0013-7227/78/1031-0229$02.00/0 Endocrinology Copyright © 1978 by The Endocrine Society
Vol. 103, No. 1 Printed in U.S.A.
Prolactin and Growth Hormone Responses to Photoperiod in Heifers* ROBERT R. PETERS AND H. ALLEN TUCKER Animal Reproduction Laboratory, Department of Dairy Science, Michigan State University, East Lansing, Michigan 48824 ABSTRACT. Serum PRL, GH, and progesterone were measured at 1300 h twice weekly in heifers exposed to natural length or 16-h light (16L)-8-h dark (8D) during Nov. 11 to March 9 (n = 20) and April 30 to Aug. 13 (n — 18). Heifers exposed to 16L-8D received fluorescent light between 0600-2200 h daily. Serum PRL increased 4-fold in response to 16 h of artificial illumination between Nov. 11 and March 9 (P < 0.01), and increased 1.6-fold between April 30 and Aug. 13 (P < 0.01). Serum PRL fluctuations in natural and 16L-8D photoperiods were synchronized and positively correlated (r = 0.36-0.53) with changes in ambient temperature. Overall, serum PRL concentrations increased (P < 0.01) within 16L-8D and natural length photoperiods as ambient temperatures increased from —7 to +29 C. When ambient temperatures were below 0 C, serum PRL concentrations were similar in heifers exposed to natural and 16L-8D photoperiods. As ambient temperatures increased, serum PRL concentrations increased synergistically in animals exposed to 16L-8D photoperiods.
C
ONCENTRATIONS of PRL in sera are highest during summer and lowest during winter in cattle (1-3), goats (4, 5), and sheep (6) in northern latitudes. Increased ambient temperatures increased basal concentrations of serum PRL in prepubertal calves (7, 8) and mature rats (9). Longer daily exposure to light increased PRL in sera from prepubertal bulls (10) and sheep (11, 12) and pituitary concentrations of PRL in hamsters (13). However, whether temperature and photoperiod synergize in regulation of concentrations of PRL in sera is unknown. Objectives of the present study were to determine: 1) if low concentrations of serum PRL normally
Thus, the interactions between length of photoperiod (16L-8D vs. natural) and ambient temperature were significant (P < 0.02) between Nov. 11 and March 9 and between April 20 and Aug. 13. Serum GH concentrations did not respond to 16L-8D photoperiods or changes in ambient temperature during the fall-winter or springsummer seasons. As determined from serum progesterone concentrations, 3 of 10 heifers exposed to 16L-8D photoperiods and 0 of 10 exposed to natural photoperiods initiated estrous cycles between Nov. 11 and March 9 (P = 0.10). Photoperiods of 16L-8D did not affect onset of estrous cycles between April 30 and Aug. 13. We conclude that 16 h of illumination daily increase serum PRL in comparison with heifers exposed to natural length photoperiods of 9-12 h. However, ambient temperatures below 0 C suppress the ability of 16L-8D photoperiods to increase serum PRL, but as temperatures increase, serum PRL increases synergistically in response to 16L-8D photoperiods. (Endocrinology 103: 229, 1978)
observed in fall and winter could be increased with 16 h of light (16L)-8 h of dark (8D) daily, 2) if a daily photoperiod of 16L-8D increases serum PRL concentrations during spring and summer, and 3) if photoperiod affected concentrations of GH in serum. Materials and Methods
Received October 25, 1977. Address requests for reprints to: Dr. H. Allen Tucker, Michigan State University, College of Agriculture and Natural Resources, Department of Dairy Science, 230 Anthony Hall, East Lansing, Michigan 48824. * Michigan Agricultural Experiment Station Journal Article 8279. This research was supported in part by USPHS Grants HD-09883 and AM-15899.
