energy for lactation. All treatments consisted of 2 infusions, one into the rumen and the other into the duodenum, with one of these infusions being a control. All infused solutions were isoosmotic with osmolarities around 340 and 330 mmol/L for rumen and duodenum, respectively. Treatments were compared using 4 dairy cows in mid lactation according to a 4 × 4 Latin square design replicated twice during 8 periods of 7 d each. Cows were housed in tie stalls and fed ad libitum with fresh perennial ryegrass cut every morning during the spring at 28 d of regrowth. Intake and feeding behavior were measured, as well as concentrations of ruminal fermentation products and some blood metabolites. The pepsin-cellulase organic matter digestibility of the offered herbage averaged 0.76 ± 0.011. The average dry matter intake of herbage was 15.5 ± 0.52 kg/d. The glucose infusions decreased dry matter intake by 0.95 kg/d compared with the control, but had the same satiating effect regardless of site or dose of infusion. The average concentration of volatile fatty acids in rumen fluid was 97.9 ± 2.03 mmol/L and the molar proportion of propionate was 21.6 ± 0.19 mmol/100 mmol. Glucose infusions into the rumen led to a decrease in the molar proportions of acetate from 64.4 on the control treatment to 60.9 mmol/100 mmol on R2.5 and increased the molar proportions of butyrate from 10.2 (control) to 13.5 mmol/100 mmol on R2.5, and minor acids (valerate and caproate), from 1.27 (control) to 2.54 mmol/100 mmol on R2.5, proportionally to the dose infused. These results suggested that energy nutrients can limit intake in dairy cows fed high-digestibility ryegrass and that butyrate and minor acids would have a limited satiating effect compared with propionate.     

Effects of varying dietary protein and energy levels on the production of lactating dairy cows

Forty-five multiparous and 18 primiparous Holstein cows were fed three levels of crude protein (CP), each at three levels of neutral detergent fiber (NDF), to identify optimal dietary CP and energy. Cows were blocked by parity and days in milk into seven groups of nine and randomly assigned to an incomplete 9 x 9 Latin square trial with four, 4-wk periods. Diets were formulated from alfalfa and corn silages, high-moisture corn, soybean meal, minerals, and vitamins. Forage was 60% alfalfa and 40% corn silage on all diets; NDF contents of 36, 32, and 28% were obtained by feeding 75, 63, and 50% forage, respectively. Dietary CP contents of 15.1, 16.7, and 18.4% were obtained by replacing high-moisture corn with soybean meal. Production data were from the last 2 wk of each period. Spot fecal and urine samples were collected from 36 cows to estimate N excretion using fecal indigestible acid detergent fiber (ADF) and urinary creatinine as markers. There were no interactions (P > or = 0.08) between dietary CP and NDF for any trait; thus, effects of CP were not confounded by NDF or vice versa. Intake of DM and fat yield were lower on 15.1% CP than at higher CP. There were linear increases in milk urea and urinary N excretion and linear decreases in N efficiency with increasing CP. Increasing CP from 15.1 to 18.4% reduced milk N from 31 to 25% of dietary N, increased urinary N from 23 to 35% of dietary N, and reduced fecal N from 45 to 41% of dietary N. Decreasing NDF gave linear increases in BW gain, yield of milk, protein, true protein, lactose, and SNF, and milk/DM intake and milk N/N intake, and linear decreases in milk urea. However, fat yield was lower on 28% than 32% NDF. Reducing NDF from 36 to 28% increased purine derivative excretion by 19%, suggesting increased microbial protein. Increasing CP by adding soybean meal to diets fed cows averaging 34 kg/d of milk increased intake and fat yield but depressed N efficiency. Increasing dietary energy by reducing forage improved milk yield and efficiency and decreased excretion of environmentally labile urinary N.     

Effect of casein and propionate supply on mammary protein metabolism in lactating dairy cows

The effects of casein (CN) and propionate (C3) on mammary AA metabolism were determined in 3 multiparous Holstein cows fitted with both duodenal and ruminal cannulas and used in a replicated Youden square with six 14-d periods. Casein (743 g/d in the duodenum) and C3 (1,041 g/d in the rumen) infusions were tested in a factorial arrangement. For each period, l-[1-¹³C]Leu (d 11) and NaH[¹³C]O₃ (d 13) were infused into a jugular vein, and blood samples were taken from the carotid artery and the mammary vein to determine Leu kinetics and net uptake of AA. Both CN and C3 treatments separately increased milk protein concentration and yield. With CN there was a general response in mammary protein metabolism, involving increases in Leu net uptake (30%), the uptake:output ratio (8%), protein synthesis (11%), secretion in milk protein (21%), and oxidation (259%). In contrast, C3 treatments tended to increase only Leu in milk protein (7%) and, when in combination with CN, to reduce Leu used for protein synthesis (5%). Across all treatments, most Leu uptake by the mammary gland was accounted for as Leu in milk or oxidized, and the Leu balance was therefore achieved without involvement of either net peptide use or production. Mammary uptake of group 1 AA increased to match milk output with all infusions. In contrast, mammary uptake of group 2 AA exceeded output to a greater extent with CN than with C3 infusions, whereas the increment in uptake of group 3 AA increased with C3 treatments. Overall, these data suggest that different mechanisms operate to improve milk protein production when either protein or energy is supplied.     

