First Half Of Lactation Research Articles

Breed and Parity Effects on Energy Balance Profiles Through Lactation: Evidence of Genetically Driven Body Energy Change

The aim of this study was to characterize patterns of energy balance through lactation of cows kept under constant feeding conditions. Danish Holstein, Danish Red, and Jersey cows were studied during consecutive lactations and remained on the same dietary treatment throughout. They were fed a normal (13.55 MJ of digestible energy/kg of dry matter) or a lower energy diet (12.88 MJ of digestible energy/kg of dry matter) ad libitum throughout lactation. Energy balance was calculated using the effective energy (EE) system in such a way that energy balance equated to body energy reserve change. In the EE system the energy values assigned to feeds are directly equivalent to the energy requirements of the animal; 1 MJ of EE supply has the same energy value as 1 MJ of lipid loss from the body. The resulting body energy change data were analyzed using a linear spline model. There was no evidence to suggest that different combinations of breed and parity required different knot placements. The Holstein mobilized significantly more body energy in early lactation than the Danish Red and Jersey breeds. Parity 1 cows mobilized significantly less than parity 2 and 3 cows. There was a significant interaction between breed and parity in the first half of lactation due to parity 1 Jersey cows having a greater mobilization than would be expected of the difference between parities in the other breeds. As lactation progressed, the differences between parities and between breeds decreased. Cows on the higher energy diet had a more positive energy balance. Within breed and parity, the following possible predictors of individual differences in body energy change were examined: fatness-corrected live weight, condition score at calving, and genotype. There was no difference in the predicted cow effect or residual energy balance profile when grouped according to quartiles of corrected live weight or according to condition score at calving. During the period of most negative energy balance (d 14) there was no significant relationship between live weight and intake, suggesting that, within diet type, the systematic patterns of body energy change through lactation in cows that were kept under stable and sufficient nutritional conditions cannot be accounted for by environmental factors such as constrained intake or condition score at calving. Thus, these patterns appear to have a genetic basis. The proportion of the phenotypic variation (remaining after accounting for fixed effects) accounted for by additive genetic effects varied through lactation from 4.2 to 13.0%. Genetic correlations between early and late lactation energy balances were low and close to zero, suggesting that body energy changes in early and late lactation are genetically independent traits.

Zum Einsatz von Luzernesilage bei Kühen mit hoher Milchleistung

Studies on the use of lucerne silage as a forage source for high-yielding dairy cows. It was the main objective of this study to investigate effects of lucerne silage inclusion in maize silage-based rations on dry matter intake, milk yield and metabolic parameters of high yielding dairy cows. Three total mixed rations (TMR) were used. They contained as the main forage source on a dry matter basis 41.5% maize silage and 12% grass silage (ration maize) or 18% maize silage and 29% of either grass silage (ration grass) or lucerne silage (ration lucerne). Organic matter digestibility, determined with wether sheep, was 77 and 65% for the grass and the lucerne silage, respectively. Each TMR was fed to 4 wether sheep to determine crude nutrient digestibilities. The content of net energy for lactation (NEL), as calculated from digestible crude nutrients, was (MJ/kg dry matter (DM)): 7.1 (maize), 7.2 (grass) and 6.8 (lucerne). Based on the results of a chemical fractionation of crude protein it was calculated that the content of utilisable crude protein (nXP) was not below 167 g/kg DM and that the N balance in the rumen was not negative for all 3 TMR. The TMR were fed for 169 days to at least 28 dairy cows per treatment which were in their first half of lactation. Feed intake, milk performance data and physiological indicators were measured for each individual cow. The DM intake was significantly higher in treatment lucerne (23.2 kg/d) than in treatments grass (21.2 kg/d) and maize (22.6 kg/d). Daily intakes of NEL and nXP were significantly lower in treatment grass than in the other two treatments. Milk yield was significantly higher in treatment maize (41.7 kg/d) than in treatments grass (37.3 kg/d) and lucerne (38.5 kg/d). This was associated with a significantly lower milk fat content in treatment maize (3.5%) in comparison with the other two treatments (3.9% each). Milk protein content was unaffected by treatment (3.4% on average). Similarly, the acetone content of milk was not significantly different between treatments. The beta-hydroxybutyrate content of the deproteinized blood was significantly higher in treatment lucerne at start of lactation, but this difference disappeared in subsequent parts of the experimental period. Measurements of back fat thickness indicated mobilisation and retention of energy by cows during the experimental period to be similar in all treatments. It is concluded that lucerne silage is a suitable forage source for high yielding dairy cows in spite of its low digestibility.

