Effect of dietary energy and previous bovine somatotropin on milk yield, mastitis, and reproduction in dairy cows.

Thirty multiparous lactating Holstein cows were blocked according to time of calving and assigned to a 2 x 3 factorial arrangement of treatments in a randomized complete block design to evaluate the effects of two dietary energy concentrations either without or with bST (20.6 mg/d per cow) administered to cows that had not or had received bST during the preceding lactation. Subcutaneous injection of bST began 28 to 35 d postpartum and continued for 39 wk. The dietary energy concentration x bST interaction was not significant for any response variable. Compared with DMI of control cows, DMI was higher for cows receiving bST, being 1.6 and 2.4 kg/d higher for cows receiving bST for one and two lactations, respectively. Milk, fat, and protein yields were higher for cows receiving bST than for controls. Those receiving bST for a second lactation also produced more milk than controls until wk 20; thereafter, milk yields were similar to those of controls. Somatotropin administration had no adverse effect on udder health. Cows receiving bST tended to ovulate less regularly than controls, which may be attributed to their higher milk yield. However, BW gains during lactation were similar for all treatments, indicating that bST-treated cows built energy reserves for the subsequent lactation. Although energy concentrations of the diets had no significant impact on yield, the higher energy diet tended to depress milk fat concentration. Administration of bST to dairy cows for a second, consecutive lactation yielded responses similar for the first 20 wk of the study to those receiving bST for the first time. However, after wk 20, milk yield was less than that by cows receiving bST for the first lactation but similar to that of control cows.     

Effect of somatotropin injection and dietary protein concentration on milk yield, and kinetics of hormones in dairy cows

Four lactating Holstein cows averaging 155 d postpartum were used in a cross-over design to examine the influence of somatotropin administration (33 mg/d for 28 d) and dietary CP (11 vs. 16% CP) concentration on milk yield and kinetics of glucagon, insulin, glucose, and somatotropin. Somatotropin and dietary CP were without influence on DM intake and digestibility. Milk yields were increased by both somatotropin administration and increased dietary CP. Increased dietary CP increased turnover of glucose. Injection of somatotropin increased entry rate, pool size, and basal concentrations of somatotropin but was without effect on kinetics of glucagon, insulin, or glucose. The absence of major changes in the kinetics of hormones and glucose suggests that other factors play a role in somatotropin enhancement of milk production. However, the observed changes are consistent with the homeorhetic influence of somatotropin on milk production. Adequate dietary CP is required to obtain maximal responses to somatotropin administration.     

Effects of administration of bovine somatotropin on milk yield and composition

Thirty-two Holstein cows were assigned to four treatment groups in a continuous lactation trial to evaluate effects of daily subcutaneous injections of 0 (A), 5.15 (B), 10.3 (C), and 20.6 mg (D) of recombinantly derived bovine somatotropin monomer. Responses were milk yield and composition, component yields, net energy intake, body weight, and somatic cell counts. Treatments began in fall, 28 to 35 d postpartum, and continued for 266 d. Base ration was corn silage and concentrate. Least squares means of milk and constituent yields and gross efficiency of milk production differed among treatments, but percentage fat and protein, energy intake, body weights, and somatic cell counts did not. Least squares means of 3.5% FCM were 21.3 (A), 25.0 (B), 26.2 (C), and 28.5 kg/d (D); fat and protein yields showed similar response to treatment. Milk yields adjusted for pretreatment yields were 21.1 (A), 25.5 (B), 26.5 (C), and 29.3 kg/d (D). Orthogonal contrasts showed lower milk and constituent yields and net energy intake for control (A) than for hormone-injected cows (B, C, D), but no differences were detected between treatments B, C, and D. Gross efficiency of production (3.5% FCM/feed net energy intake) was greater for hormone-injected cows.     

