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Acknowledgment is made to the Maryland Agricul- tural Experiment Station for support of this research. 2Send reprint requests to G. E. Dahl, Department of ...
Effects of Long Daily Photoperiod and Bovine Somatotropin (Trobest) on Milk Yield in Cows1 A.R.E. MILLER,* E. P. STANISIEWSKI,† R. A. ERDMAN,* L. W. DOUGLASS,* and G. E. DAHL*,2 *Department of Animal and Avian Sciences, University of Maryland, College Park 20742 †Pharmacia and Upjohn Company, Animal Health Division, Kalamazoo, MI 49001

ABSTRACT 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-

Received November 23, 1998. Accepted March 29, 1999. 1A preliminary report appears in J. Dairy Sci. 81(Suppl. 1): 235(Abstr.). Acknowledgment is made to the Maryland Agricultural Experiment Station for support of this research. 2Send reprint requests to G. E. Dahl, Department of Animal and Avian Sciences, University of Maryland, 1415 Animal Sciences Center, College Park 20742-2311. 1999 J Dairy Sci 82:1716–1722

daily photoperiod tended to amplify the increases in milk yield observed with either treatment individually. ( Key words: photoperiod, bovine somatotropin, lactation) Abbreviation key: LDPP = long daily photoperiod, NDPP = natural daily photoperiod, NEB = net energy balance, PRL = prolactin, ST = somatotropin, IGFBP = IGF binding protein. INTRODUCTION Three methods consistently increase milk yield in dairy cows during an established lactation. The galactopoietic effects of administration of exogenous bST are well documented (3, 7). Increased frequency of milking also consistently increases milk yield (10, 14). A long daily photoperiod ( LDPP) has been shown to increase milk yield by 8 to 10% relative to that of cows on natural photoperiod (4, 11, 18, 27). Although these individual techniques have been widely investigated, information regarding combinations of bST, increased milking frequency, and LDPP is limited. The combination of milking 3× per day and administration of bST generally increased milk yield in an additive manner (23), yet no studies have attempted to examine the effects of photoperiod and bST treatment. Thus, the first objective of the present study was to determine whether a combination of bST and LDPP produce an additive effect on milk yield of lactating cows. Evidence exists that the ultimate galactopoietic response to bST is mediated by action of IGF-I at the mammary gland (13, 22). Treatment with bST is associated with increased circulating IGF-I (3, 6, 13) and altered circulating concentrations of IGF binding proteins ( IGF-BP) 2 and 3 ( 5 ) . Increases in milk yield induced by LDPP are also associated with increased circulating IGF-I ( 8 ) , yet these changes are independent of altered somatotropin ( ST) (19, 24, 28) or IGF-BP ( 8 ) status. Therefore, our second objective was to characterize effects of bST, LDPP, and

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the combination of the two treatments on the response of the somatotropic axis. MATERIALS AND METHODS At the winter solstice, 40 lactating Holstein cows (28 multiparous, 12 primiparous), >70 DIM were balanced within parity for uniformity according to DIM and milk yield. Use of cows and all procedures were approved by the University of Maryland Animal Care and Use Committee. Cows were randomly assigned to one of four treatments ( n = 10 per treatment): 1 ) natural daily photoperiod ( NDPP) (9.5 to 14.5 h of light and 14.5 to 9.5 h of darkness/d), 2 ) NDPP + bST (Trobest; Pharmacia and Upjohn Co., Kalamazoo, MI; 14 mg/d i.m.), 3 ) LDPP (18 h of light and 6 h darkness/d), or 4 ) LDPP + bST. The study consisted of 10 2-wk measurement periods including a 2-wk pretreatment adjustment period used as a covariate and a 2-wk posttreatment period for a total duration of 140 d. Cows were housed at the Central Maryland Research and Education Center (Clarksville, MD; 39° Latitude) in a curtain-enclosed tie-stall barn with controlled light exposure. Supplemental lighting of 350 lx at eye level was provided to the LDPP cows by metal halide lamps controlled by an automatic timer. Lights were turned on at 0500 h and off at 2300 h for the LDPP groups. Cows were milked daily at 0700 h and 1600 h. Milk yield was summed daily, and milk was sampled monthly (a.m. and p.m. combined) for composition analysis. Fat, protein, and lactose along with SCC were measured by automated infrared analysis (Fossomatic; Bentley Instruments, Chaska, MN) at Mid-East DHIA (Hagerstown, MD). Fat-corrected milk yield was calculated according to Tyrrell and Reid (29). Net energy balance ( NEB) was calculated as NEB = [net energy intake] – NEL – net energy maintenance (NEM) where NEL = milk (kilograms) × (0.3512 + 0.0962 × fat percentage), and NEM = 0.08 Mcal/kg0.75 × (BW (kilograms)) 0.75. Effects of pregnancy were not accounted for when calculating NEB. Cows were weighed every 2 wk. A total mixed diet (Table 1 ) was fed ad libitum once daily. Feed refusals were recorded, and data were adjusted for DMI. The diet was formulated to support maximal milk yield of 45.3 kg of milk/d in multiparous cows and 38.6 kg of milk/d in primiparous cows [NRC, 1989; (15)]. Feed samples were collected weekly, frozen at –20°C, and analyzed for composition at the end of the study by Cumberland Valley Analytical Services (Maugansville, MD). Blood was collected every 2 wk by tail vessel puncture with 10 ml sodium heparin Vacutainer tubes and 20-ga needles (Beckton Dickinson Vacutainer Systems, Franklin

