Lactation In Animals Research Articles

The production response by dairy cows to four levels of straw inclusion in grass silage based diets

High concentrate feed levels, with a consequent fall in the fibre content of the diet, are increasingly being offered to high genetic merit dairy cows. To improve fibre intake, small quantities of straw are often incorporated into the diet. This study was undertaken to examine the production response of dairy cows, offered a medium quality grass silage based diet together with relatively high levels of concentrate supplementation, to straw inclusion in the diet.Forty high genetic merit dairy cows, 20 multiparious and 20 first lactation animals (PTA95 fat + protein = 41.4 and 39.7 kg respectively) were used in this study. The study consisted of four treatments, arranged in a two period (4 week duration), partially balanced change-over design trial, with first lactation and multiparious animals being blocked separately. The experimental treatments consisted of grass silage mixed with one of four levels of straw, and offered ad libitum in the form of a mixed diet.

Management and economics of extended calving intervals with use of bovine somatotropin

The objective of this study was to investigate a reproductive management strategy that optimized profitability per cow per day of productive life when bovine somatotropin (bST) was used. Nine herds were involved with a portion of animals randomly assigned to one of four treatments: breeding at first oestrus after 60 days in milk (DIM, 13.2-month calving interval (CI)); breeding at first oestrus after 150 days in milk (16.5-month calving interval); first lactation animals never re-bred; and second lactation animals never re-bred. All herdmates not assigned to the study served as controls. All treatment animals received bST commencing at 63 days after parturition. Effects of bST supplementation with delayed pregnancy and without pregnancy were determined using the Test Day Model© (TDM). Results indicated that with bST use a 16.5-month calving interval increased ( P < 0.05) milk yield over animals managed for a 13.2-month calving interval. Milk yield response to bST increased as lactation progressed, leading to a significant increase in persistency. When days of productive life were standardized to 4.35 yrs as the typical life cycle and a calving interval of 16.5 months was used, milk was increased 4468 kg for the animals with the extended calving interval compared with a 13.2-month calving interval. Profitability was increased by approximately US$0.75 per day per animal for animals on a 16.5-month calving interval as compared to a 13.2-month calving interval. On a herd basis, through two years of study, extended calving interval resulted to fewer calvings, lower incidence of postpartum metabolic diseases, lower veterinary costs, less culling with fewer replacements needed, and an overall improvement in herd life, animal well-being and dairy farm profitability.

Oxytocin and prolactin suppress cortisol responses to acute stress in both lactating and non-lactating sheep

Cortisol response to stress appears to differ between lactating and non-lactating animals. Lactating (14 d post partum) and non-lactating sheep were fitted with probes so that drugs and hormones could be infused directly into the posterior pituitary and paraventricular nucleus of the hypothalamus. The animals were also fitted with instruments to allow monitoring of heart rate, body temperature and blood cortisol levels. Their reactions to a source of acute stress (a barking dog) were then followed, with or without drug and hormone manipulation. Results in both lactating and non-lactating animals indicated shortcomings in the use of cortisol as a stress indicator. Infusing prolactin and oxytocin into either the posterior pituitary or the paraventricular nucleus of the hypothalamus suppressed cortisol responsiveness to stress in both lactating and non-lactating animals (the latter to a greater extent). In the absence of drugs, lactating animals had a slightly higher basal level of cortisol and a lower cortisol response to stress than their non-lactating counterparts. Despite suppression of cortisol responses, with or without drugs, other indicators of stress still changed with the presence of a barking dog, suggesting the complexity of control involved in stress responses.

Bovine CD8+ suppressor lymphocytes alter immune responsiveness during the postpartum period.

