Influence of selection for milk yield on endogenous hormones and metabolites in Holstein heifers and cows

Plasma concentrations of prolactin (PRL), growth hormone (GH), insulin, glucagon, glucose, urea and free fatty acids (FFA) were measured in Holstein calves, yearlings, bred heifers and primiparous cows, either sired by bulls with high predicted differences (PD) for milk (selection group) or by bulls from an unselected random bred control population (control group; n = 6). Serial blood samples were collected before and after feeding for an 8-h period from 0900 to 1700 h. All animals were fed a complete feed at 1100 h and administered insulin (.6 IU/100 kg body weight) at 1400 h. Mean plasma PRL was greater in control animals after feeding and insulin administration, while GH was greater overall in selection cattle. Insulin remained elevated longer in selection animals after exogenous administration, and plasma glucagon was increased in the control group. While plasma glucose and urea were unaffected by genetic group, plasma FFA were elevated in selection group calves and primiparous cows compared with the control group. All hormones and metabolites differed among the pre- and post-feeding and insulin administration periods and also with age. Mean PRL and GH increased after feeding, while glucagon decreased after exogenous insulin. Plasma FFA declined after feeding, while urea and glucose were similar before and after feeding. Mean PRL increased and glucagon decreased with advancing age and plasma GH and insulin showed inverse relationships at different ages. Plasma FFA changes closely followed GH changes with age, while plasma glucose more closely followed insulin changes with age. Results indicate that all hormones measured and FFA responded to genetic selection for milk, and increases in GH are uniformly associated with increased genetic potential for milk yield.     

Relationships among growth hormone and prolactin secretory parameter estimates in Holstein bulls and their predicted differences for lactational traits

Selection of dairy sires is based on the production records of their female ancestors, half-sibs and daughters. No trait expressed by the sire is used. Concentrations of growth hormone (GH) and prolactin (PRL), hormones produced in both males and females that are fundamental in lactation, may be correlated with production. A study was conducted to determine whether measures of these hormones in the sire would be useful predictors of lactational ability of daughters. Blood samples were collected at 15-min intervals for 8 h from 26 Holstein bulls (5.5 yr of age) that had one progeny summary available. Plasma concentrations of GH and PRL were quantified and the mean and baseline concentrations and the frequency and mean amplitude of the secretory peaks were determined for each bull. Concentrations among these values and bulls' predicted differences (PD) were determined. Significant negative correlations were detected for frequency of GH peaks and PD for yield of milk, fat and protein; correlations were positive for PRL baseline concentrations and PD for fat and protein (P less than .10), and correlations were negative for frequency of PRL peaks and PD for milk, fat and protein (P less than .10). Addition of estimates of bull hormone secretory parameters to breeding values based on performance of relatives considerably improved the accuracy (R2) for predicting progeny performance from sire information. Certain characteristics of the patterns of GH and PRL secretion may be heritable and aid in identification of superior dairy animals.     

Effect of feeding thyrotropin-releasing hormone to lactating sows

Two experiments were conducted to assess the effects of feeding thyrotropin-releasing hormone (TRH) during lactation on sows. In Exp. 1, sows were fed 0, 1, 10, 100 or 1,000 mg TRH on d 10.8 +/- .4 (mean +/- SE) after parturition. Blood samples were taken from sows every 30 min from -2 h to 8 h and at 10, 12 and 18 h from feeding. Consumption of 100 or 1,000 mg TRH increased mean serum concentrations of thyroxine (T4; P less than .001), 1,000 mg TRH increased growth hormone (GH; P less than .06) and 100 or 1,000 mg TRH increased prolactin (PRL; P less than .01), but insulin (INS; P greater than .10) was unaffected by TRH. Serum concentrations of T4 were elevated within 2 to 4 h after feeding TRH and remained elevated for 12 to 18 h. Concentrations of GH and PRL began to increase immediately after feeding 100 or 1,000 mg TRH and remained elevated for 6 and 8 h, respectively. In Exp. 2, sows were fed 0 or 200 mg TRH from d 111 of gestation to weaning at 27.1 +/- .3 d of lactation. Consumption of TRH elevated concentrations of T4 at all stages of lactation and increased respiration rate on d 10 and d 20, heart rate on d 20, and milk production on d 20 of lactation. Consumption of TRH did not influence number of pigs born, number born alive, survival rate during lactation, sow body weight, heartgirth, backfat depth, feed disappearance, or milk production on d 10 of lactation. Piglets nursing sows fed TRH were similar in weight to piglets nursing sows not fed TRH on d 0 and 5 of lactation, but they were heavier on d 10 (P less than .07), 15 (P less than .001), 20 (P less than .001) and 27 (P less than .0001). Sows fed TRH took longer (P less than .001) to return to estrus after weaning than control sows. Results indicated that feeding TRH elevated T4, GH and PRL and that feeding TRH for the duration of lactation increased milk production on d 20 of lactation and increased weaning weights, but it delayed estrus after weaning.     

