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Protein and energy self-selection of turkey hens. serum prolactin and luteinizing hormone concentrations
Camp. Biochem. Physid. Vol. lWA, No. 2, pp. 495499, Printed in Great Britain
0300-9629/91 $3.00 + 0.00 0 1991 Perpmon Press plc
1991
PROTEIN AND ENERGY SELF-SELECTION OF TURKEY HENS. SERUM PROLACTIN AND LUTEINIZING HORMONE CONCENTRATIONS DENBOW,*? H. P. VAN KREY,* R. M. HULET* and M. E. EL HALAWANI~
D. A. EMMERSON,*D. M.
*Department of Poultry Science, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0332, U.S.A. Telephone: (703) 231-6843; and IDepartment of Animal Science, University of Minnesota, St. Paul, MN 55108, U.S.A. (Received 5 February 1991) Abstract-l. The incidence of broodiness was four times as high among turkey hens fed a complete control diet than among hens allowed to self-select their diet from two different feed sources, one being relatively high in protein and the other relatively high in energy (i.e. split-diet). 2. Among non-broody birds, hens fed the split-diet had a significantly lower serum prolactin concentration in the third month of production as compared to control hens. 3. Hens in their second season of egg production had significantly lower serum luteinixing hormone concentrations during the latter stages of egg production than did first season hens.
INTRODUCTION
Broodiness remains a major problem which limits the production of turkey hatching eggs (Etches and Cheng, 1982). In addition to the loss of potential hatching eggs, expenses associated with the treatment of broody hens and with the maintenance of non-egg producing hens are incurred (Nestor et al., 1986). It has been estimated that the loss of one egg per hen costs the turkey industry $1.8 million per year in the U.S. alone (El Halawani et al., 1988). An effective broody hen treatment program can result in an additional 13 eggs per hen over a 180 day production period (Nestor et al., 1986). Clearly, broodiness is a source of considerable economic loss for the turkey industry. Although the specific role of prolactin (PRL) in broodiness is not fully known, it has long been associated with the development of behavioral broodiness in domestic chickens (Riddle et al., 1935; Saeki and Tanabe, 1955; Lea et al., 1981) and turkeys (Proudman and Opel, 1981; Etches and Cheng, 1982; Lea and Sharp, 1982; Wentworth et al., 1983). Serum PRL concentrations are elevated in laying hens as compared to non-laying hens and increase gradually over the course of production (Lea and Sharp, 1982; Zadwomy et al., 1985; El Halawani et al., 1988). A 6- to IO-fold increase in circulating PRL is observed with the onset of broodiness (El Halawani et al., 1988). In contrast, luteinizing hormone (LH) concentrations increase rapidly with photostimulation but decline to very low levels during broodiness. Methods for the disruption of broodiness have been summarized by Nestor et al. (1971). The most commonly used technique involves the movement of potentially broody hens to strange surroundings without nests such as broody coops, wire floored pens tcorrespondence should be addressed to D. M. Denbow.
or cages (El Halawani et al., 1988). More recently, a pharmacological approach to the disruption of broodiness has been taken. Parachlorophenylalanine, a serotonin synthesis inhibitor, has been reported to reduce the nesting behavior that is induced by increased PRL in female turkeys (El Halawani et al., 1982). The dopamine receptor blocking agent pimozide reduced daily nest visits and increased egg production of hens showing signs of broodiness (Millam et al., 1980). Pimozide had no significant effect on these traits when routinely administered to non-broody hens. However, pharmacological agents for the control of broodiness are not currently available for commercial use (El Halawani et al., 1988). There have been very few reports of the effect of nutrition on the incidence of broodiness. In two previous studies, the incidence of broodiness was significantly attenuated among turkey hens that were allowed to self select their diet from a feed source relatively high in protein and a feed source relatively high in energy (Emmerson et al., 1990; in press). The objective of the present study was to determine the effect of this self-selection feeding regime on serum PRL and LH concentrations. MATERIALS
AND METHODS
Animals and husbandry Two experiments were conducted to determine the effect of diet self-selection on reproductive performance. Fortyeight first-season hens were used in the initial experiment. In the second experiment, 48 first-season hens and 48 hens which had been force-molted and returned to production were combined for a total of 96 hens. In both experiments, hens were housed in individual cages in a light controlled environment. Birds were randomly assigned to one of two dietary treatments. Half the hens were fed a complete diet, while the remaining birds were allowed to self select their diet from one feed source relatively high in protein and low in energy and a second feed source relatively low in protein 495