In an experiment, extending from Nov. 2 to March 9, 20 Holstein heifers ranging from 3.0-6.5 months of age were assigned randomly to one of two groups of 10, balanced for age and line of selection for genetic differences in milk yield. Each group was housed unrestrained within separate pens. Both groups of heifers were exposed to indirect sunlight through open northern windows, and no supplemental heat was provided. One group (controls) received only indirect sunlight (natural photoperiod), whereas the second group was given the natural photoperiod plus cool-white fluorescent light daily between 0600-2200 h (16L-8D). The ration consisted of 2.3 kg 14% protein pelleted concentrate daily/heifer, alfalfa hay, and water ad libitum. An adjustment period of 9 days was al-
229
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230
PETERS AND TUCKER
lowed before blood sampling began. Natural daylengths (based on sunrise to sunset intervals) for Nov. 11, Dec. 22 (winter solstice), and March 9 were 10.0,9.3, and 11.6 h, respectively. The artificial 16 h of light was chosen because it approximated the longest period of sunlight in a day in East Lansing, MI (42° 42' north latitude), and we have preliminary evidence that this length of exposure maximizes concentrations of PRL in serum. Heifers were haltered and tied to a rope 60-cm long 2 h before blood collection. At 1300 h twice weekly, blood was collected from a jugular vein by venipuncture throughout the 17-week interval. In addition, a single measurement of ambient temperature and multiple measurements of light intensity at specified sites throughout each pen were recorded immediately after blood samples were drawn. Median light intensities (at eye level of animals) among days of this experiment ranged from 11-76 (mean = 39) for heifers exposed to natural length photoperiods and from 102-323 (mean = 207) lux for heifers receiving 16L-8D photoperiods. In a study between April 20 and Aug. 13, 18 additional Holstein heifers ranging from 3.0-7.5 months of age were weighed on each of 3 consecutive days and paired by average body weight (160 and 159 kg at start of experiment) and genetic line for milk production. One of each pair of animals was assigned randomly to one of two groups. One group was exposed to natural photoperiod and the second was exposed to natural photoperiod supplemented with 16 h of artificial illumination (16L8D), as described in the first experiment. An adjustment period of 10 days after April 20 was allowed before blood sampling commenced. Blood collections and recordings of light intensity and ambient temperature were similar to those of the first experiment. A grain mixture (2.3 kg 14% protein/heifer/day), alfalfa haylage, and water {ad libitum) were fed throughout the experiment. The natural photoperiod over this 16-week period increased from 13.6 h on April 20 to 15.3 h on June 22 (summer solstice), then decreased to 14.0 on Aug. 13. Median light intensities among days of the experiment for natural and 16L-8D photoperiods were 16-59 (mean = 40) and 161-377 (mean = 294) lux, respectively. Blood was stored at room temperature for 6 h, and at 4 C for 24 h, and sera was obtained by centrifugation. Sera were stored at —20 C until PRL, GH, and progesterone were measured by double antibody RIA, as previously described from our laboratory (14-16). Bovine PRL (NIH-B3) and bovine GH (NIH-B12) were used as reference stan-
Endo • 1978 Vol 103 • No 1
dards. Time of first appearance of concentrations of progesterone in serum greater than 1 ng/ml was judged to be time of first ovulation (17). Statistical analysis of serum PRL and GH were conducted using logio transformed data, primarily to minimize heterogeneity of variance of PRL means. These transformed data were analyzed by least squares split plot analysis of variance or paired t test (18, 19). Differences in number of heifers initiating estrous cycles between treatments were determined by X-square procedures (19).