Effects of abomasal infusions of fatty acids and one-carbon donors on hepatic ceramide and phosphatidylcholine in lactating Holstein dairy cows

Our objectives were to (1) determine whether the abomasal infusion of behenic acid (C22:0) elevated hepatic ceramide relative to palmitic acid (C16:0) or docosahexaenoic acid (C22:6n-3) infusion; (2) assess whether the abomasal infusion of choline chloride or l-serine elevated hepatic phosphatidylcholine (PC) in cows abomasally infused with C16:0; and (3) characterize the PC lipidome in cows abomasally infused with C22:6n-3, relative to C16:0 or C22:0 infusion. In a 5 × 5 Latin square design, 5 rumen-cannulated Holstein cows (214 ± 4.9 DIM; 3.2 ± 1.1 parity) were enrolled in a study with 6-d periods. Abomasal infusates consisted of (1) palmitic acid (PA; 98% C16:0); (2) PA + choline chloride (PA+C; 50 g/d choline chloride); (3) PA + l-serine (PA+S; 170 g/d l-serine); (4) behenic acid (BA; 92% C22:0); and (5) an algal oil rich in docosahexaenoic acid (DHA; 44% C22:6n-3). Emulsion infusates provided 301 g/d of total fatty acids containing a minimum of 40 g/d of C16:0. Cows were fed a corn silage-based diet. Milk was collected on d ?2, ?1, 5, and 6. Blood was collected and liver biopsied on d 6 of each period. Although we did not detect differences in milk yield, milk fat yield and content were lower in cows infused with DHA relative to PA. Plasma triacylglycerol concentrations were lower with DHA treatment relative to PA or BA. Cows infused with DHA had lower plasma insulin concentrations relative to cows infused with PA only. For objective 1, hepatic ceramide-d18:2/22:0 was highest in cows infused with BA relative to other treatments. For objective 2, plasma free choline concentrations were greater in PA+C cows relative to PA; however, we did not observe this effect with PA+S. Plasma total PC concentrations were similar for all treatments. Regarding the hepatic lipidome, a total of 18 hepatic PC were higher (e.g., PC-16:1/18:2) and 25 PC were lower (e.g., PC-16:0/22:6) with PA+C infusion relative to PA. In addition, 17 PC were higher (e.g., PC-20:3/22:5) and 21 PC were lower (e.g., PC-18:0/22:6) with PA+S infusion relative to PA. For objective 3, hepatic concentrations of many individual saturated PC (e.g., PC-18:0/15:0) were lower with DHA relative to other treatments. Hepatic concentrations of highly unsaturated PC with very-long-chain fatty acids (e.g., PC-14:0/22:6) were higher in DHA-infused cows relative to PA, PA+C, PA+S, or BA. The abomasal infusion of emulsions containing palmitic acid, palmitic acid with choline chloride or serine, behenic acid, or docosahexaenoic acid influence the hepatic ceramide and PC profiles of lactating cows.     

Effects of heat stress and dietary zinc source on performance and mammary epithelial integrity of lactating dairy cows