This study examined the immunoregulatory role of CD8+ lymphocytes during the postpartum period. Peripheral blood cells were isolated from postpartum and mid to late lactating animals. Flow cytometric analysis was performed to determine the frequencies of relevant cell populations. Depletion of CD8+ lymphocytes from whole cultures significantly decreased proliferation and cytotoxic ability of cells isolated from mid to late lactating animals. Enrichment of whole cultures with CD8+ lymphocytes further decreased their proliferative ability but pure CD8+ lymphocyte had increased cytotoxic activity. In contrast, neither depletion nor enrichment of whole cultures with CD8+ lymphocytes altered the already diminished proliferative responses of cells isolated from postpartum cows. No cytotoxic activity was observed by cells isolated from postpartum animals. Cultures from mid to late lactating cows mainly expressed IFN-gamma mRNA where as IL-4 mRNA was mainly expressed by cultures isolated from postpartum animals. Flow cytometric analysis revealed that CD8+ lymphocytes have a high level of activation and expression of the beta-chain during the postpartum compared with the mid to late lactating period. These data indicate that CD8+ lymphocytes are of the suppressor compared to the cytotoxic nature immediately following parturition.

Survey of cystic ovarian disease in practice

The "DAISY" computerised recording system (University of Reading) was used to look at records of Cystic Ovarian Disease (COD) over two 12 month periods in a cattle practice in south west England. Over 24 dairy herds with good records were assessed to include 190 cows in year 1 and 180 cows in year 2 presented with COD. A definition of the disease was established as a fluid filled structure on the ovary exceeding 25mm in diameter with no palpable luteal structure present. The economic effect of the disease was compared both within herds and between herds to assess the importance of the condition. At the incidence levels recorded there was no effect on overall herd performance although the condition did affect individuals quite profoundly. Age incidence showed a lower level of COD in 1st lactation animals. Older cows had a relatively constant incidence which followed the age profile of the herds. The overall pregnancy rate was very high (81 - 89%). Treatments consisted of those with and without manual rupture of the cyst, with and without Gonadotrophin Releasing Hormone (GnRH), and with and without Progesterone treatments. Some cases received no treatment at all. The survey showed there was no difference in outcome between treatments involving cyst rupture and those not and that there was no difference between treatment and non treatment involving progesterone or GnRH. GnRH showed a longer interval to subsequent service than rupture or progesterone and there was a lower incidence of recurrence using progesterone treatments. The presenting signs of COD show that in dairy herds with a high level of routine fertility involvement nymphomania or nyphomanism is not common. The clinical presentation of COD in the study herds was 17% at a pre-breeding examination, 58% in cows presented with no observed oestrous, 12% at a negative pregnancy diagnosis, 12% with an irregular heat or service interval. Reviewing data from a variety of veterinary surgeons is difficult. Diagnosis and treatment show considerable bias between operatives and makes it difficult to reach conclusions.

Limits to Sustained Metabolic Rate: The Link between Food Intake, Basal Metabolic Rate, and Morphology in Reproducing Mice, Mus musculus

Thirty virgin female white (MF1) mice (Mus musculus) were divided into three groups: an unmated control group; a second, mated group, allowed to proceed to peak gestation; and a third, mated group, allowed to proceed to peak lactation. Nine unmated animals and eight of each of the two mated groups had their basal metabolism (oxygen consumption at 28° C) measured and were then killed by means of a CO₂ overdose, and the wet masses of 14 different body components measured. Body mass of the control animals averaged 26.5 g, not significantly different from animals in the other two groups immediately postmating. At peak gestation body mass averaged 50.5 g, immediate postpartum mass was 35.5 g, and by the end of lactation it was 38. 4 g. Basal metabolic rate (BMR) of the control animals (0.313 W) was significantly lower than the BMR of both pregnant (0.78 W) and lactating (0.95 W) animals. Food intake of controls was 97.9 kJ d⁻¹ (3.6 X BMR). At peak gestation food intake was 119.3 kJ d⁻¹ (2.4 X BMR), and at peak lactation it was 405.5 kJ d⁻¹ (5.96 X BMR). If BMR had not increased throughout reproduction, the peak food intake would have represented 12.2 times BMR. Morphological changes associated with the reproductive cycle mainly included increases in the masses of the alimentary tract, liver, kidneys, heart, and lungs. A principal components analysis revealed four major dominant axes of variability in the morphology. The first principal component was dominated by the combined alimentary effects and explained 71.8% of the variation in BMR. These data are consistent with the hypotheses that maximal sustainable metabolic rate reflects a link between structure of the alimentary tract, maximal daily food assimilation, and BMR. That is, as mice progressed through reproduction, their increasing food requirements were beyond the capacity of their guts to supply them. This led to hypertrophy of the alimentary tract and associated organs, with a consequent increase in BMR.