D. A. EMMEMONet al.
-G+Control diet 8 Split diet TM b
i
0
t
a
0
4
6
I.4
We& of ;rodub;io”
18
ICI
20
Fig. I. Per cent broodiness of turkey hens fed conventional and split diets during a 20-week production cycle. and high in energy. Further details concerning diet and management procedures have been previously described (Emmerson et al., 1990, in press). Sample colleclion Blood serum samples were coflected on a monthly basis via the bra&al vein, and frozen at -70°C for subsequent analyses. Samples were analysed for serum LW and PRL concentrations by radioimmunoassay using techniques described by Proudman and Opel (1981) and Burke et uf. (1979) for PRL and LH, respectively. Sfalistical a&y& There were no significant differences in serum PRL and LH con~trations in the two experiments so data were combined for statistical anafysis. Based on the profound effect of broodiness on PRL and LH concentrations, broody and non-broody birds were anaiysed separately in order to avoid heterogeneity of treatment variances. Serum PRL and LH concentrations were analysed using two-way analysis of variance with diet and hen age as main effects. Per mnt broody birds was analysed using Chi-square analysis. Significance implies P < 0.05 unless otherwise stated.
120 = E \ 110 -
-)c Control
8
RESlJLTS AND DlSCUSSION
The incidence of broodiness, expressed as the percentage of broody hens within a given week, is displayed in Fig. 1. The incidence of broodiness was approximately four times as high among hens fed a conventional diet as among hens fed the Split-diet. It is impossible to ascertain whether the observed difference in broodiness was the result of the self-selection regime itself or if it was due to a difference in the diet that was selected and consumed. If the reduction in broodiness reilects a greater adequacy of the selected diet in meeting the requirements of breeder hens then it would be possible to formulate a complete diet with the same ability to attenuate broodiness. If this difference is the result of the opportunity for hens to meet their individual requirements more effectively through self-selection it would not be possible to alter broody behavior except through the feeding of a split-diet. However, these data clearly demonstrate the potential to modify broody behavior through dietary treatments.
diet
Split diet
F
2
0
1
Fig, 2. Serum prolactin concentrations
MoLl of production
4
I
of turkey hens fed conventional and split diets during a 20-week production cycle.
Prolactin and LH levels
-
497
Control diet
. . . Split diet
Aa% * First season 0 Second season
20
,
1 1
0
2
Month
s
4
3
of production
Fig. 3. Serum prolactin concentrations of first and second season turkey hens fed conventional and split diets over a 20-week production cycle. Among significantly
non-broody hens, serum PRL was (P < 0.01) greater in hens fed the control
diet during the third month of production than in hens fed the split-diet (Fig. 2). This corresponds with the period when there was the largest increase in the incidence of broodiness. Therefore, the split-diet regime not only reduced the incidence of broodiness but also altered blood PRL concentrations in hens that did not go broody. It has been suggested that a positive feedback exists where PRL stimulates nesting which in turn stimulates further PRL release (El Halawani et al., 1988). Under this hypothesis, multiple factors are responsible for the induction of broodiness. Exogenous administration of estradiol, progesterone and PRL has been reported to induce ovarian regression (Opel and Proudman, 1980) and broody behavior in turkeys (El Halawani et al., 1986). However, there
are many inconsistences in the literature concerning the effectiveness of PRL in inducing broodiness. It has been suggested that several factors including the dosage of PRL administered, the genetic propensity for broodiness, and the accessibility of nesting sites predispose birds to the induction of broody behavior (El Halawani et al., 1988). In the present experiments hens were housed in cages rather than floor pens. Although control birds had elevated serum PRL concentrations, they did not have access to nesting sites. Possibly there would have been an even greater difference in the incidence of broodiness for the two dietary treatments if hens had been housed in floor pens. Although there were not significant treatment difference for serum PRL during any other period of production, self selecting hens did maintain consistently lower serum PRL concentrations throughout the egg production cycle, resulting in a significant
*
Control diet
8 Split diet
0
1
4
Moith
s
of produc;on
Fig. 4. Serum luteinizing hormone concentrations of hens fed conventional and split diets during a 20-week production cycle.