Effect of Lameness on Culling in Dairy Cows

The purpose of this study was to assess the effect of lameness on dairy cow survival. Cox's proportional hazards regression models were fitted to single-lactation data from 2520 cows in 2 New York State dairy herds. Models were controlled for the time-independent effects of parity, projected milk yield, and calving season, and for the time-dependent effects of lameness and culling. Other common diseases were found to be nonconfounding and so were not included in any of the final models. Survival was measured as the time from calving until death or sale. Cows were censored if they reached the start of the next lactation or end of the study, whichever occurred first. All models were stratified by herd. For all lameness diagnoses combined, survival in the herd decreased for those cows becoming lame during the first half of lactation, with a hazard ratio of up to 2 times that of a nonlame cow. Foot rot diagnosed during the second or third months of lactation decreased survival during the same time period (hazard ratio = 5.1; 95% confidence interval = 1.6 to 16.2). Sole ulcers diagnosed in the first 4 mo of lactation decreased survival in several subsequent periods in which the strongest association was between diagnosis in the third and fourth months of lactation and exit from the herd during that same period (hazard ratio = 2.7; 95% confidence interval = 1.3 to 6.0). Foot warts were not associated with decreased survival in this analysis. Lameness was never associated with increased survival in any of the models.

An unexpected parallelism between Vitamin A and PCBs in seal milk

Bioaccumulating pollutants such as polychlorinated biphenyls (PCBs) induce a range of adverse effects in mammals. Vitamin A metabolism is prone to such pollutant disruption which may be particularly harmful for young animals. During lactation, maternal PCBs are transferred to the offspring through the milk. Seal milk is very fatty and consequently contains large amounts of these persistent lipophilic contaminants. In the present study, we investigated the relationships between PCBs and Vitamins A and E during lactation, in free-ranging grey seals, using longitudinal samples of milk and blubber. We discovered that, unlike Vitamin E or triglycerides, the dynamics of Vitamin A matches closely those of PCBs in milk throughout lactation. Levels of Vitamin A and PCBs remain constant during the first half of lactation and then increase at late lactation, indicating that pups ingesting higher levels of PCBs also ingest higher amounts of Vitamin A.

An unexpected parallelism between Vitamin A and PCBs in seal milk

Bioaccumulating pollutants such as polychlorinated biphenyls (PCBs) induce a range of adverse effects in mammals. Vitamin A metabolism is prone to such pollutant disruption which may be particularly harmful for young animals. During lactation, maternal PCBs are transferred to the offspring through the milk. Seal milk is very fatty and consequently contains large amounts of these persistent lipophilic contaminants. In the present study, we investigated the relationships between PCBs and Vitamins A and E during lactation, in free-ranging grey seals, using longitudinal samples of milk and blubber. We discovered that, unlike Vitamin E or triglycerides, the dynamics of Vitamin A matches closely those of PCBs in milk throughout lactation. Levels of Vitamin A and PCBs remain constant during the first half of lactation and then increase at late lactation, indicating that pups ingesting higher levels of PCBs also ingest higher amounts of Vitamin A.

