A model of phosphorus digestion and metabolism in the lactating dairy cow

A dynamic, mechanistic, compartmental model of phosphorus (P) digestion and metabolism was constructed in the Advanced Continuous Simulation Language using conservation of mass principles and mass action kinetics. Phosphorus was assumed to exist in 3 forms: inorganic (Pi), phytic acid (Pp), and organic (excluding phytic acid; Po). All 3 forms were assumed to be present in the digestive tract with absorption of Pi into blood. Inputs to the model were total P intake; Pp, Po, and Pi as proportions of total P; milk yield; rate of salivation (fixed at 239 L/d); and rate of liquid passage from the rumen (fixed at 198 L/d). The model was fitted to 2 experiments from the literature. Derived parameters were well defined by the data. With a mean observed P intake of 75 g/d, total tract P digestibility was 38%. Phytic acid P digestibility in the rumen was 74%, with no additional Pp digestion in the lower tract. Inorganic P and Po digestibility in the lower tract were 48 and 89%, respectively. Flows of Po and Pi from the rumen were 2.4 and 3.0 times greater than intake, respectively. The increase in Po was apparently due to microbial growth. The increase in Pi arose primarily from secretion of Pi into the rumen via salivation where 65% of absorbed P was recycled to the rumen. Milk synthesis used 30% of absorbed Pi, and 1% was excreted in urine. This research suggested that the primary regulation points for maintaining blood P were bone deposition and resorption and absorption from the intestine. However, because bone P balance was related to both dietary P intake and ruminal phytase activity, it is critical to achieve a better understanding of phytate digestibility across several feeds if dietary P is to be reduced below current requirements.     

Increasing estrus expression in the lactating dairy cow

Using an activity monitoring system (AMS) equipped with an accelerometer, 2 experiments were conducted to test the hypotheses that (1) enhancing progesterone before inducing luteolysis or (2) exposing cows to estradiol cypionate (ECP) or testosterone propionate (TP) after luteolysis would increase occurrence and intensity of estrus. Our goal was to determine if more cows could be detected in estrus by an AMS compared with other estrus-detection aids. In experiment 1, cows (n = 154) were fitted with both an AMS collar and a pressure-sensitive, rump-mounted device (HeatWatch; HW) and assigned to 3 treatments: (1) no CL + progesterone insert (CIDR) for 5 d, (2) CL only, or (3) CL + 2 CIDR inserts for 5 d to achieve a range in concentrations of progesterone. Prostaglandin F_2α was administered to all cows upon CIDR insert removal or its equivalent. Progesterone concentration up to 72 h posttreatment was greatest in CL + 2 CIDR, followed by CL only, and no CL + CIDR cows. Estrus occurred 14 to 28 h earlier in no CL + CIDR compared with CL-bearing cows. Estrus intensity was greater for CL + 2 CIDR than for CL-only cows. The AMS and HW detected 70 and 59% of cows defined to be in estrus, respectively. In experiment 2, cows (n = 203) were equipped with both an AMS and a friction-activated, rump-mounted patch (Estrotect patch) and assigned to receive 1 mg of ECP, 2 mg of TP, or control 24 h after PGF_2α. Concentrations of estradiol 24 and 48 h after treatment were greater in ECP cows compared with controls. Estrus expression detected by AMS or patches in cows defined to be in estrus tended to be greater or was greater for ECP compared with controls, respectively. Compared with controls and in response to TP or ECP, estrus occurred 8 to 18 h earlier and was of greater intensity for ECP cows, respectively. The AMS and patches determined 73 and 76% of cows defined to be in estrus, respectively. Of cows exposed to the AMS, HW, or patches, 70, 61, and 75%, respectively, were detected in estrus and more than 93% of these subsequently ovulated. In contrast, of the residual cows not detected in estrus, 62 to 77% ovulated in the absence of detected estrus. Only ECP was successful in inducing more expression and intensity of estrus, and proportions of cows detected in estrus exceeded 80%. Given the large proportion of cows equipped with AMS collars ovulating in the absence of estrus, further research is warranted to determine if more pregnancies can be achieved by inseminating those cows not detected in estrus at an appropriate time when PGF_2α is administered to induce luteolysis.     