TABLE 1. Ingredient composition of the diet fed to experimental cows exposed to a long daily photoperiod (LDPP) or natural daily photoperiod (NDPP) with or without bST administration.1 Ingredient

Multiparous Primiparous

Ground shell corn Alfalfa haylage Corn silage Soyplus,2 Whole cottonseed Soyplus-supplement mix Dicalcium phosphate Calcitic limestone Sodium bicarbonate Tallow Salt Magnesium oxide Trace mineral and vitamin mix3

33.37 21.62 19.72 9.20 9.12 3.69 0.86 0.62 0.58 0.45 0.39 0.19 0.19

( % of DM) 30.62 23.90 21.77 11.21 9.47 ... 0.90 0.43 0.73 ... 0.49 0.24 0.24

1LDPP = 18 h of light and 6 h of darkness/d; NDPP = 9.5 to 14 h of light and 14.5 to 9.5 h of darkness/d. 2Soyplus; West Central (Ralston, IA) is 48% CP on a DM basis. 3Formulated to provide 3000 IU of vitamin A, 500 IU of vitamin D, 10 IU of vitamin E, 50 mg of Fe, 40 mg of Zn, 40 mg of Mn, 10 mg of Cu, 0.6 mg of I, 0.3 mg of Se, and 0.1 mg of Co/kg of dietary DM.

Lakes, NJ). Samples were centrifuged at 1970 × g for 30 min, and plasma was decanted and stored at –20°C until assayed for ST and IGF-I as previously described (8, 9). Prolactin ( PRL) was also measured by radioimmunoassay with a primary antibody (bPRL AFP-4835B; kindly donated by Dr. Parlow, NHPP, and NIDDK, Torrance, CA) diluted to 1: 50,000 in working solution and a final tube dilution of 1:200,000. Bovine serum depressed binding in parallel to the standard curve in the PRL assay, and spike recovery averaged 97.9%. Mean intraassay coefficients of variation (two assays) for low (3.7 ng/ ml) and high sera (25.1 ng/ml) were 12.5 and 6.1%, respectively. The interassay coefficient of variation averaged 15.5%; assay sensitivity averaged 0.26 ng/ ml. The IGF-BP were characterized and quantified as previously described ( 8 ) . Briefly, serum was diluted to 1:15, and 30 ml was loaded onto each lane of a 12% SDS-PAGE gel. Proteins were separated electrophoretically, transferred to nitrocellulose paper, and probed with either 125I-IGF-I (IGF-BP-3) or a specific anti-IGF-BP-2 antibody (anti-bovine IGF-BP-2; Upstate Biotechnology, Lake Placid, NY). Molecular mass standards were run concurrently and used to identify migration of IGF-BP-2 and IGF-BP-3. Antibody-reactive material was visualized using chemiluminescent Supersignal (Pierce, Rockford, IL) for IGF-BP2. Radioactivity in the resolved band corresponding to IGF-BP-3 and the chemiluminescent Journal of Dairy Science Vol. 82, No. 8, 1999

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TABLE 2. Treatment means and standard errors of the difference (SED) for yield, FCM, protein percentage, fat percentage, SCC, lactose percentage, DMI, net energy balance (NEB), and BW for cows exposed to a long daily ( L D ) or a natural daily ( N D ) photoperiod ( P P ) with or without bST administration.