Test-Day Genetic Analysis of Condition Score and Heart Girth in Holstein Friesian Cows

This study aimed to estimate heritability for condition score and heart girth using a test-day model, to investigate the genetic relationships between condition score, heart girth, and milk yield traits and to analyze the genetic relationships of condition score and heart girth measured at different stages of lactation. Cows from 25 dairy herds were scored for body condition and measured for heart girth at 3-mo intervals for 2 yr. Approximately 5000 test-day observations on condition score, heart girth, and milk fat and protein yield from 1344 Italian Friesian cows were analyzed using two approaches: 1) repeated observations for a trait were considered repeated measurements of the same trait; 2) observations for a trait collected in different stages of lactation (dry period, 1 to 75, 76 to 130, 131 to 210, and 211 to 300 DIM) were treated as different traits. (Co)variance components and related parameters were estimated using REML multiple-trait procedures and animal models with unequal design for different traits. Heritability estimates for fat and protein test-day yield and for test-day condition score and heart girth were 0.22, 0.18, 0.29, and 0.33, respectively. Condition score was negatively correlated with yield traits and positively correlated with heart girth, whereas genetic relationships between heart girth and milk yield traits were negligible. Heritability estimates were 0.27 for condition score recorded in the first half of lactation (1 to 75 and 76 to 130 DIM), 0.36 for condition score in the second half of lactation (131 to 210 and 211 to 300 DIM) and 0.32 for condition score recorded on dry cows. Genetic correlations between condition scores measured in different lactation stages were generally high (0.85 or more), with the exception of the relationships between the first and the last stage of lactation (0.74) and between the first half of lactation and the dry period (0.7). Heritability estimates for heart girth in different lactation stages ranged from 0.31 to 0.40, and genetic correlations between high girth measured in different lactation stages were higher than 0.80.

Chapter 13 Non-metabolic and metabolic factors causing lactational anestrus: rat models uncovering the neuroendocrine mechanism underlying the suckling-induced changes in the mother

Follicular development and ovulation are strongly inhibited during lactation. Administration of a high dose of estrogen induces luteinizing hormone (LH) surges in ovariectomized lactating rats, suggesting that brain mechanisms regulating cyclic LH release remain intact in lactating mothers. On the other hand, tonic LH release is profoundly suppressed in lactating rats. This suggests that lactational anestrus is mainly due to suppression of the mechanism regulating pulsatile gonadotropin-releasing hormone secretion in the hypothalamus, which is responsible for follicular development and steroid production. Both metabolic and non-metabolic factors are involved in suppressing pulsatile LH secretion throughout lactation in rats. During the first half of lactation, pulsatile LH secretion is strongly suppressed, even if milk production is attenuated by pharmacological blockade of prolactin secretion in ovariectomized lactating rats. Pulsatile LH release quickly recovers by removing pups or blocking neuronal input by hypothalamic deafferentation during the period. These data suggest that the suckling stimulus itself is responsible for suppression of LH release during the first half of lactation. During the second half of lactation, negative energy balance, which is caused by the milk production, appears to play a dominant role in suppressing LH secretion. Blockade of milk production by inhibiting prolactin release causes a gradual increase in LH release even if the vigorous suckling stimulus by foster pups remains. In conclusion, the suckling stimulus itself predominantly suppresses LH pulses during the first half of lactation and metabolic factors take over the role of the suckling stimulus during the second half of lactation.

Genetic aspects of condition score, heart girth and milk-yield traits in Italian Friesian cows