Effects of genetic selection for milk yield on somatotropin, insulin-like growth factor-I, and placental lactogen in Holstein cows

Cows from static, low-merit control (CL) and contemporary, high-merit select (SL) lines that differed in milk yield by more than 4,000 kg/305-d lactation (SL > CL) were used to determine effects of selection for milk yield on blood serum concentrations of somatotropin (ST), insulin-like growth factor (IGF-I), and placental lactogen (PL). Cows were exposed to the same environment and management conditions and fed the same diets. Serum and milk samples were collected from primiparous (18 CL, 18 SL) and multiparous (12 CL, 18 SL) cows relative to day of lactation (from −28 to 280 d for nonpregnant cows and to subsequent calving for cows that conceived). Data were analyzed as repeated measures using mixed model procedures. Serum ST increased at calving, remained elevated for a longer interval in SL than in CL cows, and was greater in SL than in CL cows. Serum IGF-I decreased at calving, remained low through 14 DIM, and gradually returned to precalving concentrations as lactation progressed. Postpartum concentrations of IGF-I were less in SL than CL through 84 DIM and were similar through the remainder of lactation, resulting in a line by day interaction. Serum IGF-I and PL were not affected by merit during gestation. There was an interaction of merit and postconception interval on IGF-I, with the difference in IGF-I concentration between lines decreasing as gestation progressed. Change in serum IGF-I and PL appeared to be synchronous. Results indicate that selection for milk yield increased serum ST, prolonged the postpartum reduction in serum IGF-I, and did not alter serum PL. Results also indicate a positive relationship between PL and IGF-I and support the concept that PL plays a role in the regulation of serum IGF-I during gestation.     

Injection of synthetic human growth hormone-releasing factors in dairy cows. 1. Effect on feed intake and milk yield and composition

Eighteen multiparous Holstein cows in the second half of their lactation were used to determine the effect of human growth hormone-releasing factor (1-44)NH2 and a fragment of growth hormone-releasing factor (1-29)NH2 on lactational performance and feed intake. Saline, the 44-amino acid peptide or the 29-amino acid fragment, at the same dose per injection (.2 nmol.kg-1) was injected intravenously at 4-h intervals for 10 d. Average milk yield, milk composition, feed intake, and feed efficiency were compared for the second half of each 10-d preinjection, injection, and post-injection period. Injections of the 44-amino acid peptide and the 29-amino acid fragment increased milk yield 18.6 and 14.6%, respectively. Feed intake was not changed, but feed efficiency was increased 23.9 and 18.8% over control following 44-amino acid peptide and the 29-amino acid fragment injection, respectively. The lactational response was not different between the two peptides for any of the variables measured. This study demonstrates the feasibility of using a growth hormone-releasing factor fragment as an alternative method of elevating milk yield in cattle via somatotropins.     

Growth hormone-releasing factor stimulates milk production and sustains growth hormone release in Holstein cows

Serum growth hormone was determined in lactating cows following repeated intravenous injections of growth hormone-releasing factor. A given dose was injected every 4 h for 24 h in a 4 (cow) X 4 (d) Latin square. Growth hormone increased similarly above controls after 10, 20, or 40 micrograms releasing factor/100 kg body weight. In another experiment the effects on lactational performance and growth hormone responses of cows to repeated injections of releasing factor for 10 d were determined in a 2 (cow) X 2 (period) Latin square crossover. Administration of 20 micrograms releasing factor/100 kg body weight to 16 Holstein cows (lactating 4.5 to 7.5 mo) every 4-h for 10 d increased milk yield from 25.4 to 27.7 kg/d and increased total fat, protein, and lactose 11% above controls. Releasing factor did not affect milk composition or feed intake. Peak growth hormone response to releasing factor was similar between d 1 (19.9 ng/ml) and 10 (24.4 ng/ml). Exogenous growth hormone-releasing factor administered to lactating Holstein cows at the doses tested: 1) increases growth hormone consistently, although the response is not dose dependent, 2) is galactopoietic, 3) causes an apparent increase in feed to milk conversion, and 4) increases growth hormone to at least the same magnitude on d 10 as on d 1.     