Milk, kg/d 3.5% FCM, kg/d Protein, % Fat, % SCC, ×103/ml Lactose, % DMI, kg/d NEB, Mcal/d BW, kg

NDPP1 25.2 27.7 3.3 4.2 300 4.7 22.9 10.0 619

P

LDPP

NDPP +bST

LDPP +bST

SED

bST

PP

bST × PP

27.1 29.6 3.2 4.1 130 4.8 23.7 10.2 628

30.5 33.4 3.3 4.2 420 4.8 22.4 6.1 626

31.7 35.4 3.2 4.4 210 4.9 24.4 6.7 616

1.7 1.62 0.079 0.19 190 0.050 0.60 0.87 43

0.01 0.01 0.62 0.26 0.47 0.04 0.84 0.01 0.94

0.22 0.12 0.21 0.62 0.17 0.01 0.01 0.56 0.97

0.78 0.54 0.79 0.15 0.88 0.14 0.19 0.84 0.75

1NDPP 2The

= 9.5 to 14 h of light and 14.5 to 9.5 h of darkness/d; LDPP = 18 h of light and 6 h of darkness/d. SED for NDPP + bST = 2.4 because of heterogeneous variance.

signal from the band corresponding to IGF-BP-2 were quantified using Imagequant software (Molecular Dynamics, Inc., Sunnyvale, CA). Data were analyzed as a split unit repeated measures design with a 2 × 2 factorial treatment arrangement (bST, 0 or 14 mg/d; LDPP or NDPP). Data were summarized into 10 14-d periods with the initial period being pretreatment for the repeated measures variable. The model contained the fixed effects of bST, photoperiod, period, parity, and parity by treatment and the interactions of bST and photoperiod. In addition, the initial pretreatment value (time = 0 ) for the dependent variable was used as a covariate. The effect of cow within bST and photoperiod treatment was modeled as a random effect. Several different repeated measures covariance structures were examined, and Akaike’s Information Criteria were used to select the best fitting structure for each dependent variable. For FCM yield, heterogeneous variances were also fit across periods. In addition to the model interactions, the analysis included contrast to test the bST by photoperiod interactions for each time period. Parity effects were nonsignificant; therefore, data were averaged across primiparous and multiparous cows. One primiparous cow from the NDPP + bST treatment was removed from the study because of disease (Lymphosarcoma), and her data were not included in the analysis. RESULTS Milk and Milk Components Treatment with LDPP increased FCM by 1.9 kg/d relative to NDPP. Bovine somatotropin increased FCM by 5.7 kg/d relative to NDPP, and LDPP + bST Journal of Dairy Science Vol. 82, No. 8, 1999

increased FCM by 5.8 kg/d relative to LDPP (Table 2). The LDPP treatment tended to increase milk yield relative to NDPP during experimental period 5 ( P < 0.19). The LDPP + bST treatment tended to increase milk yield relative to treatment with NDPP

Figure 1. Group means for FCM of cows exposed to long daily photoperiod (LDPP = ◊; n = 10) (18 h of light and 6 h of darkness/ d ) or natural daily photoperiod (NDPP = ⁄; n = 10) (9.5 to 14 h of light/d and 14.5 to 9.5 h of darkness/d) with or without bST administration (LDPP + bST = ◊, n = 10; NDPP + bST = ♦, n = 9 ) by days of experiment (DAY). Each symbol represents the mean yield of cows within that group for the 14-d period. The shaded bar indicates the time of photoperiod and bST treatment. Standard error of the difference for comparison among groups for LDPP, NDPP, and LDPP + bST = 1.6; NDPP + bST = 2.4.