Mustard bran in lactating dairy cow diets

Two trials using lactating Holstein cows were conducted to evaluate effects of a diet containing oriental mustard bran on dry matter intake (DMI), milk production, milk components, and organoleptic properties. In experiment 1, 34 lactating cows (24 multiparous and 10 primiparous; days in milk ≥50 d) were used in a switchback design to determine the lactational response and organoleptic quality of milk when the diet contained 8% oriental mustard bran (MB) versus a control diet (CON). Mustard bran replaced a portion of soybean meal and all the beet pulp in the CON diet. Milk yields were greater for cows fed the MB diet; however, no differences were found in DMI, 3.5% fat- (FCM) or solids-corrected milk. Milk components and components production were not affected by treatment. Milk organoleptic qualities were not affected by diet. In experiment 2, 22 lactating cows (16 multiparous and 6 primiparous; days in milk ≥21 d) were assigned randomly within parity to receive MB or CON from wk 4 to 19 postpartum in a randomized complete block design. Cows were fed CON wk 1 to 3 postpartum. The MB diet contained the same ingredients as the CON, except sunflower seed and a portion of soybean meal were replaced with mustard bran. Milk and components data were collected during wk 3 postpartum and used as covariates to adjust treatment means. Intake was greater for cows fed the MB diet; however, daily milk, 3.5% FCM, and solids-corrected milk yields were not different between diets. Milk components and component yields were not affected by treatment. Milk urea concentration was less for cows fed the MB diet. Although cows fed the MB diet had greater DMI, this was not translated into a higher milk 3.5% FCM/DMI production efficiency ratio. During experiment 2, many cows fed MB experienced minor to severe hemolysis with bloody urine. This hemolysis believed to be caused by the S-methyl-cysteine sulfoxide contained in mustard bran could have affected milk production efficiency. The increased milk yield observed in experiment 1 was not observed in experiment 2. Adding 8% of MB to lactating cow diets had a mixed effect on DMI and milk production. Milk component yields and milk quality were not affected. Feeding this level of MB presents a hemolytic danger to lactating dairy cows.     

Dietary lipid metabolism in lactating dairy cows

Effects of feeding an oil seed supplement treated with formalin upon lipid patterns of blood and synthesis of milk fat were evaluated. Percentages and yields of fatty acids of milk fat with chain lengths between 6 and 16 carbons were decreased while percentages and yields of stearate and linoleate were increased when the lipid supplement was fed. Calculations in cows fed control and supplement, 60% and 80%, respectively, of fatty acids of milk were derived from lipids of blood were supported by arterial-venous differences. Comparisons of the fatty acid compositions of triacylglycerol of plasma and milk fat suggested that triacylglycerol may not be the sole source of linoleate transferred from blood to milk fat. A preliminary evaluation of supplement effects upon lipoprotein patterns of serum indicated two peaks in the low density lipoprotein class and that the increase in total cholesterol of blood caused by feeding lipid supplement is due to increases in cholesterol content of the low density and high density lipoprotein classes.     

Hormonal alterations in the lactating dairy cow in response to thermal stress

The influence of acute exposure to thermal stress on the secretion of progesterone, estradiol, cortisol, and LH was monitored in 14 lactating Holstein cows. Eight cows were maintained throughout the summer in a refrigerated air-conditioned tie stall barn. An additional eight cows were maintained in outdoor corrals with access only to shade. Rectal temperatures and respiration rates of cows under heat stress conditions were elevated above the cows maintained under air conditioning. Cows in both environments exhibited similar serum concentrations of progesterone and estradiol throughout the estrous cycle. Serum cortisol concentrations were higher in heat-stressed cows compared to cows maintained under cooling. The number of LH pulses on d 5 of the estrous cycle was greatly reduced in the heat-stressed cows compared to the cows under cooling. No differences in the number of pulses of LH were observed on d 12 of the estrous cycle between cows in the two environments. Data suggest that heat stress can suppress anterior pituitary release of LH without having a measurable influence on ovarian steroid hormone secretion.     