Endocrine and metabolic responses to transportation in periparturient donkeys

Periparturient equidae are particularly susceptible to hyperlipaemia, a severe metabolic disorder that can be induced and aggrevated by stressful situations. To define maternal endocrine and metabolic adaptations to the peripartum period, plasma concentrations of progesterone, oestradiol, 13,14-dihydro-15-keto-prostaglandin F 2 α (PGFM), triglyceride, cholesterol, glucose, total protein, urea, cortisol and insulin were examined in two donkey mares throughout late gestation (−50 days), parturition (Day 0) and early lactation ( + 100 days), and in one non-pregnant mare over six oestrous cycles. Maternal progesterone and oestradiol concentrations increased towards term, to maxima of 50–60 nmol 1 −1 and 130–170 pmol 1 −1 , respectively, and decreased rapidly from 2 days prepartum to < 5 nmol 1 −1 and < 15 pmol 1 −1 at parturition. Plasma PGFM concentrations of 8–11 nmol 1 −1 over 1 week prepartum decreased to < 1 nmol 1 −1 1 day postpartum. Maternal triglyceride concentrations increased from 1–2 nmol 1 −1 over late gestation to a maximum of 2–4 mmol 1 −1 at parturition. Thereafter, values rapidly decreased to <0.5 mmol 1 −1 by 3 days postpartum. Plasma cholesterol and total protein profiles also followed this pattern: no consistent changes were evident in glucose, urea, cortisol or insulin concentrations. To assess endocrine and metabolic responses to a stressor in animals under gestational or lactational energy demands, plasma cortisol, triglyceride, cholesterol, glucose and insulin concentrations were examined in the same two donkey mares transported for approximately 4 h at 3–10 weeks prepartum and 2–9 weeks postpartum. These responses were compared with those observed in non-pregnant donkeys. Transportation increased cortisol concentrations in pregnant (maximally by 215 and 130%), lactating (32 and 85%) and non-pregnant animals (145±9%). Plasma glucose concentrations remained stable in all but one lactating donkey; the hyperglycemic effect may have been an indirect indicator of a reduced tissue sensitivity to insulin. Stressor-induced suppression of circulating insulin was apparent in postpartum (−57 and −51%) and non-pregnant animals (−46 ± 4%). Triglyceride and cholesterol concentrations in all donkeys were unchanged by transportation. The observed shifts in lipid metabolism, possibly facilitated by gestational changes in concentrations of progesterone and oestradiol, are likely to make the peripartum mare vulnerable to the development of equine hyperlipaemia. However, an additional environmental stressor (4 h transportation), which stimulated an adrenocortical response in periparturient donkeys, was unable to induce any consistent alteration in metabolic function or evidence of potential dysfunction.

An investigation of the suitability of three support matrices for the culture of cells derived from the secretory alveoli of the bovine mammary gland.

The suitability of three support matrices, (thick collagen gels, aluminium oxide and cellulose ester membranes, the latter two both thinly coated with collagen) for the production of primary cultures of bovine mammary epithelial cells was investigated. Single secretory alveoli were isolated from mammary tissue of animals in early lactation by enzymatic digestion and differential filtration. Cell growth was monitored by light and scanning electron microscopy. The cellulose ester membrane was found to give the best results, allowing growth of monolayers with a morphology closely resembling that of the natural epithelium of the gland. There were low levels of fibroblast contamination, and the membranes could be easily manipulated for further studies.