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TABLE 3. Treatment means and standard errors of the difference (SED) for circulating concentrations of somatotropin ( S T ) and IGF-I for cows exposed to a long daily ( L D ) or a natural daily ( N D ) photoperiod ( P P ) with or without bST administration. P

Treatment1

NDPP2

LDPP

NDPP +bST

LDPP +bST

SED

bST

PP

bST × PP

ST, ng/ml IGF-I, ng/ml PRL, ng/ml IGF-BP-2, rdu IGF-BP-3, rdu

1.3 58 8.7 76.9 118.0

1.5 61 9.5 76.4 125.7

6.7 115 9.6 86.2 145.5

7.2 106 8.6 82.1 145.6

1.4 7.8 2.2 12.9 15.7

0.01 0.01 0.99 0.42 0.05

0.62 0.54 0.84 0.80 0.73

0.76 0.28 0.36 0.85 0.74

1PRL

= Prolactin; Rdu = relative densitometric units. = 9.5 to 14 h of light and 14.5 to 9.5 h of dark/d; LDPP = 18 h of light and 6 h of darkness/d.

2NDPP

+ bST during experimental periods 6 and 7 ( P < 0.17). Overall, treatment with LDPP + bST produced the highest yields of FCM (Figure 1). Milk components generally did not differ between photoperiod or bST treatments; however, lactose percentage was slightly lower in the NDPP treatment group (Table 2). There were no differences in milk fat percentage, protein percentage, or SCC among treatments.

3 ) did not differ between NDPP and LDPP treatments. Treatment with bST significantly decreased energy balance relative to NDPP and LDPP treatments alone. DISCUSSION Previously observed differences in responses to bST and LDPP treatments suggest that the treatments

ST, IGF-I, PRL, and IGF-BP Relative to non-bST-treated cows, bST increased circulating ST (Table 3). Photoperiod did not influence ST concentrations (Table 3). Compared with NDPP treatment, NDPP + bST treatment increased circulating IGF-I in cows, as did LDPP + bST treatment when compared with LDPP treatment (Table 3). However, concentrations of IGF-I were not affected by photoperiod treatment. There was no effect of bST on PRL concentrations. Photoperiod did not affect PRL concentrations overall (NDPP and NDPP + bST treatments averaged 7.3 ng/ml, and the LDPP and LDPP + bST treatments averaged 8.4 ng/ml) ( P = 0.9). However, on d 72 of the experiment the LDPP treatment groups had an increase in PRL concentrations of 2.9 ng/ml above the NDPP treatment groups ( P < 0.13). Photoperiod treatment had no effect on IGF-BP-2 or IGF-BP-3. Treatment with bST increased IGF-BP-3 but had no effect on IGF-BP-2 (Table 3). DMI, NEB, and BW Treatment with LDPP increased DMI 0.8 kg/d relative to NDPP treatment. Relative to NDPP + bST treatment, LDPP + bST treatment increased DMI by 2.0 kg/d (Table 2, Figure 2). Body weight was not affected by photoperiod or bST treatment, and no difference was found among the groups throughout the study (Table 2). Energy balance (Table 2, Figure

Figure 2. Group means for DMI of cows exposed to long daily photoperiod (LDPP = ◊; n = 10) (18 h of light and 6 h of darkness/ d ) or natural daily photoperiod (NDPP = ⁄; n = 10) (9.5 to 14 h of light/d and 14.5 to 9.5 h darkness/d) with or without bST administration (LDPP + bST = ◊, n = 10; NDPP + bST = ♦, n = 9 ) by days of experiment (DAY). The shaded bar indicates the time of photoperiod and bST treatment. Each symbol represents the mean yield of the 10 cows within that group for the 14-d period. Standard error of the difference for comparison among groups = 0.6. Journal of Dairy Science Vol. 82, No. 8, 1999