Results Serum PRL (Nov. 11 to March 9) Serum PRL averaged (± SE) 10.8 ± 1.7 and 41.9 ±8.1 ng/ml in heifers exposed to natural and 16L-8D photoperiods (P < 0.01), respectively, between Nov. 11 and March 9 (Fig. 1). However, at the beginning, on several other days and between days 67-92 (Jan. 16 to Feb. 10), PRL concentrations in sera were similar in both groups of heifers. Days 67-92 correspond to the coldest period of the experiment when ambient temperature consistently ranged from —7-0 C. The correlation between average PRL concentrations in the two groups of heifers between Nov. 11 and March 9 was 0.22 (P > 0.05). The correlations between PRL in sera and ambient temperatures were 0.36 and 0.42 -A 16 USD photoperiod - # Natural photoperiod East Lansing, Ml Nov. II to March 9
5
15 25 35 45
55 65
75 85 95
105 115 125
DAYS
FIG. 1. Mean serum PRL in heifers subjected to natural length or 16L-8D photoperiods between Nov. 11 and March 9 in East Lansing, MI. Standard errors pooled within treatment were 1.7 and 8.1 ng/ml, respectively.
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HORMONAL RESPONSE TO PHOTOPERIOD
(P < 0.05) for heifers treated with natural and 16L-8D photoperiods, respectively. To study these interactions, further PRL concentrations in heifers within each photoperiod treatment were arbitrarily grouped for statistical purposes according to ambient temperature as follows: -7-0, 1-7, and 8-18 C (Fig. 2, left panel). Overall, PRL concentrations increased (P < 0.01) as ambient temperatures increased from —7 to 18 C. However, the interaction of length of photoperiod (16L-8D vs. natural) with ambient temperature was significant (P < 0.01). These data suggest that the increase in PRL in sera due to increasing temperature was greater in heifers subjected to 16L-8D photoperiods than in heifers given natural length photoperiods. For example, the means of PRL concentrations in sera at temperatures between —7-0 were not different (P > 0.05) in the two photoperiod treatments (9.1 vs. 20.5 ng/ml). Compared with the natural photoperiods, 16 h of illumination daily increased serum PRL approximately 6-fold (8.2 vs. 48.9 ng/ml) and 3-fold (18.5 vs. 53.7 ng/ml) when ambient temperatures were 1-7 and 8-18 C, respectively. Serum PRL (April 30 to Aug. 13) Serum PRL of heifers given natural or 16L8D photoperiods between April 30 and Aug. 13 averaged 48.4 ± 5.9 and 78.2 ± 15.0 ng/ml, respectively (P < 0.01; Fig. 3). Serum PRL in both groups of heifers generally paralleled the Nov II to March 9
I6L : 8D photoperiod
April 30 to Aug 13
Natural photoperiod
80 (36)
E J1 60 c 8 40
(110)
-7toO
(160)
I to 7 8tol8 9tol4 I5to2l Ambient Temperature ( C )
22 to 29
FIG. 2. Interactions of ambient temperature and photoperiod length on mean (±SE) serum PRL in heifers in two seasons. Difference due to 16L-8D photoperiod was suppressed at -7-0 C and increased (P < 0.05) at 22-29 C.
231
140
120
100
80
60
40
20
--AI6U8D photoperiod Natural photoperiod East Lansing, Ml April 30 to Aug 13 15 25 35 45 55 65 75 85 95 105 DAYS
FIG. 3. Mean serum PRL in heifers subjected to natural length or 16L-8D photoperiods between April 30 and Aug. 13 in East Lansing, MI. Standard errors pooled within treatment were 5.9 and 15 ng/ml, respectively.
changes in ambient temperature. The correlation between average PRL across days of the experiment in the two groups of heifers throughout the experiment was 0.82. Thus, daily fluctuations in PRL were closely synchronized in both groups of heifers. Ambient temperature was correlated (P < 0.01) with serum PRL in heifers given natural and 16L8D photoperiods (r = 0.49 and 0.53, respectively). Concentrations of PRL in heifers in both photoperiod treatments were arbitrarily grouped according to ambient temperatures of 9-14, 15-21, and 22-29 C (Fig. 2, right panel). Overall, PRL increased as ambient temperature increased (P < 0.01) from 9 to 29 C. Furthermore, the overall interaction between length of photoperiod and ambient temperature was significant (P < 0.02). Thus, the increase in PRL in sera due to increased temperature was greater in heifers given 16L-8D as compared with natural length photoperiods. To illustrate, the means of serum PRL at temperatures between 9-14 and 15-21 C were not affected significantly by photoperiod, whereas at temperatures between 22-29 C, serum PRL was greater (P = 0.05) for heifers in 16L-8D photoperiods (90 ng/ml) as compared with those in natural photoperiods (56 ng/ml).