Growth hormone-releasing factor effects on pituitary function, growth, and lactation

Growth and lactation are complex processes controlled by several metabolic hormones such as insulin, glucagon, and thyroid hormones but most notably growth hormone. Growth hormone secretion is regulated by two hypothalamic hormones, somatostatin, an inhibitory regulatory factor, and growth hormone-releasing factor. The quantity and pattern of growth hormone secretion is ultimately regulated in concert by the secretion of both regulatory factors from the hypothalamus through the hypothalamo-hypophyseal portal system, where they exert their actions at unique pituitary receptors. Because the potential use of exogenous growth hormone administration for the stimulation of growth efficiency and lactation has been demonstrated, recent efforts have been directed toward the enhancement of production through manipulation of endogenous growth hormone secretion via its releasing factor. Thus far, releasing factor-stimulated growth and lactation has not been achieved to the same extent as that of exogenous growth hormone. Growth hormone-releasing factor has stimulated growth in two growth hormone-deficient children, as well as female, but not male, rats. Although all food-producing species tested to date respond to growth hormone-releasing factor with the appropriate growth hormone response, continuous or pulsatile administration of releasing factor has not resulted in increased growth rate in sheep, chicks, or hogs. Despite levels of circulating growth hormone that would be expected to produce a 30% increase in milk production if given exogenously to dairy cows, releasing factor-stimulated growth hormone secretion has resulted in only a 3 to 9% increase. It is clear from these studies that further developments are necessary to demonstrate the practical application of growth hormone-releasing factor.     

Effects of long daily photoperiod and bovine somatotropin (Trobest) on milk yield in cows.

Bovine somatotropin (bST) and exposure to long daily photoperiod increase milk yield of dairy cattle. We tested the hypothesis that long daily photoperiod and bST treatment would increase milk yield in an additive manner in lactating cows. At winter solstice, 40 lactating cows were started on a 140-d experiment; cows were greater than 70 d in milk (DIM) and were balanced for uniformity of DIM and milk yield within parity. Cows were randomly assigned to one of four treatments (10/treatment): 1) natural photoperiod, 2) natural photoperiod + bST (14 mg/d Trobest i.m.), 3) long daily photoperiod (18 h of light and 6 h of darkness/d), or 4) long daily photoperiod + bST. Long daily photoperiod increased fat-corrected milk (FCM) yield 1.9 kg/d versus natural photoperiod. Treatment with bST increased FCM 5.7 kg/d versus natural photoperiod, and long daily photoperiod + bST increased FCM 5.8 kg/d versus long daily photoperiod. Long daily photoperiod + bST was additive, increasing FCM 7.7 kg/d versus natural photoperiod. Serum somatotropin increased with bST, but not photoperiod, and bST increased serum insulin-like growth factor-I. Long daily photoperiod tended to increase prolactin; bST had no effect. Long daily photoperiod + bST increased dry matter intake (DMI) relative to natural photoperiod and natural photoperiod + bST; long daily photoperiod increased DMI relative to natural photoperiod + bST. Photoperiod had no effect on net energy balance; however, bST decreased net energy balance. Generally, body weight and milk composition did not differ among treatments. In conclusion, combination of bST with long-daily photoperiod tended to amplify the increases in milk yield observed with either treatment individually.     

Effect of additional cobalt, copper, manganese, and zinc on reproduction and milk yield of lactating dairy cows receiving bovine somatotropin.

The objective of this study was to determine whether organically complexed Co, Cu, Mn, and Zn would improve the reproductive performance and milk and milk component production in lactating dairy cows that began receiving bovine somatotropin in the ninth week of lactation. Holstein (n = 50) and Jersey (n = 10) cows were blocked by breed, lactation number, and incidence of retained fetal membranes. Two diets assigned within blocks and fed from parturition until 154 d of lactation were control or control supplemented daily with 26 mg of Co as Co glucoheptonate, 125 mg of Cu as Cu-Lys, 199 mg of Mn as Mn-Met, and 359 mg of Zn as Zn-Met. Cows were fitted with electronic pressure-sensing devices in the second week of lactation for detection of estrus. Ovarian structures were determined via transrectal ultrasonography at 7-d intervals from parturition until observation of the first corpus luteum. Blood samples were taken at 7-d intervals and analyzed for plasma concentrations of progesterone, insulin, and urea nitrogen. Onset of luteal activity was identified by progesterone concentrations > or = 1 ng/ml. Retained fetal membranes increased days to first estrus (detected via electronic estrous detection), first luteal activity, and first corpus luteum in control cows but not in supplemented cows. Days to first observed estrus were greater for control cows than for supplemented cows. Days to first service, days open, days from first service to conception, services per conception, milk yield, milk components, and somatic cell counts were similar for control and supplemented cows. Supplementation with complexed trace minerals effectively reduced days to first estrus.     