AbstractFunctional traits related to costs are currently of interest for selection and management of dairy cattle. The present study was aimed to estimate heritability for body condition score (BCS) and heart girth (HG), to investigate the genetic relationships between BCS, HG and milk-yield traits using a test-day model and to analyse the consistency of the estimates in different lactation stages. Cows from 25 dairy herds were scored for BCS and measured for HG at 3-month intervals for 2 years. Approximately 5000 test-day observations on BCS, HG and milk fat and protein yield from 1429 Italian Friesian cows were analysed using two approaches: (1) repeated observations were treated as repeated measurements of the same trait, both within and across lactations; (2) observations collected in different stages of lactation (dry period, 1 to 75 days in milk (DIM), 76 to 130 DIM, 131 to 210 DIM, 211 to 300 DIM) were treated as different traits. (Co)variance components and related parameters were estimated using REML multiple-trait procedures and unequal design animal models.Heritability estimates (approach 1) for fat and protein test-day yield, BCS and HG were 0.22, 0.18, 0.29 and 0.33, respectively. BCS was negatively correlated with yield traits (-0.43 and -0.48 for fat and protein yield, respectively) but positively correlated (0.33) with HG. Genetic relationships between HG and milk-yield traits were negligible. Heritability estimates (approach 2) were 0.28 and 0.27 for BCS recorded in the first half of lactation (1 to 75 and 76 to 130 DIM, respectively), 0.36 for BCS measured on cows in the second half of lactation and 0.32 for BCS recorded on dry cows. Heritability estimates for HG in different lactation stages ranged from 0.31 to 0.40. Genetic correlations between BCS measured in different lactation stages were generally high (0.85 or more), with the exception of the correlation between the first and the last stage of lactation (0.74) and of the relationships between the beginning of lactation and the dry period (0.7). Genetic correlations between HG measured in different lactation stages were mostly higher than 0.80.

Diet Choice by Dairy Cows. 1. Selection of Feed Protein Content During the First Half of Lactation

Effects of weeks in milk and milk yield on diet choice were recorded during the first half of lactation. Low and high protein feeds were used, and both consisted of 30% concentrate and 70% grass silage (fresh matter basis). Both feeds contained similar amounts of energy, but the crude protein contents were 131 and 185 g/kg of dry matter (DM), respectively, for the low and high protein feeds. In a nutrient flow experiment with three cannulated lactating cows, the metabolizable protein yields of the low and high protein feeds were 75 and 114 g/kg of DM, respectively. Thirty-seven cows were divided into control groups for the low and high protein feeds, and a choice group had access to both the low and the high protein feeds. Intake of DM and milk yield by cows in the control group fed the high protein feed were higher than those by cows in the control group fed the low protein feed, but these measurements did not differ from those of cows in the choice group. Cows in the choice group consumed a mean of 683g of high protein feed/kg of total intake, which differed from what would be considered random intake (500 g/kg total intake). Diet choice did not systematically change during the experiment and was not correlated with weeks in milk, milk yield, or milk protein output. We concluded that diet selection differed significantly from what would be considered random, which allowed cows in the choice group to perform well. However, diet choice did not reflect the estimated metabolizable protein requirements of the cows.

Ermittlung der passage eines markers im verdauungstrakt von milchkühen mit hilfe unterschiedlicher schätzverfahren: Einfluss von laktationsstadium, fütterungsniveau und rationszusammensetzung

An 12 Milchkühen wurde der Einfluß von Laktationsstadium, Fütterungsniveau und Rationszusammensetzung auf die Passage des Verdauungsbreis aus dem Reticulorumen untersucht. Es wurden gemischte Konzentratfutter: Rauhfutter‐Rationen verfüttert, wobei die Rauhfutterkomponenten aus Maisganz‐pflanzensilage und extensiv bzw. intensiv produzierter Grassilage im Verhältnis 40: 60 (Frischsubstanzbasis) bestanden und das durchschnittliche Konzentratfutter: Rauhfutter‐Verhältnis der Rationen 34: 66 in der Trockensubstanz betrug. Als Marker wurde Titan(IV)‐oxid (TiO2) eingesetzt, dessen Konzentration im Kot nach oraler Verabreichung über 120 h hinweg ermittelt wurde. Probenahmen erfolgten in vier Untersuchungszeiträumen (Perioden) ‐ ante partum (1 Periode) und in der ersten Laktationshälfte (3 Perioden) ‐, die jeweils einen bestimmten physiologischen Zustand charakterisierten. Die Fütterung der Tiere erfolgte leistungsgerecht mit durchschnittlichen Trockensubstanzaufnahmen von 12,0; 16,8; 22,6 und 21,2 kg in den jeweiligen Untersuchungszeiträumen. Passageparameter wurden mit Hilfe unterschiedlicher Schätzverfahren ermittelt, wobei ein Modell mit Gamma‐Zeitabhängigkeit (G4G1), ein exponentielles Modell sowie ein Modell, in dem die mittlere Verweildauer des Markers im gesamten Verdauungstrakt als Mittelwert aller Punkte der Marker‐Exkretionskurve berechnet wird, verwendet und verglichen wurden. Die Ergebnisse zeigten eine deutliche Beschleunigung der Passage des Markers aus dem Reticulorumen von ante partum nach post partum. Zwischen G4G1 und dem Modell, das auf die Verwendung des Mittelwertes Bezug nimmt, zeigte sich eine gute Übereinstimmung. Dagegen war die Übereinstimmung zwischen G4G1 und dem exponentiellen Modell schlechter. Effekte von Periode und Ration waren post partum nur für die Passagerate aus dem und für die mittlere Verweildauer im schnellen Kompartiment (beide geschätzt nach G4G1) nachzuweisen. TiO2 scheint als externer Marker zur Ermittlung von Passageraten gut einsetzbar zu sein, wobei die Größenordnung der Werte im Bereich der Passage von Konzentratfutterteilchen liegen dürfte.