Endocrine changes in sows exposed to elevated ambient temperature during lactation

Seven sows were placed into one of two environmental chambers at 22 C, 5 d prior to farrowing. On day 9 of lactation, one chamber was changed to 30 C (n = 4) and the other remained at 22 C (n = 3). On days 24 and 25, blood samples were collected every 15 min for 9 hr and 7 hr, respectively. On day 24, thyrotropin releasing hormone (TRH) and gonadotropin releasing hormone (GnRH) were injected iv at hour 8. On day 25 naloxone (NAL) was administered iv at hour 4 followed 2 hr later by iv injection of TRH and GnRH. Milk yield and litter weights were similar but backfat thickness (BF) was greater in 22 C sows (P less than .05) compared to 30 C sows. Luteinizing hormone (LH) pulse frequency was greater (P less than .003) and LH pulse amplitude was less (P less than .03) in 22 C sows. LH concentrations after GnRH were similar on day 24 but on day 25, LH concentrations after GnRH were greater (P less than .05) for 30 C sows. Prolactin (PRL) concentrations were similar on days 24 and 25 for both groups. However, PRL response to TRH was greater (P less than .05) on both days 24 and 25 in 30 C sows. Growth hormone (GH) concentrations, and the GH response to TRH, were greater (P less than .0001) in 30 C sows. Cortisol concentrations, and the response to NAL, were less (P less than .03) in 30 C sows. NAL failed to alter LH secretion but decreased (P less than .05) PRL secretion in both groups of sows. However, GH response to NAL was greater (P less than .05) in 30 C sows. Therefore, sows exposed to elevated ambient temperature during lactation exhibited altered endocrine function.     

Evaluation of crossbred calf and cow types for the coastal plain of North Carolina

Data in this experiment consisted of 418 lactation records, and weaning and birth weight records from 600 crossbred calves. The traits evaluated included birth weight, weaning weight, weaning weight per cow exposed, weaning weight per weight of cow, weaning weight per weight of cow exposed, and predicted milk yield. Angus, Brangus, and Gelbvieh sires were mated to purebred Hereford cows. Yearling and 2-yr-old Angus-Hereford, Brangus-Hereford, and Gelbvieh-Hereford daughters then were bred to Polled Hereford bulls (Data Set 2). Later-parity Angus-Hereford, Brangus-Hereford, and Gelbvieh-Hereford daughters were mated to Salers or Simmental sires (Data Set 3). Differences between Gelbvieh- and Brangus-sired calves or Gelbvieh-Hereford and Brangus-Hereford daughters were never significant for weaning weight, birth weight, or milk yield. Angus crosses had the lowest weaning weight, birth weight, and milk yield, but the highest kilograms of calf weaned per cow exposed in all data sets. Angus-Hereford and Brangus-Hereford dams had higher weaning weight per weight of cow exposed than Gelbvieh-Hereford dams (P < .01) in Data Set 3. There were no other significant differences related to cow weight.     

Endocrine adaptations of periparturient swine to alteration of dietary energy source