Butyrophilin and xanthine oxidase occur in constant molar proportions in milk lipid globule membrane but vary in amount with breed and stage of lactation

Butyrophilin and xanthine oxidase, major proteins of milk lipid globule membrane, both accounted for significantly higher percentages of total protein in membrane samples from Holstein than from Jersey animals. Both were high in membranes from animals in early lactation, both decreased in amount as lactation progressed to the midpoint, and then both rose in amount toward the end of lactation. In samples from both Holstein and Jersey animals, butyrophilin and xanthine oxidase were present in constant molar proportions of about 4∶1. These proteins co-enriched together with low molecular weight GTP-binding proteins in a high salt and nonionic detergent insoluble fraction of milk lipid globule membrane. Butyrophilin and xanthine oxidase content of membranes was not related to milk lipid globule diameter, suggesting that these proteins alone may not be involved solely in anchoring the membrane to the lipid globule surface. However, the possibility that a complex composed in part of butyrophilin and xanthine oxidase serves an anchoring function remains a possibility.

Factors associated with concentrations of immunoglobulins in colostrum at the first milking post-calving

The concentration of immunoglobulins (Ig) in colostrum at first machine-milking was monitored in 2045 cows during a 29-month period in a Central Florida dairy. Estimates of colostral Ig were performed immediately after cow milking in the parlor using a colostrometer. Factors associated with the concentrations of colostral Ig were evaluated using analysis of variance and logistic regression. The majority of samples, 79.8% (1632), tested in the low-quality range (less than 20 mg ml −1), while 13.5% (276) were classified as intermediate (20–50 mg ml −1). The mean colostrum score for first lactation animals was 9.3 mg ml −1 which was significantly lower than that observed for second and greater lactation cows. Mean colostrum scores by month were significantly higher during August and September than other months of the year. Holsteins were twice as likely to have high concentrations of colostral Ig (more than 50 mg ml −1) in comparison to other breeds represented. The concentration of Ig in colostrum increased significantly in cows when body condition score increased from dry-off to calving in contrast to when it remained stable or decreased. These results demonstrate the importance of monitoring colostrum quality in operations which practice the hand feeding of colostrum to neonatal calves for the purpose of optimizing passive immunity.

Ovarian aromatase cytochrome P-450 mRNA levels correlate with enzyme activity and serum estradiol levels in anestrous, pregnant and lactating rats