D. A. EMME~S~Net al.
498
-x- First semm 8 Second season
-I
2.4
E 2
cn 2.2
i 2
I
0
I
2
3
4
5
Month of production
Fig. 5. Serum luteinizing hormone concentrations of first and second Season turkey hens.
difference in the average PRL concentrations of the two treatment groups (P < 0.01). There was a significant effect on hen age on serum PRL levels for control but not self selecting hens in the second and fourth months of production resulting in a diet by age interaction for these periods (Fig. 3). Thus, second season hens appeared to be more sensitive to the effect of dietary regime on serum PRL concentrations. This might indicate age as another factor predisposing turkey hens to the induction of broody behavior. As expected, serum LH concentrations mirrored PRL levels. Hens fed the split diet had a significantly higher serum LH concentration than controls in the third month of production (Fig. 4). First season hens generally had higher serum LH concentrations throughout the production period and significantly higher serum levels in the fourth and fifth months of production when the LH concentrations of the first and second season hens diverged (Fig. 5). This observation provides further evidence of a beneficial effect of the self-selection treatment on the reproductive physiology of the turkey hen. There were no significant effects of diet or hen age on either serum PRL or LH among hens that did go broody (data not presented). Thus, diet influenced the incidence of broodiness and blood PRL and LH concentrations of non-broody hens but did not have any effect on these blood measures in broody hens. Traditionally, nutritionists have attempted to formulate diets in a manner to support optimal animal performance. The present study demonstrates the opportunity to alter the physiology and behavior of the bird through nutritional means. Genetic, physiologic and management procedures have been the most thoroughly investigated methods to reduce broodiness. This report indicates the potential of nutritional treatments as an additional approach for the control of broodiness. REFERENCES
Burke W. H., Licht P., Papkoff H. and Ona Gallo A. (1979) Isolation and characterization of luteinizing hormone and
follicle-stimulation hormone from pituitary glands of the turkey (Meleagris gallopavo). Gen. and Comp. Endow. 31, 508-520.
El Halawani M. E., Burke W. H., Dennison W. H. and Silsby J. L. (1982) Neuropharmacological aspects of neural regulation of avian endocrine function. In A&CIS of Avian Enubcrinoloav (Edited bv Scanes C. G.. Ottinaer M. A., Kenny A. L?; Balthaxart-J., Cronshaw J: and J&s I. C.). Texas Tech Press, Lubbock. El Halawani M. E., Silsby J. L., Behnke E. J. and Fehrer S. C. (1986) Hormonal induction of incubation behavior in ovariectomixed female turkeys (Meleagris gallopavo). Biol. Reprod. 36, 59-67.
El Halawani M. E., Fehrer S., Hargic B. M. and Porter T. M. (1988) Incubation behavior in the domestic turkey: physiological correlates. CRC Critical Rev. Poultry Biol. 1, 285-314.
Emmerson D. E., Denbow D. M. and Hulet R. M. (1990) Protein and energy self-selection 1. Reproductive nerformance. Brit. Poult. Sci. 31. 283-292. Emmerson D. E., Denbow D. M., Hulet R. M. and Potter L. Self-selection of dietary protein and energy by turkey breeder hens. Brit. Poult. Sci. (In press).