Lipid compared with carbohydrate calories in the diet of peripartal sows was examined for specific metabolic effects. Blood samples from second-parity Yorkshire sows fed isocaloric-isonitrogenous diets that differed by either glucose or corn oil supplying 20% of the daily metabolizable energy intake were assayed for growth hormone (GH), insulin and prolactin (PRL) concentrations. On d 97 to 103 of gestation, serum GH levels were reduced following feed intake; the reduction was greater (P less than .05) in glucose-fed sows during the postprandial period. Insulin concentration was increased following consumption of either diet, but to a greater (P less than .05) extent in glucose-fed sows corresponding to the greater (P less than .05) blood glucose response. Basal, fasting concentrations of PRL were not affected by dietary energy source; however, thyrotropin-releasing hormone (TRH) infusion resulted in a greater (P less than .05) PRL response of fat-fed sows. Samples obtained on d 8 to 11 of lactation revealed that TRH stimulated (P less than .05) GH secretion and the response was greater (P less than .05) in fat-fed sows. Plasma insulin concentration was increased (P less than .05) following feeding, and the increase was greater (P less than .05) in glucose-fed sows. Similar to the response measured in gestating sows, TRH-induced increases in PRL was greater in fat-fed, lactating sows. Sham-nursing did not influence maternal PRL concentration compared with the immediate increase noted following natural nursing. No effect of dietary energy source on reproductive performance characteristics was noted. Results of this study have shown that dietary lipid, compared with carbohydrate, selectively increased the sensitivity of the pituitary gland to a GH and PRL secretagogue such as TRH. The beneficial effects of lipid in the diet of peripartal swine may be attributable, in part, to the effect of dietary lipid on maternal pituitary PRL synthesis and(or) release.     

Dopaminergic-like activity in toxic fescue alters prolactin but not growth hormone or thyroid stimulating hormone in ewes

Studies were conducted to determine the specificity and cause of altered pituitary hormone secretion when ewes ingest endophyte-infected (Acremonium coenophialum) GI-307 tall fescue (toxic fescue). Plasma concentrations of prolactin (PRL) but not growth hormone (GH) or thyroid stimulating hormone (TSH) in ewes grazing toxic fescue were significantly lower (P < .01) than concentrations measured in ewes grazing orchardgrass (OG). Comparing hormone secretory responses of ewes grazing each grasstype, ewes on toxic fescue released less PRL following thyrotropin releasing hormone (TRH) challenge than ewes on OG. TSH responses to TRH were not affected by grasstype. At this dose of TRH, GH secretion was not significantly affected in either group of ewes. In a separate study, dopamine hydrochloride (DA) was infused into control ewes to define the effect of a pure dopamine agonist on basal and TRH-stimulated secretion of PRL, GH and TSH. DA depressed both basal and TRH-stimulated secretion of PRL without affecting the basal concentrations or responses of GH or TSH. Based on the assumption that the active agent in toxic fescue responsible for the observed hypoprolactinemia was a dopaminergic agonist, haloperidol (HAL), a DA receptor blocking drug, was administered to ewes grazing toxic fescue or OG. HAL evoked significant PRL secretion unaccompanied by any GH or TSH effect in both toxic fescue and OG ewes. Administration of HAL resulted in a gradual increase over 4 hr in PRL in toxic fescue ewes and prolonged the duration of the PRL response to TRH. No differences in circulating plasma concentrations of DA, epinephrine or norepinephrine were measured in ewes on troxic fescue or OG.

Alterations in pituitary hormone secretion due to toxic factors in fescue were confined to PRL. Hormone secretory responses to TRH and HAL suggest that the effects on PRL are mediated through dopamine-like activity in toxic fescue.     

Low Fertility in Daughters of Bulls with 1/29 Translocation

Non-return rates at different stages after insemination were compared in daughters (n = 21,212) of five translocation heterozygous sires with all cows and heifers (n = 610,714) represented in the Norwegian Red Cattle (NRF) breeding statistics during the period October 1970 to September 1972. The daughters of the carrier bulls showed statistically significant lower non-return rates for 0–30, 30–60, 60–90, 90–120, 120–150, 150–180 and 180–270 days as compared with the control animals. The decline in non-return percentage for the daughters of translocation sires was greater than in the control animals during the period from 0–30 to 60–90 days indicating higher losses of fetuses during the early period after insemination.Daughter groups of carrier bulls consisting of adult cows only showed higher non-return rates than groups comprising both cows and heifers.     

Comparison of crossbred cows containing various proportions of Brahman in spring or fall calving systems: II. Milk production

Monthly estimates of 24-h milk yield were obtained on 160 spring-calving and 153 fall-calving crossbred cows containing various proportions (0, 1/4 or 1/2) of Brahman breeding, all bred to Limousin sires. Milk production was measured using weigh-suckle-weigh procedures. Interactions between crossbred group and season of calving were not significant. Across seasons, milk production rarely was affected by proportion of Brahman breeding; however, milk yield, averaged over six monthly measurements, was greater (P less than .10) for cows out of Hereford dams than for cows out of Angus dams. Averaged over six monthly measurements, estimated 24-h milk yield ranged from 5.3 kg for Hereford x Angus to 6.2 kg for Brahman x Hereford cows. Season of calving affected milk yield during the first and fourth measurement periods. Fall-calving cows produced more (P less than .01) milk during the first period whereas spring-calving cows produced more milk during the fourth period. Lactation patterns for the two seasons differed. Spring-calving cows had a more typical lactation curve, whereas the curve for fall-calving cows showed more variability in milk yield throughout lactation. Phenotypic correlations between monthly measurements of 24-h milk yield and calf performance tended to be moderate and positive within the spring group. Correlations for the fall group, although positive, tended to be weaker than those found in the spring group.     