We examined the changes in P-450AROM mRNA, aromatase enzyme activity and serum estradiol levels (E2) in anestrous, pregnant mare's serum gonadotropin (PMSG)-treated immature, pregnant, and lactating rats to determine if: (a) the various mRNA species encoding P-450AROM in rat ovarian tissue are differentially expressed during different hormonal states, and (b) a positive relationship exists between P-450AROM mRNA and enzymatic activity in rat ovarian tissue and serum estradiol levels from the same animals.Utilizing three different cDNAs encoding rat P-450AROM, levels of P-450AROM mRNA were determined by RNA blot analysis and scanning densitometry. Probe 1, a 5' probe, detects all three P-450AROM mRNA species in rat ovarian tissue (i.e. at 1.7, 2.2 and 2.7 kb). Probe 2 contains an unspliced intronic sequence in place of the heme-binding domain at its 3' terminus and thus the mRNA detected by this probe must encode a nonfunctional aromatase protein. Only the two smaller (i.e. nonfunctional) mRNA species at 1.7 and 2.2 kb are detected by probe 2. Probe 3 contains the heme-binding region and hybridizes to principally the largest mRNA transcript at 2.7 kb (but hybridizes also to a 1.7 kb mRNA transcript). Aromatase enzyme activity was measured by using a saturating concentration of [1β-3H] testosterone as substrate in the [3H]water-release assay while serum estradiol levels were determined by radioimmunoassay.In immature rats (IR) or lactating animals (LA) P-450AROM mRNA was not detectable along with low serum estradiol (IR ≈ 2.8 pg/ml; LA ≈ 0.2 pg/ml) and aromatase activity levels (IR ≈ 0.8 pmol/h per mg protein; LA < 0.5 pmol/h per mg protein). Anestrous animals treated with 5 IU of PMSG resulted in a clear increase (24 h later) in P-450AROM mRNA levels, in concert with a 4-fold increase in serum E2 (≈ 12.5 pg/ml) and aromatase activity (≈ 4.2 pmol/h per mg protein). During pregnancy, all three mRNA species were clearly detectable, but low serum E2 levels (≈ 0.6 pmol/ml) and P-450AROM mRNA abundance were observed at 3 days of gestation (DG). Levels of all three P-450AROM mRNA species increased markedly at 15 and 18 DG; thereafter, the levels declined at 20 DG and further decreased at 22 DG. However, regardless of the probe utilized (probe 1, 2 or 3) in the RNA blot analyses, the mRNA transcripts detected by each probe were expressed in a concerted fashion with respect to abundance and pattern. Aromatase activity at 15 DG was high (≈3.0 pmol/h per mg protein) and remained at relatively high levels until 20 DG (≈4.0 pmol/h per mg protein), thereafter the activity decreased perinatally, at 22 DG (≈1.8 pmol/h per mg protein). Estradiol levels during late gestation (at 15 DG) were at high levels (≈9.0 pg/ml) and continued to increase to 18.0 pg/ml until 19 DG, thereafter, E2 levels decreased at 20 and 22 DG (≈7.0 to 8.0 pg/ml). A correlation calculated between the normalized P-450AROM mRNA levels (using probe 3) and aromatase activities revealed a strong correspondence between the P-450AROM mRNA pattern of abundance and enzyme activity in anestrous, pregnant and lactating rats, r = 0.93, p = 0.002. Also a significant positive correlation between ovarian aromatase activity and serum E2 levels was obtained, r = 0.88, p = 0.01. These findings suggest that as many as four different species of P-450AROM mRNA in ovarian tissue appear to be expressed in a concerted fashion regardless of the hormonal state or developmental stage of the animal, and (b) in anestrous, pregnant and lactating rats, a significant positive correlation exists between aromatase activity and serum E2 levels which are regulated, in part, by the levels of P-450AROM mRNA encoding the aromatase enzyme.

An examination of the effect of type of concentrate and method of concentrate feeding on the milk production of british friesian dairy cows offered grass silage based diets

In a previous study at this Institute, significant increases in total dry matter intake were obtained with first lactation animals offered grass silage and concentrates as a complete diet rather than in separate feeds. However, the increased DM intake associated with complete diet feeding was not reflected in improved animal performance. Similar results have also been reported by Phipps, Bines, Fulford and Weller (1984) and Cassel, Merrill, Milligan and Guest (1984).The aim of this experiment was to investigate the effect on animal performance of offering two types of concentrate in addition to grass silage, with concentrates offered either as a complete diet or in three feeds per day.