Etches R. J. and Cheng K. W. (1982) A homologous radioimmunoassay for turkey prolactin: changes during the renroductive and ovulatorv cvcle. PO&. Sci.. 61.1354-1362. Leak. W., Dods A. S., Sharp-P. J. and Chadwick A. (1981) The possible role of prolactin in the regulation of nesting behavior and the secretion of luteinixing hormone in broody bantams. J. Endocr. 91, 88-97. Lea R. W. and Sharp P. J. (1982) Plasma prolactin concentrations in broody turkeys: lack of agreement between homologous chicken and turkey radioimmunoassays. Brit. PO&. Sci. 23, 451-459.
Millam J. R., Burke W. H., El Halawani M. E. and Ogren L. A. (1980) Preventing broodiness in turkey hens with a dopamine receptor blocking agent. Poult. Sci. 59, 1126-1131. Nestor K. E., Bacon W. L. and Renner P. A. (1971) Influence of light intensity and length of light day in the broody nen on broodiness and egg production of turkeys. -- _ Pouk-&ci. 50, 1689-1693. Nestor K. E.. Bacon W. L. and Renner P. A. (1986) The influence of time of application of broody hen treatments on egg production of turkeys. Poulr. Sk 65, 1405-1409. One1 H. and Proudman J. A. (1980) Failure of mammalian brolactin to induce incubation behavior in chickens and turkeys. Poult. Sci. 59, 2550-2558.
Prolactin and LH levels Proudman J. A. and Opel H. (1981) Turkey prolactin: validation of radioimmunoassay and measurement of changes associated with broodiness. Biol. Reprod. 25, 573-580. Riddle O., Bates R. W. and Lahr E. L. (1935) Prolactin induced broodiness in fowl. Am. J. Physiol. 111,352-360. Saeki Y. and Tanabe Y. (1955) Changes in prolactin content of fowl pituitary during broody periods and some experiments in the induction of broodiness. Pot&. Sci. 34, 909-919.
499
Wentworth B. C., Proudman J. A., Opel H., Wineland M. J., Zimmerman, N. G. and Lapp A. (1983) Endocrine changes in the incubating and brooding turkey hen. Biol. Reprod. 29, 81-92.
Zadworny D., Walton J. S. and Etches R. J. (1985) The relationship between plasma concentrations of prolactin and consumption of feed and water during the reproductive cycle of the domestic turkey. Poulr. Sci. 64, 401-410.
0300-9629/91 $3.00 + 0.00 0 1991 Perpmon Press plc
1991
PROTEIN AND ENERGY SELF-SELECTION OF TURKEY HENS. SERUM PROLACTIN AND LUTEINIZING HORMONE CONCENTRATIONS DENBOW,*? H. P. VAN KREY,* R. M. HULET* and M. E. EL HALAWANI~
D. A. EMMERSON,*D. M.
*Department of Poultry Science, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0332, U.S.A. Telephone: (703) 231-6843; and IDepartment of Animal Science, University of Minnesota, St. Paul, MN 55108, U.S.A. (Received 5 February 1991) Abstract-l. The incidence of broodiness was four times as high among turkey hens fed a complete control diet than among hens allowed to self-select their diet from two different feed sources, one being relatively high in protein and the other relatively high in energy (i.e. split-diet). 2. Among non-broody birds, hens fed the split-diet had a significantly lower serum prolactin concentration in the third month of production as compared to control hens. 3. Hens in their second season of egg production had significantly lower serum luteinixing hormone concentrations during the latter stages of egg production than did first season hens.