Effect of monensin in lactating dairy cow diets at 2 starch concentrations

The objective of this study was to determine the effects of monensin (M) supplementation on lactation performance of dairy cows fed diets of either reduced (RS) or normal (NS) starch concentrations as total mixed rations. One hundred twenty-eight Holstein and Holstein × Jersey cows (90 ± 33 d in milk) were stratified by breed and parity and randomly assigned to 16 pens of 8 cows each in a randomized controlled trial. Pens were then randomly assigned to 1 of 4 treatments in a 2 × 2 factorial arrangement of treatments. A 4-wk covariate adjustment period preceded the treatment period, with all pens receiving NS supplemented with 18 g of monensin/t of dry matter (DM). Following the 4-wk covariate adjustment period, cows were fed their assigned treatment diets of NS with M (18 g of monensin/t), NS with 0 g of monensin/t (C), RS with M, or RS with C for 12 wk. Actual starch concentrations for the RS and NS diets were 20.4 and 26.9% (DM basis), respectively. Mean dry matter intake (DMI; 27.0 kg/d) was unaffected by the treatments. Feeding M compared with C and NS compared with RS increased milk yield by 1.3 and 1.5 kg/d per cow, respectively. Milk protein percentage and yield and lactose yield were increased and milk urea nitrogen was decreased for NS compared with RS. Feeding M increased actual and component-corrected milk feed efficiencies (component-corrected milk yield/DMI) and lactose yield and tended to increase milk urea nitrogen compared with C. Milk protein percentage was decreased for M compared with C, but milk fat percentage and yield, protein yield, and lactose percentage were unaffected by M. We observed a tendency for a starch × monensin interaction for milk feed efficiency (actual milk yield/DMI); M tended to increase efficiency more for NS than for RS. Starch and monensin had minimal effects on milk fatty acid composition and yields. Feeding RS decreased milk and protein yields, but component-corrected milk yields and feed efficiencies were similar for RS and NS. Monensin increased feed efficiency and lactation performance for both dietary starch concentrations.     

Interactions of energy and predicted metabolizable protein in determining nitrogen efficiency in the lactating dairy cow

Lactating cows are relatively inefficient in converting dietary N to milk N compared with the efficiency of N use for growth in simple-stomached animals. The majority of productive N losses occur in the postabsorptive system. The aim of the study was to test whether predicted metabolizable protein (MP) and dietary energy exerted independent effects on milk protein synthesis and postabsorptive N efficiency. If true, postabsorptive N efficiency would be expected to be greater when animals are fed high-energy diets. Forty mid-lactation cows (32 multiparous Holstein and 8 primiparous Holstein × Jersey crossbreds) were used in a complete randomized design with a 2 × 2 factorial arrangement of diets. Cows were assigned to 1 of 4 dietary treatments: high-energy, high-protein (HE/HP); high-energy, low-protein (HE/LP); low-energy, high-protein (LE/HP); and low-energy, low-protein (LE/LP). Energy concentrations were 1.55 (HE/HP and HE/LP) or 1.44 (LE/HP and LE/LP) Mcal of net energy for lactation (NE_L)/kg of dry matter (DM). Changes in predicted MP were achieved by feeding diets with 6.6 (HE/HP and LE/HP) or 4.6% (HE/LP and LE/LP) ruminally undegradable protein (DM basis). Ruminally degradable protein was held constant at 10.1% of DM. All cows were fed the HE/HP diet from d 1 to 21 followed by the respective treatments from d 22 to 43 (n = 10). Milk protein yield was reduced as dietary energy was reduced. Milk yield followed a similar pattern as milk protein yield. There was a trend for decreased milk yield as crude protein was reduced. There were no interactions between dietary energy and protein for either milk or protein yield. Plasma amino acid concentrations were not affected by treatment. Milk urea N was affected by energy and protein with a significant interaction (HE/HP = 17.2, HE/LP = 12.2, LE/HP = 21.0, LE/LP = 12.2 mg/dL). Nitrogen efficiency calculated from predicted MP supply was affected by energy and protein supplies with no apparent interaction and ranged from a low of 31% (LE/HP) to a high of 43% (HE/LP). The National Research Council model would predict N efficiency more accurately if a representation of the effects of energy on N efficiency were included in the postabsorptive system.     