Leptin, GH, PRL, Insulin and Metabolic Parameters Throughout the Dry Period and Lactation in Dairy Cows

Leptin may play a role in the endocrine-metabolic processes that guarantee the physiological course of lactation in dairy cattle. This study was aimed at determining the changes in plasma concentrations of leptin and some of the main hormones and metabolites involved in the lactogenetic process in high-yielding dairy cows throughout lactation; we also wanted to assess whether leptin secretion is subjected to seasonal influences. Blood samples were collected from 23 Italian Friesian dairy cows from the end of a lactation to the ninth month of the subsequent one; in addition, blood was sampled from 47 dairy cows in different phases of lactation during February and July. Plasma concentrations of leptin, growth hormone (GH), insulin, prolactin (PRL), glucose, non-esterified fatty acids (NEFA) and urea were quantified by either validated radioimmunoassay (RIA) or enzymatic colorimetric methods. At the beginning of lactation, GH concentrations significantly increased, while a significant reduction occurred in leptin and insulin. This endocrine condition, such as the significant increase in NEFA plasma concentrations, is indicative of a marked lipid mobilization. In the more advanced stages of lactation, when both energy and protein balances become positive, leptin plasma concentrations increased, whereas GH and NEFA concentrations declined. During the summer months, a significant increase in leptin plasma concentrations, irrespective of the phase of lactation, was observed. Collectively, our findings suggest that, in dairy cows, leptin may represent a 'metabolic signal' of animal's status of fattening and nutritional level; in addition, leptin seems to be influenced by photoperiod and environmental temperature.     

Negative energy balance increases periprandial ghrelin and growth hormone concentrations in lactating dairy cows

The reported effects of feeding on growth hormone (GH) secretion in ruminants have been inconsistent, and are likely influenced by energy status of animals. High-producing dairy cows in early lactation and late lactation were used to assess the effects of energy balance on temporal variation of plasma metabolites and hormones. Cows were fed a single diet once daily, and feed was withdrawn for 90 min prior to feeding. Beginning at the time of feed withdrawal, plasma samples were collected via jugular catheters hourly for 24h. Concentrations of non-esterified fatty acids and GH were measured for all samples, while insulin, glucose, and acylated (active) ghrelin were quantified for four sample times around feeding. As expected, calculated energy balance was significantly lower in early lactation than late lactation cows (-43.5 MJ retained/day versus 7.2 MJ retained/day). Following the primary meal of the day, a GH surge was observed in early lactation but not in late lactation cows. This difference was not explained by temporal patterns in non-esterified fatty acid, insulin, or glucose concentrations. However, a preprandial ghrelin surge was observed in early lactation only, suggesting that ghrelin was responsible for the prandial GH surge in this group. Results of a stepwise regression statistical analysis showed that both preprandial ghrelin concentration and energy balance were significant predictors of prandial GH increase over baseline. Adaptations to negative energy balance in lactating dairy cattle likely include enhanced ghrelin secretion and greater GH response to ghrelin.     

Plasma growth hormone and insulin concentrations in, and free fatty acid release from adipose tissue cultured in vitro from Holstein cows of differing cow index during early and late lactation