Relationships among milk yield, metabolism, and reproductive performance of primiparous Holstein cows treated with somatotropin

Thirty-two primiparous Holsteins were assigned to receive 0, 5.15, 10.3, or 16.5 mg/d recombinant bST. Treatment began between 28 and 35 d postpartum and continued until 60 d prior to next expected calving or 400 d postpartum. Metabolic hormones and metabolites were measured in blood weekly during the first 10 wk of treatment, and progesterone was measured in milk twice weekly to assess ovarian activity until conception. Milk production (305 d, actual) was 8350 +/- 562, 8348 +/- 515, 9571 +/- 515, and 9070 +/- 515 for cows given 0, 5.15, 10.3, or 16.5 mg/d and did not differ statistically. Insulin, glucose, NEFA and blood urea nitrogen were not influenced by dose of bST. However, insulin-like growth factor-I increased linearly with dose of bST. Days from parturition to first detected estrus, days open, and services per conception did not differ among groups; however, days from parturition to first service increased with dose of bST. Rate of detection of estrus decreased with dose of bST, resulting in a longer interval to first insemination. The lower rate of detection of estrus in bST-treated cows may have been associated with reduced expression of estrus. Additional statistical analyses were conducted to determine the relationship between metabolism, milk production, and reproduction over all doses of bST. There was a negative correlation between 305-d milk yield and glucose (r = -.44) and insulin (r = -.46) concentrations during 30 to 100 d postpartum. In stepwise regression analyses, insulin accounted for 21% of the variance in 305-d milk yield. Glucose and NEFA accounted for 18% of the variance in days open, and there was a negative correlation (r = -.32) between glucose and days open. Days open and milk production were not correlated.     

Effect of dry period length and dietary energy source on energy balance,milk yield,and milk composition of dairy cows

The objective of this study was to evaluate the effects of dry period length and dietary energy source in early lactation on milk production, feed intake, and energy balance (EB) of dairy cows. Holstein-Friesian dairy cows (60 primiparous and 108 multiparous) were randomly assigned to dry period lengths (0, 30, or 60 d) and early lactation ration (glucogenic or lipogenic), resulting in a 3 × 2 factorial design. Rations were isocaloric and equal in intestinal digestible protein. The experimental period lasted from 8 wk prepartum to 14 wk postpartum and cows were monitored for milk yield, milk composition, dry matter intake (DMI), energy balance, and milk fat composition. Prepartum average milk yield for 60 d precalving was 13.8 and 7.7 ± 0.5 kg/d for cows with a 0- and 30-d dry period, respectively. Prepartum DMI and energy intake were greater for cows without a dry period and 30-d dry period, compared with cows with a 60-d dry period. Prepartum EB was greater for cows with a 60-d dry period. Postpartum average milk yield until wk 14 was lower for cows without a dry period and a 30-d dry period, compared with cows with a 60-d dry period (32.7, 38.7, and 43.3 ± 0.7 kg/d for 0-, 30-, and 60-d dry period, respectively). Postpartum DMI did not differ among treatments. Postpartum EB was greater for cows without a dry period and a 30-d dry period, compared with cows with a 60-d dry period. Young cows (parity 2) showed a stronger effect of omission of the dry period, compared with a 60-d dry period, on additional milk precalving (young cows: 15.1 kg/d; older cows: 12.0 kg/d), reduction in milk yield postcalving (young cows: 28.6 vs. 34.8 kg/d; older cows: 41.8 vs. 44.1 kg/d), and improvement of the EB postcalving (young cows: 120 vs. −93 kJ/kg^0.75·d; older cows: −2 vs. −150 kJ/kg^0.75·d. Ration did not affect milk yield and DMI, but a glucogenic ration tended to reduce milk fat content and increased EB, compared with a more lipogenic ration. Reduced dry period length (0 and 30 d) increased the proportion of short- and medium-chain fatty acids in milk fat and omitting the dry period decreased the proportion of long-chain fatty acids in milk fat. In conclusion, shortening and omitting the dry period shifts milk yield from the postpartum to the prepartum period; this results in an improvement of the EB in early lactation. An increased energy status after a short dry period can be further improved by feeding a more glucogenic ration in early lactation.     