INTRODUCTION
Broodiness remains a major problem which limits the production of turkey hatching eggs (Etches and Cheng, 1982). In addition to the loss of potential hatching eggs, expenses associated with the treatment of broody hens and with the maintenance of non-egg producing hens are incurred (Nestor et al., 1986). It has been estimated that the loss of one egg per hen costs the turkey industry $1.8 million per year in the U.S. alone (El Halawani et al., 1988). An effective broody hen treatment program can result in an additional 13 eggs per hen over a 180 day production period (Nestor et al., 1986). Clearly, broodiness is a source of considerable economic loss for the turkey industry. Although the specific role of prolactin (PRL) in broodiness is not fully known, it has long been associated with the development of behavioral broodiness in domestic chickens (Riddle et al., 1935; Saeki and Tanabe, 1955; Lea et al., 1981) and turkeys (Proudman and Opel, 1981; Etches and Cheng, 1982; Lea and Sharp, 1982; Wentworth et al., 1983). Serum PRL concentrations are elevated in laying hens as compared to non-laying hens and increase gradually over the course of production (Lea and Sharp, 1982; Zadwomy et al., 1985; El Halawani et al., 1988). A 6- to IO-fold increase in circulating PRL is observed with the onset of broodiness (El Halawani et al., 1988). In contrast, luteinizing hormone (LH) concentrations increase rapidly with photostimulation but decline to very low levels during broodiness. Methods for the disruption of broodiness have been summarized by Nestor et al. (1971). The most commonly used technique involves the movement of potentially broody hens to strange surroundings without nests such as broody coops, wire floored pens tcorrespondence should be addressed to D. M. Denbow.
or cages (El Halawani et al., 1988). More recently, a pharmacological approach to the disruption of broodiness has been taken. Parachlorophenylalanine, a serotonin synthesis inhibitor, has been reported to reduce the nesting behavior that is induced by increased PRL in female turkeys (El Halawani et al., 1982). The dopamine receptor blocking agent pimozide reduced daily nest visits and increased egg production of hens showing signs of broodiness (Millam et al., 1980). Pimozide had no significant effect on these traits when routinely administered to non-broody hens. However, pharmacological agents for the control of broodiness are not currently available for commercial use (El Halawani et al., 1988). There have been very few reports of the effect of nutrition on the incidence of broodiness. In two previous studies, the incidence of broodiness was significantly attenuated among turkey hens that were allowed to self select their diet from a feed source relatively high in protein and a feed source relatively high in energy (Emmerson et al., 1990; in press). The objective of the present study was to determine the effect of this self-selection feeding regime on serum PRL and LH concentrations. MATERIALS
AND METHODS
Animals and husbandry Two experiments were conducted to determine the effect of diet self-selection on reproductive performance. Fortyeight first-season hens were used in the initial experiment. In the second experiment, 48 first-season hens and 48 hens which had been force-molted and returned to production were combined for a total of 96 hens. In both experiments, hens were housed in individual cages in a light controlled environment. Birds were randomly assigned to one of two dietary treatments. Half the hens were fed a complete diet, while the remaining birds were allowed to self select their diet from one feed source relatively high in protein and low in energy and a second feed source relatively low in protein 495
D. A. EMMEMONet al.
-G+Control diet 8 Split diet TM b
i
0
t
a
0
4
6
I.4
We& of ;rodub;io”
18
ICI
20
Fig. I. Per cent broodiness of turkey hens fed conventional and split diets during a 20-week production cycle. and high in energy. Further details concerning diet and management procedures have been previously described (Emmerson et al., 1990, in press). Sample colleclion Blood serum samples were coflected on a monthly basis via the bra&al vein, and frozen at -70°C for subsequent analyses. Samples were analysed for serum LW and PRL concentrations by radioimmunoassay using techniques described by Proudman and Opel (1981) and Burke et uf. (1979) for PRL and LH, respectively. Sfalistical a&y& There were no significant differences in serum PRL and LH con~trations in the two experiments so data were combined for statistical anafysis. Based on the profound effect of broodiness on PRL and LH concentrations, broody and non-broody birds were anaiysed separately in order to avoid heterogeneity of treatment variances. Serum PRL and LH concentrations were analysed using two-way analysis of variance with diet and hen age as main effects. Per mnt broody birds was analysed using Chi-square analysis. Significance implies P < 0.05 unless otherwise stated.