Early (EL) and late (LL) lactation Holstein cows were segregated into three Cow Index (CI) groups (high, HG; medium, MG; low, LG; n = 47). Feed intake by lactation group, individual milk yield data and blood samples, obtained by puncture of the coccygeal vein or artery at 12-hr intervals, were collected for 7 d. Cows were fed alfalfa hay top dressed with grain mixture. On day 7, 5 g of subcutaneous adipose tissue were removed from the tail-head region. Tissue was minced into 10-15 mg pieces in Krebs-Ringer bicarbonate buffer with 5 ng/ml insulin added (KRB). Triplicate 100-mg aliquots were incubated in KRB + 3% essentially fatty-acid-free bovine serum albumin with either 50 ng/ml growth hormone (G), 5 micrograms/ml epinephrine (EPI), both (G+E) or neither (CON) at 37 C for 2 hr. Early lactation cows averaged greater (P less than .05) daily milk production (33.4 vs 22.1 kg), greater (P less than .05) plasma growth hormone (GH) concentrations (3.9 vs 3.0 ng/ml) but lesser (P less than .01) insulin (INS) concentrations (.49 vs .73 ng/ml) than LL cows. Adipose tissue FFA release in vitro was greater (P less than .01) when media contained EPI (EPI: 8.10; G+E: 8.05 microE/l/g tissue) than when EPI was not present (CON: 1.33; G: 1.39 microE/l/g tissue), but was not affected by stage of lactation. Including hormonal data in the model as covariates indicated that increased plasma INS concentrations before biopsy reduced subsequent FFA release in vitro when tissue was incubated with added EPI, but not when incubation media lacked EPI. Increased GH concentrations had the opposite effect. Further, FFA release was greatest from HG cow adipose when incubated in media lacking EPI, but greatest from LG cow adipose when incubated in media containing EPI.     

Somatotroph and lactotroph function in relation to growth in six-week-old pigs reared in a hot or cool environment

The influences of thermal environment and individual growth rate on somatotroph and lactotroph function were examined in 6-week-old barrows reared entirely in a hot (H: 27–32°C, n = 8) or cool (C: 21°C, n = 10) environment. Growth hormone (GH) and prolactin (PRL) cell contents and responses to growth hormone-releasing hormone (GHRH) or thyrotropin-releasing hormone (TRH) were evaluated in cultured pituitary cells from each animal. Plasma GH, PRL, and insulin-like growth factor-1 (IGF-1) concentrations also were monitored. Thermal environment did not affect in vitro GH secretion, cellular GH content, or plasma GH concentrations. Stimulated in vitro GH release (GHRH-basal) and plasma GH were inversely related to average daily gain (ADG, r = −.76, p < .005 and r = −.51, p < .05, respectively). Cellular GH content also declined as ADG increased (r = −.57, p < .05). Plasma IGF-1 concentrations were not affected by thermal environment and were not related to ADG. Pituitary cells from H animals secreted and contained more PRL than cells from C animals (p < .05). Plasma PRL values were correlated with ADG (r = .54, p < .05), but did not differ between thermal groups. Stimulated in vitro PRL (TRH-vehicle) secretion was positively related with ADG only in the H group (r = .97, p < .001). In contrast, cellular PRL content decreased with ADG in cells from the H barrows (r = −.8, p < .05). Lactotroph function was not related to growth in cells from C pigs. In summary, 1) heat enhanced PRL secretion and cell content; 2) growth and somatotroph function were inversely related; and 3) serum PRL and the PRL response to TRH in cells from H barrows were positively related to growth.     

Effects of breeding season length and calving season on range beef cow productivity

A 5-yr study was conducted beginning in 1983 with 460 cows to evaluate the effects of three breeding seasons (30, 45, and 70 d in length) and two times of spring calving, March (early) and April (late), on cattle production under Nebraska Sandhills range conditions. Criteria evaluated included pregnancy and weaning percentages, calving date and distribution, cow weights and body condition at four intervals, calf birth and weaning weights, and cow productivity. The 30-d breeding season included a 10-d estrus synchronization and AI period; in the other breeding seasons only natural breeding was used. The same sires were used over the entire study period. Percentage of cows pregnant and percentage of calves weaned were lower (P less than .01) for cows bred for 30 d than for cows bred for 45 or 70 d. Average calving dates were similar among the breeding groups within the early and late calving herds. Pregnancy rates from AI were higher (P less than .01) for the cows calving in April (64%) than for the cows calving in March (41%). Cows calving in April lost less weight between precalving and prebreeding and were heavier (P less than .05) at prebreeding time than the cows calving in March. Calf weaning weights were not different (P greater than .10) among any of the breeding season groups or between the two calving herds when calves were weaned at a similar age. Cow productivity (calf weaning weight per breeding female) was highest (P less than .05) for the cows bred for 70 d (186 kg), intermediate for the cows bred for 45 d (172 kg), and lowest for cows bred for 30 d (162 kg).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of Kisspeptin on Regulation of Growth Hormone and Luteinizing Hormone in Lactating Dairy Cows