Dietary Zn and heat stress alter gut integrity in monogastric animals. However, effects of Zn on mammary epithelial integrity in heat-stressed lactating dairy cows have not been studied. Multiparous lactating Holstein cows (n = 72) were randomly assigned to 1 of 4 treatments with a 2 × 2 factorial arrangement to study the effects of environment and Zn source on performance and mammary epithelial integrity. Treatments included 2 environments [cooled (CL) or not cooled (NC)] and 2 Zn sources [75 mg/kg of supplemental Zn as Zn hydroxychloride (IOZ) or 35 mg/kg of Zn hydroxychloride + 40 mg/kg of Zn-Met complex (ZMC)]. The experiment was divided into baseline and environmental challenge phases of 84 d each. All cows were cooled during the baseline phase (temperature-humidity index = 72.5), whereas NC cows were not cooled during environmental challenge (temperature-humidity index = 77.7). Mammary biopsies were collected on d 7 and 56 relative to the onset of environmental challenge to analyze gene expression of claudin 1, 4, and 8, zonula occludens 1, 2, and 3, occludin, and E-cadherin and protein expression of occludin and E-cadherin. Deprivation of cooling increased respiration rate (64.8 vs. 73.9 breaths/min) and vaginal temperature (39.03 vs. 39.94°C) and decreased dry matter intake (26.7 vs. 21.6 kg/d). Energy-corrected milk yield decreased for NC cows relative to CL cows (24.5 vs. 34.1 kg/d). An interaction between environment and Zn source occurred for milk fat content as CL cows fed ZMC had lower milk fat percentage than other groups. Relative to CL cows, NC cows had lower concentrations of lactose (4.69 vs. 4.56%) and solids-not-fat (8.46 vs. 8.32%) but a higher concentration of milk urea nitrogen (9.07 vs. 11.02 mg/mL). Compared with IOZ, cows fed ZMC had lower plasma lactose concentration during baseline and tended to have lower plasma lactose concentration during environmental challenge. Plasma lactose concentration tended to increase at 3, 5, and 41 d after the onset of environmental challenge in NC cows relative to CL cows. Treatment had no effect on milk BSA concentration. Cows fed ZMC tended to have higher gene expression of E-cadherin relative to IOZ. Compared with CL, NC cows had increased gene expression of occludin and E-cadherin and tended to have increased claudin 1 and zonula occludens 1 and 2 gene expression in the mammary gland. Protein expression of occludin and E-cadherin was unchanged. In conclusion, removing active cooling impairs lactation performance and affects gene expression of proteins involved in the mammary epithelial barrier, and feeding a portion of dietary zinc as ZMC improves the integrity of the mammary epithelium.     

Effects of feed intake and thermal stress on mammary blood flow and other physiological measurements in lactating dairy cows

Six midlactation Holstein cows were exposed to treatments of thermal comfort environments with ad libitum or restricted (70% of ad libitum) DM intake and a thermal stress environment with ad libitum intake in two balanced 3 X 3 Latin squares to evaluate effects on mammary blood flow and other physiological measurements. Daily DM intake decreased from 17.8 kg in thermal comfort with ad libitum intake to 12.5 kg in thermal comfort with restricted intake and to 14.8 kg in thermal stress. Daily milk production decreased from 23.9 kg in thermal comfort ad libitum to 22.2 kg in thermal comfort restricted and 21.6 kg in thermal stress. Mammary blood flows (half udder) over the entire sampling interval (18 h) were 5.1, 4.3, and 4.5 L/min for treatments: thermal comfort, ad libitum intake; thermal comfort, restricted intake; and thermal stress, ad libitum intake. Mammary blood flows did not differ significantly among between treatments. Relationship of hald udder blood flow (L/min) to previous day's DM intake (kg/d) was described by the linear equation: L/min = .49 + (.27 kg/d); r2 = .46; and the quadratic equation: L/min = 6.04 - (.54 kg/d) + (.03 [kg/d]2); r2 = .55. Mammary blood flows 10 min before through 10 min after milking machine attachment were greater for cows at thermal comfort and ad libitum intake than for cows at thermal comfort and restricted intake or thermal stress and ad libitum intake (5.5, 4.8, and 4.8 L/min).     

Effects of lasalocid on performance of lactating dairy cows.

Sixty lactating dairy cows (30 multiparous and 30 primiparous) were used in a completely randomized block design to determine the effect of lasalocid supplementation on dairy cow performance. Starting wk 2 prepartum and lasting through wk 17 of lactation, cows received one of three experimental diets. The experimental total mixed rations were control (CD), control + 10 mg/kg of lasalocid (CD + 10) and control + 20 mg/kg of lasalocid (CD + 20). The alfalfa-based control diet (40:60; forage:concentrate) was formulated to contain 18% crude protein, 35% nonstructural carbohydrates, 31% neutral detergent fiber, and 6.6% ether extract. Lasalocid supplementation linearly decreased dry matter intake (DMI) without affecting milk production or milk composition. Mean milk production and percentages of fat and protein were 30.0, 30.8, and 28.6; 3.56, 3.51, and 3.63; 3.06, 3.05, and 3.09; respectively for treatments CD, CD + 10, and CD + 20. Lasalocid supplementation decreased milk urea N (MUN) when compared to control cows, and increasing supplementation caused a significant linear decrease in MUN. For the primiparous cows, lasalocid supplementation decreased DMI and MUN while increasing body condition score and feed efficiency. Results from this study indicate that lactating dairy cows and, in particular, primiparous cows may benefit from lasalocid supplementation in terms of more efficient utilization of nutrients for milk production, reduced MUN levels, reduced body condition loss, and higher margin over feed cost.     