120 = E \ 110 -
-)c Control
8
RESlJLTS AND DlSCUSSION
The incidence of broodiness, expressed as the percentage of broody hens within a given week, is displayed in Fig. 1. The incidence of broodiness was approximately four times as high among hens fed a conventional diet as among hens fed the Split-diet. It is impossible to ascertain whether the observed difference in broodiness was the result of the self-selection regime itself or if it was due to a difference in the diet that was selected and consumed. If the reduction in broodiness reilects a greater adequacy of the selected diet in meeting the requirements of breeder hens then it would be possible to formulate a complete diet with the same ability to attenuate broodiness. If this difference is the result of the opportunity for hens to meet their individual requirements more effectively through self-selection it would not be possible to alter broody behavior except through the feeding of a split-diet. However, these data clearly demonstrate the potential to modify broody behavior through dietary treatments.
diet
Split diet
F
2
0
1
Fig, 2. Serum prolactin concentrations
MoLl of production
4
I
of turkey hens fed conventional and split diets during a 20-week production cycle.
Prolactin and LH levels
-
497
Control diet
. . . Split diet
Aa% * First season 0 Second season
20
,
1 1
0
2
Month
s
4
3
of production
Fig. 3. Serum prolactin concentrations of first and second season turkey hens fed conventional and split diets over a 20-week production cycle. Among significantly
non-broody hens, serum PRL was (P < 0.01) greater in hens fed the control
diet during the third month of production than in hens fed the split-diet (Fig. 2). This corresponds with the period when there was the largest increase in the incidence of broodiness. Therefore, the split-diet regime not only reduced the incidence of broodiness but also altered blood PRL concentrations in hens that did not go broody. It has been suggested that a positive feedback exists where PRL stimulates nesting which in turn stimulates further PRL release (El Halawani et al., 1988). Under this hypothesis, multiple factors are responsible for the induction of broodiness. Exogenous administration of estradiol, progesterone and PRL has been reported to induce ovarian regression (Opel and Proudman, 1980) and broody behavior in turkeys (El Halawani et al., 1986). However, there
are many inconsistences in the literature concerning the effectiveness of PRL in inducing broodiness. It has been suggested that several factors including the dosage of PRL administered, the genetic propensity for broodiness, and the accessibility of nesting sites predispose birds to the induction of broody behavior (El Halawani et al., 1988). In the present experiments hens were housed in cages rather than floor pens. Although control birds had elevated serum PRL concentrations, they did not have access to nesting sites. Possibly there would have been an even greater difference in the incidence of broodiness for the two dietary treatments if hens had been housed in floor pens. Although there were not significant treatment difference for serum PRL during any other period of production, self selecting hens did maintain consistently lower serum PRL concentrations throughout the egg production cycle, resulting in a significant
*
Control diet
8 Split diet
0
1
4
Moith
s
of produc;on
Fig. 4. Serum luteinizing hormone concentrations of hens fed conventional and split diets during a 20-week production cycle.
D. A. EMME~S~Net al.
498
-x- First semm 8 Second season
-I
2.4
E 2
cn 2.2
i 2
I
0
I
2
3
4
5
Month of production
Fig. 5. Serum luteinizing hormone concentrations of first and second Season turkey hens.
difference in the average PRL concentrations of the two treatment groups (P < 0.01). There was a significant effect on hen age on serum PRL levels for control but not self selecting hens in the second and fourth months of production resulting in a diet by age interaction for these periods (Fig. 3). Thus, second season hens appeared to be more sensitive to the effect of dietary regime on serum PRL concentrations. This might indicate age as another factor predisposing turkey hens to the induction of broody behavior. As expected, serum LH concentrations mirrored PRL levels. Hens fed the split diet had a significantly higher serum LH concentration than controls in the third month of production (Fig. 4). First season hens generally had higher serum LH concentrations throughout the production period and significantly higher serum levels in the fourth and fifth months of production when the LH concentrations of the first and second season hens diverged (Fig. 5). This observation provides further evidence of a beneficial effect of the self-selection treatment on the reproductive physiology of the turkey hen. There were no significant effects of diet or hen age on either serum PRL or LH among hens that did go broody (data not presented). Thus, diet influenced the incidence of broodiness and blood PRL and LH concentrations of non-broody hens but did not have any effect on these blood measures in broody hens. Traditionally, nutritionists have attempted to formulate diets in a manner to support optimal animal performance. The present study demonstrates the opportunity to alter the physiology and behavior of the bird through nutritional means. Genetic, physiologic and management procedures have been the most thoroughly investigated methods to reduce broodiness. This report indicates the potential of nutritional treatments as an additional approach for the control of broodiness. REFERENCES
Burke W. H., Licht P., Papkoff H. and Ona Gallo A. (1979) Isolation and characterization of luteinizing hormone and
follicle-stimulation hormone from pituitary glands of the turkey (Meleagris gallopavo). Gen. and Comp. Endow. 31, 508-520.