Kisspeptin (KP),a neuroendocrine regulator of reproduction,is hypothesized to be an integrator of metabolism and hormones critical to the regulation of reproduction.Lactation is associated with enhanced growth hormone (GH) responsiveness and reduced fertility.Our study was designed to determine the effects of lactation on KP-stimulated GH and luteinizing hormone (LH) secretion.Five nonlactating and five lactating dairy cows were used in the study.Experiments were conducted with lactating cows at weeks 1,5 and 11 after parturition.The experimental treatments (saline and KP [ 100 and 400 pmol/kg body weight] ) were given intravenously and blood was collected and plasma was stored until later assay to determine concentrations of GH,LH,progesterone and non-esterified fatty acids.We found that neither dose of KP stimulated an increase in GH secretion.The low dose of KP increased (P ＜0.05)LH concentrations only in lactating cows.The higher dose of KP elicited an increase in circulating LH concentrations in both lactating and non-lactating cows.The lower dose of KP increased ( P ＜ 0.05 ) the area under the curve for LH only in cows during week 5 of lactation,and the area under the curve of LH following the highest dose of KP was greater ( P ＜ 0.05 ) in cows during week 5 of lactation than that for the other groups of cows.In summary,lactation status and stage of lactation did not change the sensitivity of the GH system to KP.However,an effect of stage of lactation on KP-stimulated LH secretion was detected in the dairy cows.Study of the KP system during lactation in dairy cows may provide critical insights into the mechanisms for lactation-associated changes in the reproductive axis.     

Influence of nutrition and body condition on pituitary, ovarian, and thyroid function of nonlactating beef cows

Nonpregnant Hereford cows (n = 70) were used to determine the effect of nutrient intake and body condition on reproductive and thyroid function. Body condition scores (BCS; 1 = emaciated; 9 = obese) of cows averaged 5.0 +/- .2 on July 1, and cows were fed for 4 mo either to lose weight and BCS (thin; n = 22), to maintain weight and BCS (moderate; n = 24), or to gain weight and BCS (fat; n = 24). After November 1, cows received a complete ration to maintain weight and BCS. Cows were slaughtered in December (six thin, eight moderate, and eight fat cows) or the subsequent March (16 cows per group). Before slaughter, cows were given two injections of prostaglandin F2 alpha (PGF) 11 d apart. Six days after the second PGF injection, cows were simultaneously treated with 100 micrograms of gonadotropin releasing hormone (GnRH; i.m.) and 100 micrograms of thyrotropin releasing hormone (TRH; i.v.) and serum samples were obtained. The BCS of cows at slaughter (8 d after PGF) averaged 3.4, 5.3, and 7.1 (P less than .01) and carcass energy content averaged 243, 432, and 714 Mcal (P less than .01) for thin, moderate, and fat cows, respectively. Wet ovarian (P less than .001) and corpora lutea (P less than .01) weights were heavier for fat cows. Content of LH in the pituitary gland and concentrations of thyroxine (T4) in serum after GnRH/TRH were not influenced by nutrient intake or BCS. However, thin cows had greater concentrations (P less than .05) of LH in serum after GnRH/TRH than did moderate or fat cows. We conclude that nutrient intake and body energy reserves of beef cows influenced ovarian function and LH in serum after treatment with GnRH.     

Effects of body condition score at parturition and postpartum supplemental fat on metabolite and hormone concentrations of beef cows and their suckling calves