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increase milk yield through separate mechanisms. For example, the effect of LDPP on IGF-I appears to be independent of circulating ST and IGF-BP (8, 24). Also, the time course of the response to bST and LDPP are different. Increases in yield because of bST administration are evident within days of treatment and disappear rapidly after bST injections are discontinued ( 3 ) . In contrast, increases because of LDPP treatment take longer to develop ( 2 to 4 wk) and have a carry-over effect after removal of supplemental light ( 8 ) . However, the combination of LDPP + bST treatment had not been previously examined for effects on milk yield or effects on the somatotropic axis. Results of the present experiment showed that bST and LDPP treatments increased milk yield in lactating cattle when used independently, although the response to LDPP was smaller than that reported in previous studies (3, 8, 11, 18, 27). Further, milk yield responses observed suggest that the combination of bST and LDPP had additive effects with regard to milk yield. Milk yield was highest in the combined treatment group relative to groups in which bST or LDPP treatments were used alone. Further, the divergence tended to increase toward the end of the experiment. The lag in development of any additive effect on milk yield is consistent with the temporal delay in galactopoietic response to LDPP treatment (i.e., 2 to 4 wk), relative to bST (i.e., 2 to 4 d). Milk composition remained consistent among treatments throughout the course of the study. Previously, decreases in fat in response to LDPP treatment have been observed (27); yet bST and LDPP were generally without effect on the major milk components measured. Although not consistently, slight elevations in lactose percentage during bST treatment have been observed previously (2, 26). However, the percentages of lactose in the present experiment were similar among treatments. Consistent with many reports (1, 4, 6, 7, 8, 17) bST and LDPP did not affect milk composition in the present experiment. As expected, increases in milk yield with the use of bST were paralleled by increases in serum ST and IGF-I concentrations. Plasma concentrations of ST increased on d 1 of treatment, and an increase in IGFI was observed at the time of the next blood sample collection (i.e., d 14). The present data confirm previous results (19, 28, 30) that LDPP did not increase serum ST concentrations. No difference was found in the serum ST concentrations between the NDPP and LDPP groups or between NDPP + bST and LDPP + bST treatments. Of interest, LDPP failed to increase IGF-I in the present study. Previous reports (8, 24, Miller and Journal of Dairy Science Vol. 82, No. 8, 1999

Figure 3. Group means for net energy balance of cows exposed to long daily photoperiod (LDPP = ◊, n = 10) (18 h of light and 6 h of darkness/d) or natural daily photoperiod (NDPP = ⁄, n = 10) (9.5 to 14 h of light/d and 14.5 to 9.5 h of darkness/d) with or without bST administration (LDPP + bST = ◊, n = 10; NDPP + bST = ♦, n = 9 ) by days of experiment (DAY). The shaded bar indicates the time of photoperiod and bST treatment. Each symbol represents the mean yield of the 10 cows within that group for the 14-d period. Standard error of the difference for comparison among groups = 0.87.

Dahl, unpublished) have shown evidence that IGF-I is increased by LDPP in dairy cattle across a range of physiologic conditions, including lactating cows, heifers, and dry cows. Lack of IGF-I response has been observed when nutrition was limited, but the cows in the present study were fed identical diets in amounts sufficient to support higher levels of yield observed with bST and photoperiod treatments. Thus, a dietary limitation of the IGF-I response seems unlikely. The lack of an IGF-I response to LDPP when bST was also provided may be related to the maximal response of IGF-I to bST. That is, the IGF-I response was maximal with the 14-mg dose of bST, precluding detection of a photoperiod-induced additive response. In contrast to previous data ( 8 ) , the results of the present study do not provide support for the hypothesis that increases in IGF-I are responsible for the galactopoietic effect of LDPP. Increases in milk yield that were observed without the expected significant increases in serum IGF-I concentrations allow an alternate explanation for the galactopoietic effect of LDPP. Possibly, another hormone, alone or in addition to IGF-I, may be responsi-

PHOTOPERIOD, BOVINE SOMATOTROPIN, AND MILK YIELD

ble for the LDPP increase in milk yield. For example, LDPP increases serum PRL (16, 20, 25). Further, recent evidence suggests that PRL and ST may interact to produce galactopoietic responses in rodents (12). Increased PRL concentrations in response to LDPP may be interacting with ST independent of IGF-I to increase milk yield in lactating cows. The role of PRL is tempered, however, by the observation of Plaut et al. ( 2 1 ) that exogenous PRL had no effect on established lactation in cattle. However, the duration of that study was less than the expected time required ( 4 wk) to observe a long-day effect on milk yield. Consistent with earlier studies, LDPP tended to increase PRL concentrations in the present study. Overall, there were no significant differences in PRL concentrations among groups, but the transient increases in PRL concentrations, and the potential for a galactopoietic effect, in the LDPP cows cannot be completely discounted. Insulin-like growth factor-1 in the blood circulation is influenced by the IGF-BP. Circulating IGF-BP were measured in half of the cows on this experiment. Consistent with previous reports, no change in IGFBP levels were observed with photoperiodic treatment [Table 3; (8)]. This result suggested that alterations of the clearance rate of IGF-I was not responsible for the LDPP effect on milk yield ( 8 ) . In contrast, bST increased IGF-BP-3 as previously observed ( 3 ) and had no effect on IGF-BP-2, regardless of photoperiod treatment. The cows that produced the most milk generally consumed the most feed. Within groups of cows that demonstrated increased milk yield, DMI subsequently increased to support the higher demand for yield. This result is consistent with those of previous works (17, 27) showing that increases in DMI generally follow, rather than precede, increases in milk yield. Manipulation of photoperiod had no effect on NEB, although NEB decreased in cows treated with bST. Body weight did not differ among groups. All cows followed a typical pattern of increased BW as gestation progressed and they approached the end of lactation. Collectively, the effects on DMI and NEB provide further evidence of dissimilar mechanisms of bST- and LDPP-induced increases in milk yield. With bST, it appears that cows mobilize body energy reserves to support increased milk yield, which depresses energy balance and is eventually compensated by increased DMI. Such effects would be consistent with the lipolytic effect of bST on adipose tissue (3, 6, 7). In contrast, LDPP-induced increases in milk yield were supported energetically by increased DMI, consistent with a lack of effect of photoperiod on