Genetics of growth, feed intake, and milk yield in Holstein cattle.

Heritabilities, genetic and phenotypic correlations among growth, forage consumption, and BW changes of heifers and feed consumption, BW changes, and yields of first lactation cows were estimated. Data were from 1266 Holstein progeny of 74 sires born from 1972 to 1985 at three Agriculture Canada research herds. Heavier heifers at 26 wk consumed more feed from 26 to 34 wk than smaller heifers but gained the same BW. The BW gain and feed consumption heritabilities were .17 and .23, respectively; genetic correlation was .44, and phenotypic correlation was .27. During first lactation, feed intake from 8 to 16 wk and measures of milk yield are very tightly intercorrelated both phenotypically and genetically (.78 to .98). Precalving BW gain and BW at calving were genetically uncorrelated with measures of milk yield (-.09 to +.05). Loss of BW during the first 8 wk of first lactation was moderately heritable (.29) and correlated genetically and phenotypically with measures of milk yield in early lactation (.32 to .39) and feed consumption (.26). From 8 to 16 wk, average BW changes were small and had low heritability and weak phenotypic correlations with measures of milk yield or feed intake. The BW at 26 wk and BW gain from 26 to 34 wk were very poor indicators of early first lactation milk yield. Heifer feed intake was weakly correlated phenotypically (-.07 to .16) but moderately genetically correlated (.17 to .23) with early first lactation milk yield and feed consumption.     

Effects of bovine somatotropin on milk yield and composition, body weight, and condition score of Holstein and Jersey cows

Forty cows (20 Holstein, 20 Jersey) were administered 0, 5, 10, 15, or 20 mg of recombinantly derived bST daily to determine the effect on milk yield, milk composition, body weight, and body condition score. Administration of bST was from 75 +/- 7 d through 305 d postpartum. A total mixed diet of 45% corn silage and 55% of a concentrate mixture (dry basis) was provided for ad libitum intake. Milk yield of Holstein and Jersey cows administered 20 mg of bST increased 25.3 and 22.8%, respectively, over controls. Fat-corrected milk from Holsteins and Jerseys that were administered 20 mg of somatotropin increased 32.2 and 18.7% over controls, but Jersey response was greatest when 15 mg of bST were administered (27.1% over controls). Dry matter intake of the 20 mg bST group was 13.5% greater than DM intake of controls. Apparent efficiency of production increased linearly with increasing somatotropin. There was no significant change in body weight, but body condition score declined linearly with increasing somatotropin. Most milk composition measures were unaffected by somatotropin. Jersey and Holstein milk yield increased quadratically and linearly, respectively, with somatotropin dose.     

Effect of long-term infusion with recombinant growth hormone-releasing factor and recombinant bovine somatotropin on development and function of dominant follicles and corpora lutea in Holstein cows.

The objective of this study was to evaluate the effects of recombinant bovine growth hormone-releasing factor (rGRF) or recombinant bovine somatotropin (rbST) on growth and function of the first-wave dominant follicle and corpus luteum. Primiparous Holstein cows (117 d postpartum) were infused with 12 mg/d of rGRF (n = 10) or 29 mg/d of rbST (n = 10) for 63 d, and non-infused cows (n = 10) were controls. At slaughter on d 5 of an estrous cycle, blood and ovaries were collected and data from cows with a corpus luteum were analyzed (control, n = 8; rGRF, n = 5; rbST, n = 6). Treatment with rGRF or rbST increased somatotropin (ST) and IGF-I in serum similarly compared with controls. In contrast, rbST-treated cows had higher concentrations of ST in follicular fluid (FF) compared with rGRF-treated and control cows. In addition, rbST, but not rGRF, increased the number and decreased the size of estrogen-active follicles (EA; estradiol > progesterone concentrations in FF), increased the abundance of IGF binding proteins-2, -3, and -4 in FF from EA follicles, and increased the number but decreased the size of corpora lutea and decreased concentration of progesterone in serum compared with controls. Based on these results, we concluded that long-term infusion of rbST alters growth and function of the first-wave dominant follicle and the corpus luteum in cattle.     