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PETERS AND TUCKER
232
16 L : 8D photoperiod Natural photoperiod East Lansing, Ml Nov II to March 9
20
10
w
5
15 25 35 45
55 65 75 85 95 105 115 125 DAYS
FIG. 4. Mean serum GH in heifers subjected to natural length or 16L-8D photoperiods between Nov. 11 and March 9 in East Lansing, MI. Standard errors pooled within treatment were 0.4 and 0.8 ng/ml, respectively.
Serum GH (Nov. 11 to March 9 and April 30 to Aug. 13) Serum GH concentrations from Nov. 11 to March 9 averaged 7.7 ± 0.4 and 8.0 ± 0.7 ng/ml (P > 0.05) in heifers subjected to natural and 16L-8D photoperiods, respectively (Fig. 4). Similarly, between April 30 and Aug. 13, GH concentrations were not different in heifers exposed to natural and 16L-8D photoperiods, averaging 12.4 ± 2.0 and 11.9 ± 2.9 ng/ml {P > 0.05), respectively (daily averages not shown). Serum progesterone (Nov. 11 to March 9 and April 30 to Aug. 13) The 3 oldest of 10 heifers exposed to 16 h of illumination daily between Nov. 11 and March 9 initiated estrous cycles before the end of the experiment, as determined from serum progesterone concentrations which averaged 3.8 ng/ml in heifers during the luteal phase of the estrous cycle. In contrast, none of the heifers exposed to natural photoperiods exhibited cyclic serum progesterone patterns (average progesterone,
Vol. 103, No. 1 Printed in U.S.A.
Prolactin and Growth Hormone Responses to Photoperiod in Heifers* ROBERT R. PETERS AND H. ALLEN TUCKER Animal Reproduction Laboratory, Department of Dairy Science, Michigan State University, East Lansing, Michigan 48824 ABSTRACT. Serum PRL, GH, and progesterone were measured at 1300 h twice weekly in heifers exposed to natural length or 16-h light (16L)-8-h dark (8D) during Nov. 11 to March 9 (n = 20) and April 30 to Aug. 13 (n — 18). Heifers exposed to 16L-8D received fluorescent light between 0600-2200 h daily. Serum PRL increased 4-fold in response to 16 h of artificial illumination between Nov. 11 and March 9 (P < 0.01), and increased 1.6-fold between April 30 and Aug. 13 (P < 0.01). Serum PRL fluctuations in natural and 16L-8D photoperiods were synchronized and positively correlated (r = 0.36-0.53) with changes in ambient temperature. Overall, serum PRL concentrations increased (P < 0.01) within 16L-8D and natural length photoperiods as ambient temperatures increased from —7 to +29 C. When ambient temperatures were below 0 C, serum PRL concentrations were similar in heifers exposed to natural and 16L-8D photoperiods. As ambient temperatures increased, serum PRL concentrations increased synergistically in animals exposed to 16L-8D photoperiods.
