Effect of a synthetic corticoid on milk yield and composition and on blood metabolites and hormones in dairy cows

Food intake regulation in late pregnancy and early lactation

The dip in food intake, which starts in late pregnancy and continues into early lactation, has traditionally been interpreted as a depression in intake due to physical constraints. However, the rôle of physical constraints on intake has been overemphasized, particularly in early lactation. There is mounting evidence that the presence and mobilization of body reserves in early lactation play an important rôle in regulating intake at this time.

Conceptually, the dip in intake in early lactation observed when cows have access to non-limiting foods can be accounted for by assuming that the cow has a desired level of body reserves. When the cow is not compromised, the changes with time in body reserves and the dip in intake represent the normal case and provide the basis against which to assess true depressions in intake which may occur when the cow is compromised by limiting nutrition or environment.

The regulation of body reserves and intake in the periparturient cow is orchestrated through nervous and hormonal signals. Likely factors that are involved in intake regulation are reproductive hormones, neuropeptides, adrenergic signals, insulin and insulin resistance and leptin. Furthermore, oxidation of NEFA in the liver may result in feedback signals that reduce intake. The relative importance of these is discussed. A better understanding of the physiological signals involved in intake regulation and their interrelations with body weight regulation may provide important indicators of the degree of compromise that periparturient cows may experience.

Integration of metabolism and intake regulation: a review focusing on periparturient animals

There has been great interest in dry matter intake regulation in lactating dairy cattle to enhance performance and improve animal health and welfare. Predicting voluntary dry matter intake (VDMI) is complex and influenced by numerous factors relating to the diet, management, housing, environment and the animal. The objective of this review is to identify and discuss important metabolic factors involved in the regulation of VDMI and their integration with metabolism. We have described the adaptations of intake and metabolism and discussed mechanisms of intake regulation. Furthermore we have reviewed selected metabolic signals involved in intake regulation. A substantial dip in VDMI is initiated in late pregnancy and continues into early lactation. This dip has traditionally been interpreted as caused by physical constraints, but this role is most likely overemphasized. The dip in intake coincides with changes in reproductive status, fat mass, and metabolic changes in support of lactation, and we have described metabolic signals that may play an equally important role in intake regulation. These signals include nutrients, metabolites, reproductive hormones, stress hormones, leptin, insulin, gut peptides, cytokines, and neuropeptides such as neuropeptide Y, galanin, and corticotrophin-releasing factor. The involvement of these signals in the periparturient dip in intake is discussed, and evidence supporting the integration of the regulation of intake and metabolism is presented. Still, much research is needed to clarify the complex regulation of VDMI in lactating dairy cows, particularly in the periparturient animal.

The bovine pituitary-adrenocortical axis and milk yield

A review is given of the available literature concerning the relationship between the bovine pituitary-adrenocortical axis and milk yield in dairy cattle. A severe drop in milk yield (more than 50%) can be induced by a single or repeated intramuscular injection of at least 200 IU ACTH or by a single intramuscular injection of 14.6 mg dexamethasone. Sixty minutes after an intravenous injection, both 200 IU ACTH and 100 mg cortisol are equivalent to a plasma cortisol concentration of at least 31 ng/ml. Thus the decrease in milk yield after an intramuscular injection of more than 200 IU ACTH can hardly be induced by cortisol only. The fact that bovine plasma hardly binds any dexamethasone, in sharp contrast with bovine mammary epithelial tissue, is a possible explanation of the special part which dexamethasone plays in milk yield.

Physiological responses to intramammary or intravenous treatment with endotoxin in lactating dairy cows

Twenty-one, middle to late lactation Holstein cows were assigned to one of three treatments in a completely randomized design to examine physiological changes associated with intramammary or intravenous administration of Escherichia coli endotoxin. Treatments were 1) Hank's balanced salt solution infusion in two contralateral quarters (control), 2) E. coli endotoxin infusion in two contralateral quarters, and 3) intravenous infusion of E. coli endotoxin. Blood was sampled and rectal temperature was measured at 30-min intervals. Endotoxin treatment was at 0900 h and sampling continued until 1700 h. Serum prolactin, cortisol, and 13,14-dihydro-15-keto-prostaglandin F2 alpha were measured. A pyretic response was observed in intravenous and intramammary treatment groups after endotoxin treatment. Response peak was higher (41.1 vs. 40.3 degrees C) and occurred later (6 vs. 4.5 h posttreatment) in the intramammary than the intravenous treatment group. Significant prolactin peaks were observed also in intravenous and intramammary endotoxin treatment groups. Prolactin peaked higher (288 vs. 112 ng/ml) and occurred sooner (1 vs. 4 h posttreatment) in the intravenous than in the intramammary treatment group. Cortisol followed a trend similar to prolactin. Cortisol peaked higher (100 vs. 82 ng/ml) and sooner (2.5 vs. 4.5 h posttreatment) in the intravenous than in the intramammary treatment group. Concentrations of 13,14-dihydro-15-keto-prostaglandin F2 alpha increased rapidly posttreatment in the intravenous group only.

Quantitative estimates of mammary growth during various physiological states: a review

The parenchymal portion of the mammary gland is immature at birth and begins to grow at a faster rate than the whole body shortly before occurrence of puberty. This accelerated or allometric growth rate is maintained for several estrous cycles, then returns to a growth rate equal to general body growth. Allometric growth of the mammary gland returns at conception and continues in most species for a variable period after parturition. Elevated secretion of estradiol and progesterone throughout pregnancy drives the allometric mammary growth during pregnancy. However, mammary growth during lactation in cows is independent of ovarian secretions and prolactin. Mammary cell numbers during lactation eventually decline as milk production decreases. Concurrent pregnancy reduces mammary cell numbers during lactation, but during the dry period concurrent pregnancy markedly increases mammary cell numbers over those in nonpregnant animals. Dry periods that are short reduce the increments in mammary cell numbers, which normally occur during early stages of the next lactation. Because numbers of mammary epithelial cells are a major determinant of milk yield, understanding the mechanisms that stimulate mammary epithelial cell numbers has the potential to lead to new methods for increasing efficiency of milk production.