Effects of somatotropin on milk yield and physiological responses during summer farm and hot laboratory conditions

The effects of bST on performance and physiological responses of lactating cows was studied under farm summer and laboratory heat conditions. Twelve cows, 90 to 50 d postpartum, were injected with either bST or vehicle solution for 30 d under farm summer and 10 d under either laboratory thermoneutral or heat conditions. Somatotropin increased milk yield by 6.1 (21%), 8.1 (32%), and 7.3 kg (35%) under the farm summer, laboratory thermoneutral, and heat conditions, respectively. Somatotropin also increased milk fat by 15 and 19% and dry matter intake by 16 and 18% under laboratory thermoneutral and heat conditions, respectively. Somatotropin increased the efficiency of feed conversion into milk without any significant changes in body weight and temperatures. Somatotropin reduced plasma concentrations of triiodothyronine and cortisol and had no effect on plasma prolactin and insulin concentrations. Somatotropin did not increase water intake; however, hematocrit was decreased. The results suggest that stimulatory effects of bST on milk production are still observed on heat-stressed cows without any significant indications of additional heat stress.     

Effect of exogenous bovine somatotropin on mammary lipid metabolism and milk yield in lactating dairy cows

Eight lactating Holstein cows were used to study the effect of bovine somatotropin on mammary lipid metabolism, milk production, and milk composition. Eight cows were injected with either excipient (n = 4) or 50 IU somatotropin/d (n = 4). A 2-wk adjustment period preceded a 10-d period when treatments were administered. On d 9, serial blood samples were collected from 2 to 5 h post-injection to determine concentrations of somatotropin and metabolites in plasma. Mammary tissue biopsies were obtained 2 to 3 h after a.m. milking on d 10 to determine lipoprotein lipase activity and in vitro rates of acetate incorporation into fatty acids and acetate oxidation by mammary tissue slices. Activity of lipoprotein lipase in milk and mammary tissue and rate of acetate metabolism by mammary tissue were not affected by treatment. The following means for placebo and somatotropin, respectively, were: milk production (21.5 and 24.8 kg/d), milk fat (3.8 and 4.0%), and 4% FCM (20.7 and 25.1 kg/d). Plasma concentrations of somatotropin (3.0 and 43.4 ng/ml) and free fatty acids (105.6 and 219.5 mu eq/L) were significantly elevated with somatotropin. The injection of somatotropin into lactating dairy cows did not affect the mechanisms of fatty acid synthesis or metabolism evaluated in this study.     

Development and validation of a short-term, serum-free culture system for bovine granulosa cells: evaluation of the effects of somatotropin and growth hormone-releasing factor on estradiol production

The objective of this study was to develop and validate a short-term, serum-free culture system to determine whether recombinant bovine somatotropin (rbST) or recombinant bovine growth hormone-releasing factor (rbGRF) altered the estradiol-producing capacity of bovine granulosa cells isolated from dominant or subordinate follicles of the first follicular wave. Thus, ovaries were obtained at an abattoir from cows that were between d 2 to 5 or 6 to 10 of the estrous cycle. Three size classes of follicles were isolated from each cow's ovaries: small (2 to 5 mm in diameter), medium (6 to 14 mm), or the largest (6 to 19 mm). In vivo steroid-producing capacity of follicles was assessed by measuring concentration of estradiol, progesterone, androstenedione and 5alpha-dihydrotestosterone in each follicle. In vitro steroid-producing capacity was assessed by culturing granulosa cells from the different follicle sizes for 48 h in serum-free media with 19-OH androstenedione and measuring the estradiol and progesterone concentrations in media at the end of culture. The effect of different doses of FSH, rbST, or rbGRF on estradiol and progesterone production by granulosa cells from each follicle size class during d 2 to 5 or 6 to 10 was also evaluated. A high percentage (91.7%) of the largest follicles obtained on d 2 to 5 was estrogen-active (estradiol > progesterone) compared with other follicle classifications (d 2 to 5, small = 0%, medium = 13.8%; d 6 to 10, small = 0%, medium = 3.3%, largest = 33.3%). Estradiol was highest (P < 0.05) in the largest follicle on d 2 to 5 and correlated positively with follicle diameter. The pattern of in vitro production of estradiol by granulosa cells from the different follicle size classes reflected the original in vivo capacity of follicles to produce estradiol. However, only granulosa cells from the largest estrogen-active follicle on d 2 to 5 produced more estradiol than progesterone in vitro. Progesterone production by granulosa cells from all follicle classifications was increased by FSH, but FSH only enhanced estradiol production by granulosa cells from the largest estrogen-active follicles on d 2 to 5. Recombinant bST blocked the FSH-induced increase in estradiol by granulosa cells from the largest estrogen-active follicles on d 2 to 5, whereas rbGRF had no effect on steroid production. Based on these results, we concluded that short-term, serum-free culture of bovine granulosal cells obtained from first-wave follicles at an abattoir could be used to reflect reliably the original in vivo estradiol-producing capacity of granulosal cells, and that neither rbST nor rbGRF enhance basal or FSH-induced estradiol production by bovine granulosa cells from first-wave follicles.     