C
ONCENTRATIONS of PRL in sera are highest during summer and lowest during winter in cattle (1-3), goats (4, 5), and sheep (6) in northern latitudes. Increased ambient temperatures increased basal concentrations of serum PRL in prepubertal calves (7, 8) and mature rats (9). Longer daily exposure to light increased PRL in sera from prepubertal bulls (10) and sheep (11, 12) and pituitary concentrations of PRL in hamsters (13). However, whether temperature and photoperiod synergize in regulation of concentrations of PRL in sera is unknown. Objectives of the present study were to determine: 1) if low concentrations of serum PRL normally
Thus, the interactions between length of photoperiod (16L-8D vs. natural) and ambient temperature were significant (P < 0.02) between Nov. 11 and March 9 and between April 20 and Aug. 13. Serum GH concentrations did not respond to 16L-8D photoperiods or changes in ambient temperature during the fall-winter or springsummer seasons. As determined from serum progesterone concentrations, 3 of 10 heifers exposed to 16L-8D photoperiods and 0 of 10 exposed to natural photoperiods initiated estrous cycles between Nov. 11 and March 9 (P = 0.10). Photoperiods of 16L-8D did not affect onset of estrous cycles between April 30 and Aug. 13. We conclude that 16 h of illumination daily increase serum PRL in comparison with heifers exposed to natural length photoperiods of 9-12 h. However, ambient temperatures below 0 C suppress the ability of 16L-8D photoperiods to increase serum PRL, but as temperatures increase, serum PRL increases synergistically in response to 16L-8D photoperiods. (Endocrinology 103: 229, 1978)
observed in fall and winter could be increased with 16 h of light (16L)-8 h of dark (8D) daily, 2) if a daily photoperiod of 16L-8D increases serum PRL concentrations during spring and summer, and 3) if photoperiod affected concentrations of GH in serum. Materials and Methods
Received October 25, 1977. Address requests for reprints to: Dr. H. Allen Tucker, Michigan State University, College of Agriculture and Natural Resources, Department of Dairy Science, 230 Anthony Hall, East Lansing, Michigan 48824. * Michigan Agricultural Experiment Station Journal Article 8279. This research was supported in part by USPHS Grants HD-09883 and AM-15899.
In an experiment, extending from Nov. 2 to March 9, 20 Holstein heifers ranging from 3.0-6.5 months of age were assigned randomly to one of two groups of 10, balanced for age and line of selection for genetic differences in milk yield. Each group was housed unrestrained within separate pens. Both groups of heifers were exposed to indirect sunlight through open northern windows, and no supplemental heat was provided. One group (controls) received only indirect sunlight (natural photoperiod), whereas the second group was given the natural photoperiod plus cool-white fluorescent light daily between 0600-2200 h (16L-8D). The ration consisted of 2.3 kg 14% protein pelleted concentrate daily/heifer, alfalfa hay, and water ad libitum. An adjustment period of 9 days was al-
229
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230
PETERS AND TUCKER
lowed before blood sampling began. Natural daylengths (based on sunrise to sunset intervals) for Nov. 11, Dec. 22 (winter solstice), and March 9 were 10.0,9.3, and 11.6 h, respectively. The artificial 16 h of light was chosen because it approximated the longest period of sunlight in a day in East Lansing, MI (42° 42' north latitude), and we have preliminary evidence that this length of exposure maximizes concentrations of PRL in serum. Heifers were haltered and tied to a rope 60-cm long 2 h before blood collection. At 1300 h twice weekly, blood was collected from a jugular vein by venipuncture throughout the 17-week interval. In addition, a single measurement of ambient temperature and multiple measurements of light intensity at specified sites throughout each pen were recorded immediately after blood samples were drawn. Median light intensities (at eye level of animals) among days of this experiment ranged from 11-76 (mean = 39) for heifers exposed to natural length photoperiods and from 102-323 (mean = 207) lux for heifers receiving 16L-8D photoperiods. In a study between April 20 and Aug. 13, 18 additional Holstein heifers ranging from 3.0-7.5 months of age were weighed on each of 3 consecutive days and paired by average body weight (160 and 159 kg at start of experiment) and genetic line for milk production. One of each pair of animals was assigned randomly to one of two groups. One group was exposed to natural