Mechanisms of the effect of Icv IL-1β on oxytocin release in the anesthetized, lactating rat

Interleukin-1β stimulates the release of many hypothalamic hormones, including oxytocin. Experiments were done to examine the effect of icv IL-1β on circulating oxytocin levels in rats throughout lactation, and to determine if alpha-adrenergic mechanisms and/or prostaglandins were involved as mediators. Blood samples were taken from urethane-anesthetized, nonlactating and lactating rats before and after icv treatment with either IL-1β or PBS-BSA; or icv treatment with IL-1β following pretreatment with either phentolamine or indomethacin. Plasma was assayed using a specific oxytocin radioimmunoassay. Interleukin-1β stimulated oxytocin release in all rats tested resulting in an approx 2- to 2.5-fold increase in plasma hormone levels. Throughout the first half of lactation, oxytocin responsiveness to IL-1β increased. On d 20 of lactation, the plasma oxytocin response to IL-1β was depressed. Adrenergic mechanisms may be involved in this differential effect on oxytocin release in lactation as phentolamine pretreatment attenuated IL-1β-induced oxytocin release in early lactation and reversed the depression in IL-1β-stimulation of oxytocin release observed in late lactation. It is possible that central IL-1β is involved in the weaning process: oxytocin responsiveness to IL-1β increases throughout the first half of lactation when oxytocin demands are high and is depressed in late lactation when weaning is occurring.

Reproductive Demands and Mass Gains: A Paradox in Female Red Squirrels (Tamiasciurus hudsonicus)

We studied the response of lactating red squirrels to reproductive demands that were experimentally increased by litter-size manipulations. Females with experimentally increased reproductive demands ('augment' females) gained significantly more body mass during the first half of lactation than did unmanipulated ('control') females. However, augment females lost more mass following the mid-point of lactation than control females, so that the net mass difference between early lactation and late summer was negligible for both treatments. Measurements of total body water at the mid-point of lactation confirmed that mass gains during early lactation reflected changes in body fat levels, indicating that females adjusted their energy stores according to their reproductive demands. We also analysed the relationship between early lactation mass gain and natural litter size among a larger group of control females, studied at the same site between 1990 and 1994. There was a significant, positive relationship between natural litter size and female mass gain. Furthermore, females characterized by the largest gains in body mass had the highest levels of juvenile survival to emergence, indicating that early lactation mass gain is an important component of parental investment. These responses suggest that (i) energy storage during early lactation is used to reduce daily energy requirements during late lactation; (ii) breeders use demands during early lactation to 'forecast' requirements during late lactation; and (iii) that despite the elevated energetic demands of lactation, individuals can quickly adjust their energy budgets from slightly positive to highly positive levels.