Effect of glutamine supplementation on splanchnic metabolism in lactating dairy cows

The suggestion that glutamine (Gln) might become conditionally essential postpartum in dairy cows has been examined through increased postruminal supply of Gln. Net nutrient flux through the splanchnic tissues and mammary gland was measured in 7 multiparous Holstein cows receiving abomasal infusions of water or 300 g/d of Gln for 21 d in a crossover design. Milk yield increased significantly (by 3%) in response to Gln supplementation, but the 2.4% increase in milk protein yield was not statistically significant. Glutamine treatment had no effect on portal or hepatic venous blood flows. Net portal appearance of Gln and Glu was increased by Gln supplementation, accounting for 83% of the infused dose with, therefore, only limited amounts available to provide additional energy to fuel metabolism of the portal-drained viscera. The extra net portal appearance of Gln was offset, however, by a corresponding increase in hepatic removal such that net Gln splanchnic release was not different between treatments. Nonetheless, the Gln treatment resulted in a 43% increase in plasma Gln concentration. Infusions of Gln did not affect splanchnic flux of other nonessential amino acids or of essential amino acids. Glutamine supplementation increased plasma urea-N concentration and tended to increase net hepatic urea flux, with a numerical increase in liver hepatic O₂ consumption. There were no effects on glucose in terms of plasma concentration, net portal appearance, net liver release, or postliver supply, suggesting that Gln supplementation had no sparing effect on glucose metabolism. Furthermore, mammary uptake of glucose and amino acids, including Gln, was not affected by Gln supplementation. In conclusion, this study did not support the hypothesis that supplemental Gln would reduce glucose utilization across the gut or increase liver gluconeogenesis or mammary glutamine uptake to increase milk protein synthesis.     

A redefinition of the representation of mammary cells and enzyme activities in a lactating dairy cow model

The Molly model predicts various aspects of digestion and metabolism in the cow, including nutrient partitioning between milk and body stores. It has been observed previously that the model underpredicts milk component yield responses to nutrition and consequently overpredicts body energy store responses. In Molly, mammary enzyme activity is represented as an aggregate of mammary cell numbers and activity per cell with minimal endocrine regulation. Work by others suggests that mammary cells can cycle between active and quiescent states in response to various stimuli. Simple models of milk production have demonstrated the utility of this representation when using the model to simulate variable milking and nutrient restriction. It was hypothesized that replacing the current representation of mammary cells and enzyme activity in Molly with a representation of active and quiescent cells and improving the representation of endocrine control of cell activity would improve predictions of milk component yield. The static representation of cell numbers was replaced with a representation of cell growth during gestation and early lactation periods and first-order cell death. Enzyme capacity for fat and protein synthesis was assumed to be proportional to cell numbers. Enzyme capacity for lactose synthesis was represented with the same equation form as for cell numbers. Data used for parameter estimation were collected as part of an extended lactation trial. Cows with North American or New Zealand genotypes were fed 0, 3, or 6 kg of concentrate dry matter daily during a 600-d lactation. The original model had root mean square prediction errors of 17.7, 22.3, and 19.8% for lactose, protein, and fat yield, respectively, as compared with values of 8.3, 9.4, and 11.7% for the revised model, respectively. The original model predicted body weight with an error of 19.7% vs. 5.7% for the revised model. Based on these observations, it was concluded that representing mammary synthetic capacity as a function of active cell numbers and revisions to endocrine control of cell activity was meritorious.     