photoperiod and the second was exposed to natural photoperiod supplemented with 16 h of artificial illumination (16L8D), as described in the first experiment. An adjustment period of 10 days after April 20 was allowed before blood sampling commenced. Blood collections and recordings of light intensity and ambient temperature were similar to those of the first experiment. A grain mixture (2.3 kg 14% protein/heifer/day), alfalfa haylage, and water {ad libitum) were fed throughout the experiment. The natural photoperiod over this 16-week period increased from 13.6 h on April 20 to 15.3 h on June 22 (summer solstice), then decreased to 14.0 on Aug. 13. Median light intensities among days of the experiment for natural and 16L-8D photoperiods were 16-59 (mean = 40) and 161-377 (mean = 294) lux, respectively. Blood was stored at room temperature for 6 h, and at 4 C for 24 h, and sera was obtained by centrifugation. Sera were stored at —20 C until PRL, GH, and progesterone were measured by double antibody RIA, as previously described from our laboratory (14-16). Bovine PRL (NIH-B3) and bovine GH (NIH-B12) were used as reference stan-
Endo • 1978 Vol 103 • No 1
dards. Time of first appearance of concentrations of progesterone in serum greater than 1 ng/ml was judged to be time of first ovulation (17). Statistical analysis of serum PRL and GH were conducted using logio transformed data, primarily to minimize heterogeneity of variance of PRL means. These transformed data were analyzed by least squares split plot analysis of variance or paired t test (18, 19). Differences in number of heifers initiating estrous cycles between treatments were determined by X-square procedures (19).
Results Serum PRL (Nov. 11 to March 9) Serum PRL averaged (± SE) 10.8 ± 1.7 and 41.9 ±8.1 ng/ml in heifers exposed to natural and 16L-8D photoperiods (P < 0.01), respectively, between Nov. 11 and March 9 (Fig. 1). However, at the beginning, on several other days and between days 67-92 (Jan. 16 to Feb. 10), PRL concentrations in sera were similar in both groups of heifers. Days 67-92 correspond to the coldest period of the experiment when ambient temperature consistently ranged from —7-0 C. The correlation between average PRL concentrations in the two groups of heifers between Nov. 11 and March 9 was 0.22 (P > 0.05). The correlations between PRL in sera and ambient temperatures were 0.36 and 0.42 -A 16 USD photoperiod - # Natural photoperiod East Lansing, Ml Nov. II to March 9
5
15 25 35 45
55 65
75 85 95
105 115 125
DAYS
FIG. 1. Mean serum PRL in heifers subjected to natural length or 16L-8D photoperiods between Nov. 11 and March 9 in East Lansing, MI. Standard errors pooled within treatment were 1.7 and 8.1 ng/ml, respectively.
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HORMONAL RESPONSE TO PHOTOPERIOD
(P < 0.05) for heifers treated with natural and 16L-8D photoperiods, respectively. To study these interactions, further PRL concentrations in heifers within each photoperiod treatment were arbitrarily grouped for statistical purposes according to ambient temperature as follows: -7-0, 1-7, and 8-18 C (Fig. 2, left panel). Overall, PRL concentrations increased (P < 0.01) as ambient temperatures increased from —7 to 18 C. However, the interaction of length of photoperiod (16L-8D vs. natural) with ambient temperature was significant (P < 0.01). These data suggest that the increase in PRL in sera due to increasing temperature was greater in heifers subjected to 16L-8D photoperiods than in heifers given natural length photoperiods. For example, the means of PRL concentrations in sera at temperatures between —7-0 were not different (P > 0.05) in the two photoperiod treatments (9.1 vs. 20.5 ng/ml). Compared with the natural photoperiods, 16 h of illumination daily increased serum PRL approximately 6-fold (8.2 vs. 48.9 ng/ml) and 3-fold (18.5 vs. 53.7 ng/ml) when ambient temperatures were 1-7 and 8-18 C, respectively. Serum PRL (April 30 to Aug. 13) Serum PRL of heifers given natural or 16L8D photoperiods between April 30 and Aug. 13 averaged 48.4 ± 5.9 and 78.2 ± 15.0 ng/ml, respectively (P < 0.01; Fig. 3). Serum PRL in both groups of heifers generally paralleled the Nov II to March 9
I6L : 8D photoperiod
April 30 to Aug 13
Natural photoperiod
80 (36)
E J1 60 c 8 40
(110)
-7toO
(160)
I to 7 8tol8 9tol4 I5to2l Ambient Temperature ( C )
22 to 29
FIG. 2. Interactions of ambient temperature and photoperiod length on mean (±SE) serum PRL in heifers in two seasons. Difference due to 16L-8D photoperiod was suppressed at -7-0 C and increased (P < 0.05) at 22-29 C.