Proximate Composition of Australian Sea Lion Milk Throughout the Entire Supra-Annual Lactation Period

A total of 87 milk samples was collected from 47 Australian sea lions, Neophoca cinerea, with pups ranging from one day old to 471 days old. Analysis of proximate composition yielded an overall mean (+/- s.d.) of 30.82 +/- 9.84% lipid, 56.97 +/- 9.96% water, 9.97 +/- 2.52% protein, and 0.88 +/- 0.25% ash. Milk fat content increased during lactation and was inversely proportional to water content. There was a significant difference between the lipid content of milk collected during the first half of lactation (<250 days, range 1-120 days) (26.1 +/- 8.7%, n = 35) and that collected during the second half of lactation (>250 days, range 314-469 days) (39.1 +/- 5.5%, n = 12) (Mann-Whitney U-test, P < 0.0001). The relatively low lipid (energy) content of Australian sea lion milk is postulated to be, in part, an adaptive response to living in a low-energy marine environment where the rate of energy transfer from mother to young is decreased and the duration of maternal dependence of the young is increased.

Dietary energetics of the insectivorous Mexican free-tailed bat (Tadarida brasiliensis) during pregnancy and lactation.

Stomach content analysis of 20 pregnant (x body mass=13.4 g) and 18 lactating (x body mass=11.5 g) female Tadarida brasiliensis revealed that the diet, expressed as percent volume, consists largely of lepidopterans, coleopterans, hymenopterans, and dipterans, in decreasing order of importance. We found no significant difference in the diet of pregnant and lactating females when expressed as percent volume. However, when expressed as percent frequency, proportionately more pregnant females fed on lepidopterans, coleopterans, and dipterans than did lactating bats, and proportionately more lactating females fed on hymenopterans. We found no significant differences in the percentages of water, lean dry mass, fat, and energy density in the stomach contents of pregnant and lactating females. Water in stomach contents averaged 62.7% and fat and lean dry mass averaged 22.2% and 15.2%, respectively (expressed as percentage of wet mass); energy density averaged 31.2 kJ g-1 dry mass. This relatively high energy density of stomach contents, as compared to whole insects, can be attributed to the consumption of insects high in fat (especially flying ants) and the abdomens only of moths and beetles (other body parts being discarded). Estimates of nightly food intake increased markedly from mid- to late pregnancy, stabilized or decreased during late pregnancy, and increased again during early to mid-lactation. Average nightly feeding rate doubled from pregnancy to lactation and increased threefold during the first half of lactation. By adjusting our gravimetric estimates of nightly feeding rate upwards by 40% (based on estimates of field metabolic rate), we estimate that the average nightly energy intake of female T. brasiliensis ranges from 57 kJ day-1 in early lactation to 104 kJ day-1 in mid-lactation. These estimates represent nightly feeding rates averaging 39% and 73% of a female's body mass in the period from early to mid-lactation, respectively.

Energetics of Lactation and Growth in the Fat Sand Rat, Psammomys obesus: New Perspectives of Resource Partitioning and the Effect of Litter Size

We calculated energy budgets for lactating fat sand rats (Psammomys obesus) and their pups. Females increased metabolizable energy intake (MEI) over maintenance requirements (MEm) by up to 85% in small (one to three pups), 118% in medium (four to five pups) and up to 138% in large (six to eight pups) litters. In the first half of lactation (days 1-15) there was an increase in body energy of females by 8 and 62·5 kJ in medium and large litters, respectively, and a decrease of 74·5 kJ in females of small litters. In the second half of lactation (days 16-30) body energy of females decreased by 136, 206 and 191 kJ in small, medium and large litters, respectively, which contributed to 30-45% of milk energy production.Milk intake per pup was significantly higher in small than in large litters. In all litters, milk intake per pup increased with time and then decreased towards weaning. Maximum energy retention of pups during growth occurred in small litters. From 15 days of age, pups of all litters started to feed on Atriplex halimus leaves. Pups increased their MEI from A. halimus rapidly and concomitantly reduced their milk energy intake. Just before weaning, MEI from A. halimus reached 90%, 91% and 93% for small, medium and large litters, respectively.Efficiency of lactation is usually expressed as either (i) body energy increase in young as a fraction of the additional MEI above MEm of females or (ii) body energy produced in the litter as a fraction of milk production. Both methods do not determine the efficiency of milk energy intake for growth for they do not separate MEm of pups from heat increment of milk energy for growth. We used a two component non-linear model to calculate MEm of pups and their efficiency of utilization of milk for growth of the pups. Efficiency of utilization of milk for growth was 85-91% for the pups and therefore 9-15% of the metabolizable energy of milk, above MEm of the pups, was lost as heat increment of feeding. No such values are available for other rodents.