Calving patterns in seasonal dairy herds

Two field studies examined the calving patterns of cows in seasonal dairy herds in the Waikato (Field Study 1) and South Taranaki regions (Field Study 2). The first study examined patterns for cows commencing their second or subsequent lactation in herds which had used an inseminating service during the previous season. The second study included first lactation heifers only in 15 herds where animals had been naturally mated, and in 15 herds in which they had been synchronised and then artificially inseminated at the synchronised oestrus. The parameters describing calving patterns were based on the date for each herd's planned start of calving (PSC), which was 282 days from the date on which breeding commenced in the preceding season. The average interval from PSC to mean calving date for the 35 herds in Field Study 1 was 22 days, with individual herds ranging from 15 to 30 days. In herds with heifers which had been naturally mated (Field Study 2), it was 17.6 days compared to 11.0 days for previously synchronised animals. Calculating the intervals from PSC to median calving date and separately for the last two quartiles more effectively described a herd's calving pattern. The duration for the last quartile of the calving pattern was influenced by the extent and timing of induced calving. In Field Study 1, 88.6% of the 35 herd owners induced premature parturition in at least one cow. In these herds, 11.3% of cows were treated and calved prematurely. Only 61.7% of heifers which had previously been naturally mated calved by 3 weeks after PSC. Their calving dates were not evenly distributed over this 3-week period, with 9.8% in the first week and 25.6% in the third week. The calving pattern for heifers which had been previously synchronised showed several distinct peaks. Calvings to the synchronised mating were completed 15 days after PSC, by which time 64.7% of animals had calved. By 3 weeks after PSC, 72.9% of these heifers had calved. The results showed that there was considerable variation in calving patterns in seasonal dairy herds. This variation would have been due to differences in conception pattern, and the way induced calving had been applied. The calving pattern in heifers which had been naturally mated was less concentrated than had been expected. Synchronisation can significantly concentrate the calving pattern of these first lactation animals. The parameters used to describe calving patterns may be less applicable in herds in which a high proportion of animals is induced to calve prematurely, or where a whole herd is synchronised. Nonetheless, they do serve as an illustrative example of the variation in calving patterns among herds.

Non-esterified fatty acid and glycerol kinetics and fatty acid re-esterification in goats during early lactation

Non-esterified fatty acid (NEFA) and glycerol kinetics were studied in lactating goats to gain insight into the mechanisms by which animals in early lactation can initially mobilize and later replenish body fat reserves. Kinetic measurements were made at days 10, 38 and 76 post-partum in ten multiparous lactating does. Plasma NEFA concentrations and NEFA entry rate decreased as lactation advanced, being significantly higher at day 10 than at either days 38 or 76 of lactation. Both plasma NEFA concentrations and NEFA entry rate were negatively correlated with calculated energy balance. In contrast, glycerol entry rate was significantly higher at day 76 than at day 10 of lactation and was positively related to both calculated energy intake and energy balance. Apparent intracellular fatty acid re-esterification was lower at day 10 than at later stages of lactation and was positively related to calculated energy balance. It is suggested that during early lactation, substantial shifts in adipose tissue fat reserves can occur via altering rates of fatty acid re-esterification and de novo lipogenesis, without major changes in the rate of lipolysis.

Hypoinsulinaemia in the lactating rat is caused by a decreased glycaemic stimulus to the pancreas

An in-vitro perifusion system was devised in order to examine the secretory profiles of isolated islets of Langerhans, derived from different physiological states, when subjected to various stimuli relevant to lactation. Islets from pregnant rats secreted more insulin than did those from virgin animals; however, islets from lactating and virgin animals secreted similar amounts of insulin with all stimuli, including glucose, amino acids, cations and neurotransmitters. When virgin rats were pretreated for 5 days in vivo with GH or prolactin, insulin responses in vitro were unchanged. Cannulation of the hepatic portal vein and inferior vena cava in vivo revealed that both insulin and glucose concentrations were lower in the portal vein of the lactating rat compared with the virgin animal. It was therefore concluded that insulin concentrations are depressed during lactation as a consequence of the pancreas receiving a diminished glycaemic stimulus rather than because of any change in beta-cell sensitivity.