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serum ST. The observed effects of LDPP, bST, and the combination of both on DMI and NEB support these conclusions. CONCLUSIONS Photoperiod and bST independently caused increases in milk yield in lactating cows. Results of the present study suggest that the combination of LDPP and bST produced additive effects on milk yield and DMI. As additive effects have been observed with the use of bST and 3× daily milking and bST and LDPP, the combination of all three techniques may produce the greatest galactopoietic effect. ACKNOWLEDGMENTS Special thanks are given to Thomas Moreland, Wendy Smith, and the farm crew at the Central Maryland Research and Education Center (Clarksville) for daily care of the animals and data recording. Also, we thank Michael Tine, Clare Bonifant, Jennifer Smith, and Erin Connor for helping with sample collections. Finally, thank you to Anne Kimrey and Alisha Parlier for helping with assays and laboratory analyses. REFERENCES 1 Bauman, D. E., P. J. Eppard, M. J. DeGeeter, and G. M. Lanza. 1985. Responses of high-producing dairy cows to long-term treatment with pituitary somatotropin and recombinant somatotropin. J. Dairy Sci. 68:1352–1362. 2 Bauman, D. E., and C. J. Peel. 1987. Somatotropin and lactation. J. Dairy Sci. 70:474–486. 3 Bauman, D. E., and R. G. Vernon. 1993. Effects of exogenous bovine somatotropin on lactation. Annu. Rev. Nutr. 13:437–461. 4 Bilodeau, P. P., D. Petitclerc, N. St. Pierre, G. Pelletier, and G. J. St. Laurent. 1989. Effects of photoperiod and pair-feeding on lactation of cows fed corn or barley grain in total mixed rations. J. Dairy Sci. 72:2999–3005. 5 Cohick, W. S., M. A. McGuire, D. R. Clemmons, and D. E. Bauman. 1992. Regulation of IGF binding proteins in serum and lymph of lactating cows by somatotropin. Endocrinology 130:1508–1514. 6 Dahl, G. E., L. T. Chapin, M. S. Allen, W. M. Moseley, and H. A. Tucker. 1991. Comparison of somatotropin and growth hormone-releasing factor on milk yield, serum hormones, and energy status. J. Dairy Sci. 74:3421–3428. 7 Dahl, G. E., L. T. Chapin, W. M. Moseley, and H. A. Tucker. 1993. Galactopoietic effects of recombinant somatotropin and growth hormone-releasing factor in dairy cows. J. Dairy Sci. 76: 1550–1557. 8 Dahl, G. E., T. H. Elsasser, A. V. Capuco, R. A. Erdman, and R. R. Peters. 1997. Effects of a long daily photoperiod on milk yield and circulating concentrations of insulin-like growth factor-1. J. Dairy Sci. 80:2784–2789. 9 Elsasser, T. H., T. S. Rumsey, and A. C. Hammond. 1989. Influence of diet on basal and growth hormone-stimulated plasma concentrations of IGF-I in beef cattle. J. Anim. Sci. 67: 128–141. 10 Erdman, R. A., and M. Varner. 1995. Fixed yield responses to increased milking frequency. J. Dairy Sci. 78:1199–1203. Journal of Dairy Science Vol. 82, No. 8, 1999

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