Calcium salts of polyunsaturated fatty acids deliver more essential fatty acids to the lactating dairy cow

Recent research has focused on the importance of supplying essential fatty acids to the lactating dairy cow. The addition of essential fatty acids, specifically linoleic and linolenic acid, to dairy cow diets has been investigated as a method to increase reproductive efficiency. Rumen bacteria, however, biohydrogenate polyunsaturated fatty acids (PUFA) to saturated fatty acids. This is an important issue because it can also lead to milk fat depression when unsaturated fatty acids are fed. The formation of Ca salts has previously been shown to partially protect unsaturated fatty acids from rumen biohydrogenation. The objective of this experiment was to evaluate feed intake, milk production, and milk composition of cows fed Ca salts of palm fatty acids (CS) compared with those fed Ca salts of palm fatty acids with an increased content of PUFA (CS+PUFA). Nineteen lactating Holstein cows were used in a switchback experiment to determine any differences between CS and CS+PUFA on milk production and composition. This experiment consisted of 3 consecutive periods of 14 d. Treatments were formulated to provide 450 g/d (dry matter basis) of the Ca salt supplement and were mixed with the same basal ration. Milk weights and feed intakes were recorded daily for each cow. Milk samples were collected the last 2 d of each period and analyzed for milk composition and fatty acids. Dry matter intake [28.0 vs. 27.0 kg/d; standard error of the mean (SEM) = 0.4] and milk production (44.4 vs. 44.0 kg/d; SEM = 0.7) were not different between treatments for CS and CS+PUFA, respectively. Milk fat percentage (3.34 vs. 3.22%; SEM = 0.07) and milk protein percentage (2.78 vs. 2.80%; SEM = 0.01) were not different for CS- and CS+PUFA-fed cows. Feeding CS+PUFA reduced the concentration of palmitic acid in milk fat (28.3 vs. 26.8 wt%; SEM = 0.3). Supplementation of CS+PUFA increased the linoleic acid concentration (3.96 vs. 4.61 wt%; SEM = 0.1) of milk fat, indicating that linoleic acid was partially protected from rumen biohydrogenation. Concentrations of conjugated linoleic acid were also increased (0.44 vs. 0.52 wt%; SEM = 0.02) when cows consumed CS+PUFA, indicating that some biohydrogenation did occur. Supplementing CS+PUFA did not alter milk production, milk fat percentage, or dry matter intake when compared with CS. The CS+PUFA supplement supplied more linoleic acid to the small intestine for milk fat synthesis.     

Long-acting insulins alter milk composition and metabolism of lactating dairy cows

This study investigated the effect of 2 different types of long-acting insulin on milk production, milk composition, and metabolism in lactating dairy cows. Multiparous cows (n = 30) averaging 88 d in milk were assigned to one of 3 treatments in a completely randomized design. Treatments consisted of control (C), Humulin-N (H; Eli Lilly and Company, Indianapolis, IN), and insulin glargine (L). The H and L treatments were administered twice daily at 12-h intervals via subcutaneous injection for 10 d. Cows were milked twice daily, and milk composition was determined every other day. Mammary biopsies were conducted on d 11, and mammary proteins extracted from the biopsies were analyzed by Western blot for components of insulin and mammalian target of rapamycin signaling pathways. Treatment had no effect on dry matter intake or milk yield. Treatment with both forms of long-acting insulin increased milk protein content and tended to increase milk protein yield over the 10-d treatment period. Analysis of milk N fractions from samples collected on d 10 of treatment suggested that cows administered L tended to have higher yields of milk protein fractions than cows administered H. Milk fat content and yield tended to be increased for cows administered long-acting insulins. Lactose content and yields were decreased by treatment with long-acting insulins. Administration of long-acting insulins, particularly L, tended to shift milk fatty acid composition toward increased short- and medium-chain fatty acids and decreased long-chain fatty acids. Plasma concentrations of glucose and urea N were lower for cows administered long-acting insulins; interactions of treatment and sampling time were indicative of more pronounced effects of L than H on these metabolites. Concentrations of nonesterified fatty acids and insulin were increased in cows administered long-acting insulins. Decreased concentrations of urea N in both plasma and milk suggested more efficient use of N in cows administered long-acting insulins. Western blot analysis of mammary tissue collected by biopsy indicated that the ratios of phosphorylated protein kinase b (Akt) to total Akt and phosphorylated ribosomal protein S6 (rpS6) to total rpS6 were not affected by long-acting insulins. Modestly elevating insulin activity in lactating dairy cows using long-acting insulins altered milk composition and metabolism. Future research should explore mechanisms by which either insulin concentrations or insulin signaling pathways in the mammary gland can be altered to enhance milk fat and protein production.     