Regulation of Bovine Adipose Tissue Metabolism During Lactation. 6. Cellularity and Hormone-Sensitive Lipase Activity as Affected by Genetic Merit and Energy Intake

Subcutaneous adipose tissue cell size and density and hormone-sensitive lipase activity were determined during late pregnancy and first lactation in Holstein heifers. Animals were daughters of bulls with either +791 (high line) or +390 (low line) kg Predicted Difference (1974 base) for milk. Each line consumed either a 71%: 29% (high energy) or a 36%:63% (low energy) barley concentrate and alfalfa hay diet from 0 to 140 d lactation. Heifers fed the low energy ration ate 33% less NE1 and produced 20% less milk with 52.7% greater milk fat percentage during 28 to 140 d of lactation than heifers fed the high energy ration. Prepartum, animals of the high line had similar adipocyte volume and density to low line animals. However, during early lactation, high line animals had a greater decrease in volume and a larger increase in density than low line animals. There was a delayed recovery of volume in high line, compared with low line animals in late lactation. Adipocyte volume also was decreased by dietary energy restriction. Hormone-sensitive lipase activity was similar in both genetic lines prepartum and increased in all groups at 60 d postpartum. Animals of high line or fed low energy rations had increased activity per gram during early, but not later, lactation. Activity per cell and per milligram cytoplasm protein increased in all groups in early lactation and were highest in high line animals fed the low energy ration.

Effect of lactation and of roughage to concentrate ratios on outflow rates of protein supplements from the rumen of sheep and goats

Six dry (three ewes and three goats, Trial 1) and six lactating (three sheep and three goats, Trial 2) animals were used in a 3 × 3 Latin square design with three diets of varying barley grain to barley hay ratios (75:25, 55:45, 35:65) and three 12-day periods. In Trials 1 and 2, animals were fed to meet their energy requirements. In Trial 3, six dry (three ewes and three goats) and six lactating (three ewes and three goats) were offered a diet (55:45 grain to hay ratio) to supply them with 1.3 × their maintenance energy requirements. The objectives of the present work were to investigate the influence of lactation, roughage to concentrate ratios and species on the fractional outflow rate (FOR). Neither species nor treatment (diet) had any significant effect on FOR in dry or lactating animals. There was a trend however towards lower FOR in the high concentrate diets in both dry and lactating animals. FOR was closely related ( r 2 = 0.78) to feed dry matter (DM) intake. Feeding dry and lactating sheep and goats the same feed allowance resulted in a similar FOR. Based on the present study, it may be concluded that there is no need to consider lactation, roughage to concentrate ratios in the diet and/or species (sheep/goats) in calculating effective degradability. Finally, different values for FOR should be taken at different levels of feed intake for calculating effective protein degradability.

A new approach to the mathematical formulation of lactation curves

ABSTRACTSix alternative lactation curves were derived by considering the proportional rates of change in milk yield during lactation. Experimental data showed that rates of change do not necessarily fall to a constant value in mid and late lactation. This accounts for the systematic lack of fit reported by several authors using the derivative of the incomplete gamma function, whose second derivative has a horizontal asymptote. Curves whose derivatives are not restricted in this way described patterns of milk yield in first to third lactation animals more closely. Some statistical properties of their parameters have been examined. The curve loge (y) = a - bt'(1 + kt') + ct'2 + d/t, where t is days since calving, t' = t – 150, A: is a constnat and a, b, c and d are parameters to be estimated, is recommended. This curve was also fitted to yields of milk fat, protein and lactose.

Rat and human mammary tissue can synthesize choline moiety via the methylation of phosphatidylethanolamine.