Effect of graded duodenal infusions of glucose on yield and composition of milk from dairy cows. 2. Diets based on grass silage

We conducted two trials to study the effect of graded amounts of glucose infused into the duodenum on milk yield and composition as well as on plasma metabolites, using diets based on grass silage. In trial 1, four fistulated Holstein cows were arranged in a 4 x 4 Latin square design and received 0, 750, 1500, and 2250 g of glucose/d in the duodenum. In trial 2, five fistulated Holstein cows were arranged in a 5 x 5 Latin square design and received 0, 250, 500, 1000, and 2000 g/d of glucose. In both trials, cows were fed a basal diet of 38% grass silage, 10% dehydrated alfalfa, 49% energy concentrate, and 3% oil meal. The treatments (feed plus infusion) were isoenergetic and isonitrogenous. Increased amounts of glucose increased milk yield up to 2.4 and 1.6 kg/d in trials 1 and 2, respectively. Lactose content was not affected, while fat yield and content decreased linearly. The decrease in milk fat resulted from a reduced yield of C18, probably caused by a lower mobilization of fat. The glucose treatments significantly affected the profiles of medium-chain fatty acids, by enhancing the elongation process (up to C14). Glucose infusions induced an asymptotic response of protein yield (linear increase up to 1000 g of glucose, after ceiling). It appears that with poor postruminal starch diets, such as grass silage-based diets containing 35 to 40% of concentrate, the glucose supply to the mammary gland may be limiting for milk synthesis.     

External pudic venous reflux into the mammary vein in lactating dairy cows

A representative blood sample from the mammary vein depends on the functional integrity of the valves in the external pudic vein (EPV). To determine if the EPV valves maintain blood flow into the inguinal direction during the second and subsequent lactations, we used eight lactating cows catheterized in the EPV, the lateral branch of the cranial mammary vein (MV), and the external pudic artery (EPA). The averaged daily milk yields were 25.0 +/- 1.8 kg in cows in second lactation and 31.5 +/- 2.9 kg in older cows. The relative time taken by a pulse dose of p-amino hippuric acid (PAH) injected into the EPV, to reach the EPA and the MV, was measured in a first trial. In a second trial, we assessed the extent of alteration of the mammary PAH blood concentration with blood originating from other tissues using a continuous infusion of PAH into the EPA simultaneously with blocking or not any EPV backflux. From the first experiment, the PAH injected into the EPV appeared first in the EPA and then in the MV in cows in second lactation, suggesting that blood flow was towards the inguinal region. But in a third-lactation cow, the order of appearance was reversed. In parallel, the occlusion trial demonstrated that the concentration of PAH in the MV was diluted by 14 to 39% with blood draining nonmammary tissues only in cows in third or fourth lactation. This resulting reversed flow from the EPV towards the MV would have a detrimental impact on conclusions of mammary gland metabolism studies conducted with cows in their third lactation or higher.     

Effect of supply of metabolizable protein on whole body and splanchnic leucine metabolism in lactating dairy cows

The effect of the supply of metabolizable protein (MP) on protein metabolism across the splanchnic tissues was determined in six catheterized lactating Holstein cows. In a crossover design, two isonitrogenous (16.3% CP) diets balanced to provide a low (Lo-MP) or high (Hi-MP) supply of MP were fed over 35-d periods. After 24 d of feeding, N balance was determined over a 6-d period. On d 33, [13C] sodium bicarbonate was infused into one jugular vein for 6 h, and hourly breath samples were collected. On d 34 or 35, L[1-(13)C] leucine was infused into one jugular vein, and between 2 to 6 h of infusion, breath and blood samples were taken hourly from the portal and hepatic veins and an artery. Isotopic enrichments of plasma leucine, 4-methyl-2-oxopentanoate, and expired CO2 were determined for calculation of leucine kinetics. Net leucine absorption was greater, either on a direct basis (leucine transfer only) or corrected for portal-drained viscera metabolism to 4-methyl-2-oxopentanoate and CO2 for the Hi-MP diet. There were no effects of diet on hepatic net flux of leucine across the liver, and, thus, more leucine was available to peripheral tissues with the Hi-MP diet. Combined with an increment in portal absorption of most of essential AA, this led to increased milk protein output, although it only represented 16% of the additional available leucine. Whole body leucine oxidation was also greater for the Hi-MP diet, as was leucine used for protein synthesis. Despite these changes, MP supply did not affect irreversible loss rate of leucine by portal-drained viscera and the liver; these averaged 35 and 20% of whole body irreversible loss rate, respectively. These ratios confirm the high metabolic activity of splanchnic tissues in lactating dairy cows, which are even greater than previously reported in growing ruminants.