Somatotropin and lactation

Somatotropin, which can now be produced by biotechnology, could have an enormous impact on the dairy industry. Milk yield has been increased up to 40% with daily injections of somatotropin. Cows adjust their nutrient intake to support this increase. Somatotropin does not adversely affect cows' health, although all studies to date have been for less than one complete lactation. The search for a single biochemical or physiological event to account for the effects of somatotropin on milk production is elusive. Coordinated changes in many tissues and physiological processes occur to support the increases in the synthesis of lactose, fat, and protein in the mammary gland. Changes in the irreversible loss and oxidation rates of two key metabolites, glucose and free fatty acids, can quantitatively account for increases in lactose and milk fat during the short-term administration of somatotropin. Similarly, feed intake and live weight changes can account for increases in milk production in the longer experiments. Parallels between physiological changes that occur during somatotropin administration and differences between genetically high and lower yielding cows are highlighted, and the rates of improvement that can be expected from various new technologies are quantified. Existing data on the safety of somatotropin to both the consumer and the animal are evaluated.

Influence of adrenocorticotropin upon milk production, milk constituents, and endocrine measures of dairy cows

Ten lactating Holstein cows were injected intramuscularly with either 200 IU of adrenocorticotropin or saline every 6 h for 42 h. Blood concentrations of corticosteroids, glucose, sodium, and potassium, and milk yield and constituents were measured before, during, and after the injection period. Mean corticosteroid increased eightfold for adrenocorticotropin 1 h after first injection, remained significantly higher than controls during the injection period, and then decreased to preinjection concentrations 13 h after last injection. Mean milk yields were similar at 8 h after first injection, then decreased for adrenocorticotropin during the injection period. Increases in glucose concentrations accompanied suppression of milk yield for adrenocorticotropin. After injections glucose concentrations returned to preinjection concentrations at 13 h after last injection whereas milk yield for adrenocorticotropin remained suppressed for several milkings. Milk protein yields decreased for adrenocorticotropin during and after the injection period. Fat percentage for adrenocorticotropin increased though fat yield was unaffected. Concentrations of somatic cells in milk were unchanged by adrenocorticotropin treatment, but sodium was increased and potassium decreased during injection. Mammary gland function in lactating dairy cows may be affected adversely by exaggerated increases of concentrations of corticosteroids in blood.

Plasma unesterified fatty acid and triacylglycerol concentration of the goat (Capra hircus) during pregnancy and lactation

1. The concentrations of unesterified fatty acids and triacylglycerol were measured in plasma obtained from goats at various stages of pregnancy and lactation. 2. The plasma unesterified fatty acid concentration increased during the last third of pregnancy: highest concentrations were found at parturition and during early lactation. 3. The plasma triacylglycerol concentration showed a transient increase during mid-pregnancy: the concentration fell around parturition and no evidence for hypertriglyceridemia was found during late-pregnancy. 4. Results are compared with those of other studies of ruminant and non-ruminant species.

Hormonal responses of lactating dairy cattle to TRH and ACTH in a shade management system within a subtropical environment

Lactating cows (64) were balanced by breed (54 Holstein and 10 Jersey) and assigned randomly to shade (S) or no shade (NS) management treatments for a continuous 20 wk trial beginning 5-5-76. A sub-sample of Holstein cows, five S and five NS, were fitted with jugular catheters 84 days after initiation of experiment. Thyrotropin Releasing Hormone (TRH; 100 mug) was administered intravenously at 1200 h to evaluate prolactin responses. Two days later each cow received intravenously 200 IU of ACTH at 1100 h to compare acute corticoid responses to ACTH. Mean prolactin response to TRH was greater for NS cows (291 vs 169 ng/ml; P < .01) as was peak plasma concentrations at 20 min (467 vs 267 ng/ml; P < .01). Mean corticoid response to ACTH injection was less for NS cows (52 vs 70 ng/ml; P < .10). Corticoid concentrations of plasma in both treatments had declined 65% by 7 h postinjection. These endocrine differences may be associated with thermoregulation and/or metabolic adjustments of cows exposed to different environmental systems of management during a seasonal period of thermal stress.

Periparturient endocrine changes of conceptus and maternal units in Jersey cows bred for milk yield

Control cows, sired by and bred to bulls with assumed zero estimated breeding values, and selected cows, sired by and bred to bulls of high predicted difference for milk yield, were used. Blood samples were collected via jugular venipuncture on alternate days from 35 to 14 days prepartum, from 14 to 28 days postpartum, and daily from 14 days prepartum to 14 days postpartum. We examined blood hematocrit, and concentrations in plasma of protein, estrone, estradiol, estrone sulfate, progestins, glucocorticoids, luteinizing hormone, and prolactin. Total plasma volume was measured on days -21, -7, and +11. Weekly body weights, prepartum and postpartum, were recorded. All data were analyzed by least squares analysis of variance with day and hematocrit as continuous independent variables. Hematocrit was higher for selected cows throughout the sampling period. Prepartum concentrations of progesterone were higher in selected cows, but concentrations of estrone, luteinizing hormone, and prolactin were lower than in control cows. Hormonal and physiological responses indicated that selection for milk yield influenced both the conceptus and maternal units as measured by prepartum endocrine function.