Studies on Regulatory Mechanisms for Gonadotropins Secretion in Lactating Rats

The present study was designed to clarify mechanisms responsible for regulation of FSH and LH secretion and ovarian follicular maturation during lactation in rats. Throughout lactation in rats, plasma levels of follicle-stimulating hormone (FSH) remain within the range normally found during the diestrous phase of the estrous cycle, regardless of litter size, although levels of luteinizing hormone (LH) in plasma are consistently lower in dams nursing 8 pups than in dams nursing 2 pups. All antral follicles larger than 401μm in diameter degenerate during the first half of lactation and do not reappear until the second half of lactation in rats nursing 8 pups. Healthy follicles of this size, however, are always present in mothers nursing 2 pups. These changes in follicular degeneration corresponded with a decrease in concentrations of plasma LH in the first half of lactation, and follicular development was noted with an increase in basal levels of plasma LH in the second half of lactation. Corpora lutea formed after postpartum ovulation secrete a large amount of progesterone under the influence of prolactin released as a result of suckling. Despite continued suckling, plasma concentrations of prolactin and progesterone decline gradually and a slight increase in plasma concentrations of estradiol-17β occurs during the second half of lactation. Concentrations of bioactive and immunoreactive inhibin in plasma vary with the number of healthy antral follicles and no positive correlation has been observed with luteal function throughout lactation in the rat.On the basis of the present results it is indicated that 1) tonic secretion of LH is an important factor in normal follicular maturation and maintenance in lactating rats, 2) the suckling stimulus, rather than ovarian factors, is mainly responsible for the suppression of FSH as well as LH secretion during the first half of lactation in rats nursing 8 pups. On the other hand, during the second half of lactation in rats nursing 8 pups, ovarian inhibin plays a primary role in the suppression of FSH secretion, whereas ovarian steroids act to suppress LH secretion. The present findings also indicate that 3) the suckling stimulus alone suppressed the secretion of both LH and FSH and this effect may be mediated by the inhibition of LH releasing hormone (LH-RH) secretion from the hypothalamus, 4) the suckling-induced inhibition of LH-RH may be primarily mediated by endogenous corticotropin-releasing hormone and opioid peptides and 5) the pituitary-adrenal system is capable of influencing the maintenance of a normal secretion of gonadotropins and prolactin as well as the maintenance of ovarian function during the lactation in the rat.