Effect of human growth hormone-releasing factor (1-29)NH2 on growth hormone release and milk production in dairy cows

Twelve cows (209 d in lactation, 642 kg BW) were used in an experiment conducted over four 10-d periods (one preinjection, one injection, and two postinjection). Gelatine (n = 6) or 10 mg of growth hormone-releasing factor in gelatine (n = 6) was injected subcutaneously at 1000 h every day on d 11 to 20. Data were averaged for the last 5 d of each period. During the injection period, milk, fat, and protein yields increased by 3 kg.d-1 (14.3%), .14 kg.d-1 (16.7%), and .12 kg.d-1 (15.4%), respectively. Moreover, milk, fat, and protein yields for the treated cows remained higher than for the control cows until the last postinjection period. Growth hormone response was evaluated from blood samples withdrawn from 2 h prior to 8 h postinjection on d 11, 15, and 20 and on d 40. After growth hormone-releasing factor injection, peaks and area under the curve were 24.5, 30.8, and 47.0 ng.ml-1 and 2475, 3979, and 3741 ng.ml-1.min-1 on d 11, 15, and 20, respectively. On d 40, there was no difference in growth hormone concentrations in blood between control and treated cows. These results demonstrate that 10 d of daily injection of a growth hormone-releasing factor increases milk production by 14.3% (3 kg.d-1) and still induces growth hormone release at the end of the injection period without any sign of refractoriness.     

Effect of modified dry period lengths and bovine somatotropin on yield and composition of milk from dairy cows

Dry periods of 40 to 60 d have been an industry standard because dry periods <40 d have resulted in reduced milk yields in the subsequent lactation by 10 to 30%. However, recent research has demonstrated no production losses for cows given a 30-d dry period. The current study evaluated milk production effects of shortened or omitted dry periods for cows at mature-equivalent production >12,000 kg of milk and treated with bovine somatotropin (bST). The study used 2 commercial dairies and one university dairy and included 4 treatments. Five multiparous and 5 primiparous cows from each farm were assigned to each treatment: 1) 60-d dry period, label use of bST (60DD); 2) 30-d dry period, label use of bST (30DD); 3) continuous milking, label use of bST (CMLST); and 4) continuous milking with continuous use of bST (CMCST). Per label, bST use started at 57 to 70 d in milk and ended 14 d before drying (60DD and 30DD) or expected calving date (CMLST). In primiparous cows, average milk yields during the first 17 wk of lactation were reduced for cows on treatments 30DD, CMLST, and CMCST vs. the 60DD treatment. (38.3, 35.1, and 37.5 vs. 44.1 +/- 1.3 kg/d, respectively). For multiparous cows, respective milk yields did not differ (46.6, 43.4, 46.5, and 47.7 +/- 2.1 kg/d). Shortened or omitted dry periods may impede mammary growth in primiparous cows, resulting in reduced milk yield in the subsequent lactation. In contrast, a shortened or omitted dry period with either bST protocol did not alter production in multiparous cows treated with bST. Quality aspects of prepartum milk and colostrum require additional characterization. For multiparous cows, milk income generated for short dry periods or for continuous milking might increase their profitability. At 17 wk of the subsequent lactation, estimates of the cumulative net margins of multiparous cows on the 30DD treatment and continuous milking treatments exceeded those of cows on the 60DD treatment by 40 dollars to 60 dollars per cow.