A model of net amino acid absorption and utilization by the portal-drained viscera of the lactating dairy cow

A more complete understanding of amino acid (AA) metabolism by the various tissues of the body is required to improve upon current systems for predicting the use of absorbed AA. The objective of this work was to construct and parameterize a model of net removal of AA by the portal-drained viscera (PDV). Six cows were prepared with arterial, portal, and hepatic catheters and infused abomasally with 0, 200, 400, or 600 g of casein daily. Casein infusion increased milk yield quadratically and tended to increase milk protein yield quadratically. Arterial concentrations of a number of essential AA increased linearly with respect to infusion amount. When infused casein was assumed to have a true digestion coefficient of 0.95, the minimum likely true digestion coefficient for noninfused duodenal protein was found to be 0.80. Net PDV use of AA appeared to be linearly related to total supply (arterial plus absorption), and extraction percentages ranged from 0.5 to 7.25% for essential AA. Prediction errors for portal vein AA concentrations ranged from 4 to 9% of the observed mean concentrations. Removal of AA by PDV represented approximately 33% of total postabsorptive catabolic use, including use during absorption but excluding use for milk protein synthesis, and was apparently adequate to support endogenous N losses in feces of 18.4 g/d. As 69% of this use was from arterial blood, increased PDV catabolism of AA in part represents increased absorption of AA in excess of amounts required by other body tissues. Based on the present model, increased anabolic use of AA in the mammary and other tissues would reduce the catabolic use of AA by the PDV.     

Using brown midrib 6 dwarf forage sorghum silage and fall-grown oat silage in lactating dairy cow rations

Double cropping and increasing crop diversity could improve dairy farm economic and environmental sustainability. In this experiment, corn silage was partially replaced with 2 alternative forages, brown midrib-6 brachytic dwarf forage sorghum (Sorghum bicolor) or fall-grown oat (Avena sativa) silage, in the diet of lactating dairy cows. We investigated the effect on dry matter (DM) intake, milk yield (MY), milk components and fatty acid profile, apparent total-tract nutrient digestibility, N utilization, enteric methane emissions, and income over feed cost. We analyzed the in situ DM and neutral detergent fiber disappearance of the alternative forages versus corn silage and alfalfa haylage. Sorghum was grown in the summer and harvested in the milk stage. Oats were grown in the fall and harvested in the boot stage. Compared with corn silage, neutral detergent fiber and acid detergent fiber concentrations were higher in the alternative forages. Lignin content was highest for sorghum silage and similar for corn silage and oat silage. The alternative forages had less than 1% starch compared with the approximately 35% starch in the corn silage. Ruminal in situ DM effective degradability was similar, although statistically different, for corn silage and oat silage, but lower for sorghum silage. Diets with the alternative forages were fed in a replicated 3 × 3 Latin square design experiment with three 28-d periods and 12 Holstein cows. The control diet contained 44% (DM basis) corn silage. In the other 2 diets, sorghum or oat silages were included at 10% of dietary DM, replacing corn silage. Sorghum silage inclusion decreased DM intake, MY, and milk protein content but increased milk fat and maintained energy-corrected MY similar to the control. Oat silage had no effect on DM intake, MY, or milk components compared to the control. The oat silage diet increased apparent total-tract digestibility of dietary nutrients, except starch, whereas the sorghum diet slightly decreased DM, organic matter, crude protein, and starch digestibility. Cows consuming the oat silage diet had higher milk urea N and urinary urea N concentrations. Milk N efficiency was decreased by the sorghum diet. Diet did not affect enteric methane or carbon dioxide emissions. This study shows that oat silage can partially replace corn silage at 10% of the diet DM with no effect on MY. Brown midrib sorghum silage harvested at the milk stage with <1% starch may decrease DM intake and MY in dairy cows.     