Mechanisms responsible for suppression of FSH and LH during lactation in the rat

Mechanisms responsible for suppression of FSH and LH secretion during lactation were investigated in rats, with special reference to the suckling stimulus and ovarian inhibin. Concentrations of immunoreactive inhibin in the peripheral plasma and bioactive inhibin in ovarian venous plasma were always low on days 3 and 5 of lactation in dams nursing eight pups, whereas values were always high on days 17 and 20 of lactation in dams nursing eight pups and on day 5 of lactation in dams nursing two pups. There was an FSH surge within 48 h after removal of litters on days 3 and 5 of lactation in dams nursing eight pups, whereas plasma concentrations of FSH were unchanged within 48 h by removal of litters on days 17 and 20 of lactation in dams nursing eight pups and on day 5 of lactation in dams nursing two pups. Plasma LH concentrations increased significantly compared with those of control animals within 24 h after removal of the litter on any day of lactation, regardless of the litter size. Plasma FSH levels increased within 6 h after bilateral or unilateral ovariectomy in lactating rats only on the days when plasma concentrations of inhibin were high before ovariectomy, such as day 17 of lactation in dams nursing eight pups and on day 5 of lactation in dams nursing two pups, whereas the mean concentrations of plasma LH showed no significant increase within 12 h after bilateral ovariectomy in these lactating rats. Treatment with progesterone or oestradiol-17 beta after unilateral ovariectomy did not inhibit the increase in plasma FSH levels, while the increase in plasma concentrations of FSH after surgery was completely inhibited by injecting inhibin (porcine follicular fluid). Treatment with steroid hormones inhibited the basal levels of LH in unilateral ovariectomized lactating rats. Plasma FSH concentrations increased sharply within 6 h after a single i.v. injection of anti-inhibin serum on days 10, 15 and 20 of lactation in dams nursing eight pups and on day 5 of lactation in dams nursing two pups, whereas only a small but significant increase in concentrations of FSH was noted 6 h after the antiserum treatment on day 5 of lactation in dams nursing eight pups. Concentrations of plasma LH were unchanged by treatment with antiserum in lactating rats throughout lactation.(ABSTRACT TRUNCATED AT 400 WORDS)

Milk composition in the red-necked wallaby, Macropus rufogriseus banksianus (Marsupialia)

1. 1. Milk samples were collected throughout lactation from 10 captive red-necked wallabies. 2. 2. The milk solids content increased throughout lactation and was accompanied by major changes in the relative proportions of protein, lipid and carbohydrate. 3. 3. The carbohydrate fraction consisted of oligosaccharides in the first half of lactation but changed subsequently to monosaccharides. 4. 4. The quantitative and qualitative changes observed were similar to those recorded for other macropodids.

Effect of adrenalectomy on the regulation of the secretion of gonadotrophins and prolactin in the lactating rat.

The suckling stimulus exerts an inhibitory action on the release of gonadotrophins during lactation. The possible involvement of the adrenal glands in this process was examined by studying the plasma levels of gonadotrophins in lactating rats ovariectomized on the day after parturition. It appeared that the suppression, throughout suckling, of the rise in levels of gonadotrophins in blood after ovariectomy occurred to the same extent in adrenalectomized and in sham-operated animals. It thus seems unlikely that adrenocortical hormones, albeit secreted in larger quantities during lactation, exert an inhibitory effect on the release of gonadotrophins. Adrenalectomy had a marked effect on the plasma concentrations of prolactin during the second half of lactation. Whereas plasma concentrations of prolactin in the first half of lactation were similar in adrenalectomized and sham-operated rats, the concentrations in adrenalectomized rats did not undergo the reduction found in sham-operated rats. Adrenal hormones may thus be involved in the reduction of blood levels of prolactin observed in rats and in other mammals as lactation progresses.

Lactating Dairy Cow Responses to Dietary Sodium, Chloride, and Bicarbonate During Hot Weather

Twelve Holstein and eight Jersey cows in the first half of lactation were assigned to one of four concentrate formulations to provide supplemental sodium salts in the total diet dry matter of 1) .5% sodium chloride, 2) .5% sodium chloride plus .72% sodium bicarbonate, 3) .5% sodium chloride plus .72% sodium bicarbonate, and 4) 1.44% sodium bicarbonate to measure effects on body temperature and respiration rate, milk yield and composition, and blood components that reflect acid-base balance. After data were adjusted for body weight, age, and a covariate based on differences within individuals in the standardization period, they were analyzed with a model that included breed, treatment, week, temperature-humidity index, and interactions. Cows fed sodium bicarbonate and no supplemental sodium chloride had lower body temperatures than the other groups. High sodium and chloride in basal diet and drinking water prevented the large change in dietary amount and ratio of these two electrolytes that treatments were designed to impose. More stringent control of dietary amounts and greater heat stress will be necessary to show effects of these elements on acid-base balance.