The normal mammal requires large amounts of choline for maintenance and growth of tissue mass. Since milk, the only food for neonates, has many-fold higher free choline concentration than does maternal plasma, it is possible that mammary gland can synthesize choline molecules. The only known mammalian pathway for the synthesis de novo of choline molecules is catalysed by phosphatidylethanolamine N-methyltransferase (PeMT), which synthesizes phosphatidylcholine (PtdCho) via sequential methylation of phosphatidylethanolamine (PtdEtn) using S-adenosylmethionine (AdoMet) as a methyl donor. We identified PeMT activity in rat mammary tissue, and differences in affinities for substrate, as well as in activities as a function of pH, suggest that at least two distinct enzyme activities are involved [i.e. one catalysing the methylation of PtdEtn to form phosphatidyl-N-methylethanolamine (PtdMeEtn) and the other catalysing the methylation of PtdMeEtn and phosphatidyl-NN-dimethylethanolamine (PtdMe2Etn) to form PtdMe2Etn and PtdCho, respectively]. The relationships between AdoMet concentrations and PtdCho formation from endogenous PtdEtn in rat mammary homogenate were complex: a sigmoidal component (with a Hill coefficient of 2.2), requiring 55 microM-AdoMet for half saturation (Vmax. = 9 pmol/h per mg of protein), and a high affinity component (Kapparent = 8.7 microM and Vmax. = 3.8 pmol/h per mg of protein) were identified. When exogenous PtdMe2Etn was added as substrate, PtdCho formation exhibited Michaelis-Menten kinetics for AdoMet, and its affinity for AdoMet was high (Kapparent = 9 microM, Vmax. = 85 pmol/h per mg of protein). In the presence of endogenous substrates, the rates of PeMT-catalysed PtdCho formation within homogenates of rat mammary tissue were similar in tissue from lactating and non-lactating animals. When exogenous PtdMe2Etn was added to homogenates of rat mammary tissue, tissue from lactating rats made twice as much PtdCho as did tissue from non-lactating rats. Isolated mammary epithelial cells also exhibited PeMT activity; the rate of formation of PtdCho was much greater in intact versus broken cells. We also identified PeMT activity in homogenates of mammary tissue from non-lactating humans. The rate of PtdCho formation was of similar magnitude to that seen in rat tissue. This evidence supports the hypothesis that some of the choline found in milk could have been synthesized de novo in the mammary gland.

Both type I and II insulin-like growth factor receptor binding increase during lactogenesis in bovine mammary tissue.

Bovine GH is a potent stimulant of lactation, and the insulin-like growth factors I and II (IGF-I and -II) are believed to mediate GH's growth-promoting actions. Since all of IGF's known actions are mediated through its receptor subtypes, we analyzed the distribution of IGF receptor subtypes in lactating and nonlactating bovine mammary tissue. Analysis of competition curves showed that IGF-I had greater potency than IGF-II in competing with [125I]IGF-I for binding to membranes prepared from both lactating and nonlactating animals. An insulin concentration of 4 micrograms/ml displaced less than 40% of the [125I]IGF-I bound to membranes prepared from both lactating and nonlactating animals, indicating that a high percentage of [125I]IGF-I was bound to the type II receptor. Lactation was associated with an increase in the total amount of [125I]IGF-I bound, and this change was due to an increase in binding to both receptor subtypes. Specifically, membranes prepared from lactating animals had a 3-fold increase in binding competed for by insulin and a 2-fold increase in binding not competed for by insulin. Affinity cross-linking of [125I]IGF-I to membranes prepared from both lactating and nonlactating animals, followed by polyacrylamide gel electrophoresis (PAGE) and autoradiography, showed that 260K and 135K bands were present. Competition experiments indicated that unlabeled IGF-I effectively competed for binding to the 260K band, whereas insulin did not. Binding to the 135K band could be inhibited by both IGF-I and insulin. The intensity of the labeled bands showed that type II receptors were relatively more abundant than type I receptors in membranes from both lactating and nonlactating animals. Membranes prepared from lactating animals showed both 135K and 127K species of the type I receptor, whereas nonlactating animals showed only the 135K band. We conclude that type I and II receptors are present in bovine mammary tissue, and type II predominate. Lactation is associated with increases in the concentration of both receptor subtypes, especially type I receptors. Lactation may be associated with structural changes in the type I receptor. These changes in receptor distribution could play a role in modulating the physiological effects of the IGFs on mammary tissue.