El Halawani M. E., Burke W. H., Dennison W. H. and Silsby J. L. (1982) Neuropharmacological aspects of neural regulation of avian endocrine function. In A&CIS of Avian Enubcrinoloav (Edited bv Scanes C. G.. Ottinaer M. A., Kenny A. L?; Balthaxart-J., Cronshaw J: and J&s I. C.). Texas Tech Press, Lubbock. El Halawani M. E., Silsby J. L., Behnke E. J. and Fehrer S. C. (1986) Hormonal induction of incubation behavior in ovariectomixed female turkeys (Meleagris gallopavo). Biol. Reprod. 36, 59-67.
El Halawani M. E., Fehrer S., Hargic B. M. and Porter T. M. (1988) Incubation behavior in the domestic turkey: physiological correlates. CRC Critical Rev. Poultry Biol. 1, 285-314.
Emmerson D. E., Denbow D. M. and Hulet R. M. (1990) Protein and energy self-selection 1. Reproductive nerformance. Brit. Poult. Sci. 31. 283-292. Emmerson D. E., Denbow D. M., Hulet R. M. and Potter L. Self-selection of dietary protein and energy by turkey breeder hens. Brit. Poult. Sci. (In press).
Etches R. J. and Cheng K. W. (1982) A homologous radioimmunoassay for turkey prolactin: changes during the renroductive and ovulatorv cvcle. PO&. Sci.. 61.1354-1362. Leak. W., Dods A. S., Sharp-P. J. and Chadwick A. (1981) The possible role of prolactin in the regulation of nesting behavior and the secretion of luteinixing hormone in broody bantams. J. Endocr. 91, 88-97. Lea R. W. and Sharp P. J. (1982) Plasma prolactin concentrations in broody turkeys: lack of agreement between homologous chicken and turkey radioimmunoassays. Brit. PO&. Sci. 23, 451-459.
Millam J. R., Burke W. H., El Halawani M. E. and Ogren L. A. (1980) Preventing broodiness in turkey hens with a dopamine receptor blocking agent. Poult. Sci. 59, 1126-1131. Nestor K. E., Bacon W. L. and Renner P. A. (1971) Influence of light intensity and length of light day in the broody nen on broodiness and egg production of turkeys. -- _ Pouk-&ci. 50, 1689-1693. Nestor K. E.. Bacon W. L. and Renner P. A. (1986) The influence of time of application of broody hen treatments on egg production of turkeys. Poulr. Sk 65, 1405-1409. One1 H. and Proudman J. A. (1980) Failure of mammalian brolactin to induce incubation behavior in chickens and turkeys. Poult. Sci. 59, 2550-2558.
Prolactin and LH levels Proudman J. A. and Opel H. (1981) Turkey prolactin: validation of radioimmunoassay and measurement of changes associated with broodiness. Biol. Reprod. 25, 573-580. Riddle O., Bates R. W. and Lahr E. L. (1935) Prolactin induced broodiness in fowl. Am. J. Physiol. 111,352-360. Saeki Y. and Tanabe Y. (1955) Changes in prolactin content of fowl pituitary during broody periods and some experiments in the induction of broodiness. Pot&. Sci. 34, 909-919.
499
Wentworth B. C., Proudman J. A., Opel H., Wineland M. J., Zimmerman, N. G. and Lapp A. (1983) Endocrine changes in the incubating and brooding turkey hen. Biol. Reprod. 29, 81-92.
Zadworny D., Walton J. S. and Etches R. J. (1985) The relationship between plasma concentrations of prolactin and consumption of feed and water during the reproductive cycle of the domestic turkey. Poulr. Sci. 64, 401-410.