To determine the effects of BCS at parturition and postpartum lipid supplementation on blood metabolite and hormone concentrations, 3-yr-old Angus x Gelbvieh beef cows, which were nutritionally managed to achieve a BCS of 4 +/- 0.07 (479.3 +/- 36.3 kg of BW) or 6 +/- 0.07 (579.6 +/- 53.1 kg of BW) at parturition, were used in a 2-yr experiment (n = 36/yr). Beginning at 3 d postpartum, cows within each BCS were assigned randomly to be fed hay and a low-fat control supplement or lipid supplements with either cracked high-linoleate or high-oleate safflower seeds until d 61 of lactation. The diets were formulated to be isonitrogenous and isocaloric, and the safflower seed supplements were formulated to achieve 5% DMI as fat. On d 31 and 61 of lactation, blood samples were collected preprandially and then hourly postprandially (at 0, 1, 2, 3, and 4 h). Serum insulin (P = 0.27) and glucose (P = 0.64) were not affected by BCS at parturition. The mean concentrations of plasma NEFA (P = 0.08) and beta-hydroxybutyrate (P = 0.08) tended to be greater, and serum IGF-I was greater (P < 0.001) in BCS 6 than BCS 4 cows. Conversely, serum GH was greater (P = 0.003) for BCS 4 cows, indicating that regulation of IGF by GH may have been uncoupled in BCS 4 cows. The postpartum diet did not affect NEFA (P = 0.94), glucose (P = 0.15), IGF-I (P = 0.33), or GH (P = 0.62) concentrations. Oleate-supplemented cows had greater (P = 0.03) serum insulin concentrations, whereas control cows had greater (P = 0.01) plasma beta-hydroxybutyrate concentrations. Concentrations of NEFA (P = 0.05) and glucose (P < 0.001) were greater, and beta-hydroxybutyrate tended (P = 0.07), to be greater at d 3, whereas serum IGF-I was greater (P = 0.003) at d 6 of lactation. Similar concentrations of NEFA, glucose, GH, and IGF-I indicate that the nutritional status of beef cows during early lactation was not influenced by lipid supplementation. However, perturbations of the somatotropic axis in BCS 4 cows indicate that the influence of energy balance and BCS of the cow at parturition on postpartum performance should be considered when making managerial decisions.     

Lactation Biology Symposium: effects of photoperiod on mammary gland development and lactation

Photoperiod, or the daily sequence of light and dark, has dramatic effects on many physiological systems across animal species. Light patterns alter melatonin secretion profiles and, subsequently, the release profiles and circulating concentrations of several hormones that influence a variety of physiological responses. Although the impact of photoperiod on reproductive processes is perhaps the most common example, it is often the seasonal aspects of ovulation and anestrus that are considered. However, in cattle, the final phase of reproduction, that is, lactation, is significantly influenced by photoperiod. In contrast to short days (SDPP; 8 h light:16 h dark), exposure to long days (LDPP) of 16 to 18 h of light and 6 to 8 h of darkness increases milk yield 2 to 3 kg/d, regardless of the stage of lactation. There is evidence that this LDPP effect is due to increased circulating IGF-I, independent of any effect on GH concentrations. Cows that are housed under SDPP during the dry period have increased mammary growth and produce 3 to 4 kg/d more milk in the subsequent lactation compared with cows on LDPP when dry. While cows are on SDPP, circulating prolactin (PRL) diminishes but expression of PRL receptor increases in mammary, liver, and immune cells. Moreover, PRL signaling pathways within those tissues are affected by photoperiod. Further, replacement of PRL to cows on SDPP partially reverses the effects of SDPP on production in the next lactation. Thus, effects on dry cows are mediated through a PRL-dependent pathway. Before maturity, LDPP improve mammary parenchymal accumulation and lean body growth, which lead to greater yields in the first lactation. The accumulated evidence supports the concept that photoperiod manipulation can be harnessed to improve the efficiency of production across the life cycle of the dairy cow.     

Effects of Feed in the Dry Period on Fertility of Dairy Cows Post Partum

It has been hypothesized that the decrease in fertility of dairy cows is related to the increase in milk yield and associated with fatty liver and increased vulnerability for both infection and metabolic diseases. To elucidate the mechanisms behind this complex of health and reproductive problems an Animal Model, based on overfeeding in the dry period, has been developed. In an experiment carried out in 1996 sixteen animals were fed 119 MJ/d net energy to lactation (NEL) (experimental group) and another sixteen just 49 MJ/d NEL (control group) in the last 8 weeks before parturition. During lactation the cows from both groups were given the lactation ration ad libitum. After parturition both groups differed significantly in level and duration of the NEB, in weight loss, increased concentrations of NEFAs and bHBB in the blood and TAG in the liver. Although the insulin level was lower in the experimental animals during the first week, the difference was not significant. The number of ovulations within the first 100 days pp was significant lower in the experimental group and was related to depth and duration of the NEB and the weight loss: more NEB leads to less ovulations. The interval partus-le ovulation was also significant different but related to the TAG level in the liver: more TAG leads to a longer interval. The experimental group with the fatty liver suffered more incidenses of metabolic diseases. It has been concluded that the Model leads to a severe NEB, to Fatty Liver and to more incidences of diseases. Both the NEB and the fatty liver influence the fertility of the cows pp and it can not be excluded that some diseases have an effect too. More studies are planned to unravel the factors that affect the fertility of high yielding cows.