Effects of the method of conservation of timothy on nitrogen metabolism in lactating dairy cows

Six ruminally and duodenally cannulated lactating primiparous Holstein cows were used to study the effects of different methods of conservation of timothy on N metabolism. Cows were assigned randomly to 2 replicated 3 × 3 Latin squares (35-d periods). Because of missing data from 2 cows, data were analyzed as a 3 × 4 Youden square. Diets contained a similar concentrate (44% of total ration on a dry matter basis) plus first-cut timothy conserved as hay, or as restrictively (formic) or extensively fermented silage (inoc). Crude protein contents were 10.4, 13.6, and 14.8% for hay, formic, and inoc, respectively. Hay and formic had a high soluble carbohydrate content (≥8.0% of dry matter) and formic and inoc had a high soluble protein content (≥8.0% of dry matter). Haying and restricting fermentation resulted in increased efficiency of partition to milk N (30.9, 28.2, 24.7% of N intake for hay, formic, and inoc, respectively). Despite a 14% lower N intake with hay, no effects of treatments were detected on microbial protein synthesis and apparent intestinal digestion of essential AA. Haying reduced feed protein degradation in the rumen, whereas this effect was not observed when restricting fermentation in the silage. Haying and restricting fermentation induced a lipogenic fermentation pattern in the rumen (4.55, 4.23, and 3.78 ratio of acetate to propionate for hay, formic, and inoc), but no effects on milk fat yield and plasma glucose were observed. Whole-body protein metabolism was unaffected by treatments.     

Hepatic gluconeogenic and ketogenic interrelationships in the lactating cow

Interrelationships between propionate, palmitate, and butyrate metabolism were investigated in vitro with [1-carbon-14] carboxyl substrates. Production of labeled glucose, ketone bodies, and carbon dioxide was used to estimate rates of bovine hepatic gluconeogenesis and ketogenesis. Incubations were with liver slices from eight lactating Holstein cows fed either a control or high concentrate-low fiber diet. Liver samples were acquired by trochar biopsy at 30, 60, 90, and 180 days postpartum. Ketone production from both palmitate and butyrate was highest in liver slices obtained at 30 days. Glucose production from labeled propionate was also highest in early lactation. The higher rates of gluconeogenesis and ketogenesis in early lactation were associated with higher hepatic carnitine palmitoyltransferase (EC 2.3.1.21) activity. Feeding the high concentrate enhanced gluconeogenesis from propionate and decreased ketogenesis from palmitate. Propionate addition (10 mM) to incubation media also decreased the total amount of palmitate oxidized [( carbon-14] dioxide plus [carbon-14] ketones). Diet had no effect on hepatic butyrate metabolism. Results indicated that ketogenesis is regulated via rate of long chain fatty acid transport into the mitochondria. Stage of lactation has a greater influence on long and short chain fatty acid metabolism than does diet composition.     

Reproductive disorders in the periparturient dairy cow

Incidence, predisposing factors, and implications of various reproductive disorders (dystocia, twinning, stillbirth, retained placenta, cystic ovaries, anovulation, infections of the reproductive tract, metritis, and abnormal health status) are reviewed as to their inter-relationships and collective impact on reproductive performance, milk yield and predisposition to other diseases or disorders in the periparturient dairy cow. All reproductive disorders reviewed reviewed reduce reproductive performance either directly or indirectly. Concurrent milk yield was reduced marginally in a few studies as a consequence of twinning, retained placenta, cystic ovaries, metritis, or other uterine disorders, and in cows with an abnormal health status. There is strong evidence for associated losses in milk yield following surgical delivery of a stillborn calf. We conclude that most periparturient disorders occur as a complex, rather than as a single abnormality. Cows with one disorders are at increased risk for other disorders, including metabolic ones. In contrast, actual milk yield or potential for high production generally does not predispose cows to increased risk for any of the reproductive disorders. The literature suggests that prophylactic measures to prevent occurrence of the one disorder might decrease the risk and incidence of other related disorders, either directly or indirectly.