231
140
120
100
80
60
40
20
--AI6U8D photoperiod Natural photoperiod East Lansing, Ml April 30 to Aug 13 15 25 35 45 55 65 75 85 95 105 DAYS
FIG. 3. Mean serum PRL in heifers subjected to natural length or 16L-8D photoperiods between April 30 and Aug. 13 in East Lansing, MI. Standard errors pooled within treatment were 5.9 and 15 ng/ml, respectively.
changes in ambient temperature. The correlation between average PRL across days of the experiment in the two groups of heifers throughout the experiment was 0.82. Thus, daily fluctuations in PRL were closely synchronized in both groups of heifers. Ambient temperature was correlated (P < 0.01) with serum PRL in heifers given natural and 16L8D photoperiods (r = 0.49 and 0.53, respectively). Concentrations of PRL in heifers in both photoperiod treatments were arbitrarily grouped according to ambient temperatures of 9-14, 15-21, and 22-29 C (Fig. 2, right panel). Overall, PRL increased as ambient temperature increased (P < 0.01) from 9 to 29 C. Furthermore, the overall interaction between length of photoperiod and ambient temperature was significant (P < 0.02). Thus, the increase in PRL in sera due to increased temperature was greater in heifers given 16L-8D as compared with natural length photoperiods. To illustrate, the means of serum PRL at temperatures between 9-14 and 15-21 C were not affected significantly by photoperiod, whereas at temperatures between 22-29 C, serum PRL was greater (P = 0.05) for heifers in 16L-8D photoperiods (90 ng/ml) as compared with those in natural photoperiods (56 ng/ml).
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PETERS AND TUCKER
232
16 L : 8D photoperiod Natural photoperiod East Lansing, Ml Nov II to March 9
20
10
w
5
15 25 35 45
55 65 75 85 95 105 115 125 DAYS
FIG. 4. Mean serum GH in heifers subjected to natural length or 16L-8D photoperiods between Nov. 11 and March 9 in East Lansing, MI. Standard errors pooled within treatment were 0.4 and 0.8 ng/ml, respectively.
Serum GH (Nov. 11 to March 9 and April 30 to Aug. 13) Serum GH concentrations from Nov. 11 to March 9 averaged 7.7 ± 0.4 and 8.0 ± 0.7 ng/ml (P > 0.05) in heifers subjected to natural and 16L-8D photoperiods, respectively (Fig. 4). Similarly, between April 30 and Aug. 13, GH concentrations were not different in heifers exposed to natural and 16L-8D photoperiods, averaging 12.4 ± 2.0 and 11.9 ± 2.9 ng/ml {P > 0.05), respectively (daily averages not shown). Serum progesterone (Nov. 11 to March 9 and April 30 to Aug. 13) The 3 oldest of 10 heifers exposed to 16 h of illumination daily between Nov. 11 and March 9 initiated estrous cycles before the end of the experiment, as determined from serum progesterone concentrations which averaged 3.8 ng/ml in heifers during the luteal phase of the estrous cycle. In contrast, none of the heifers exposed to natural photoperiods exhibited cyclic serum progesterone patterns (average progesterone,
