Effect of Photoperiod in the Third Trimester of Gestation on Milk Production and Circulating Hormones in Dairy Goats

Abstract

Multiparous Israeli Saanen goats (n = 8) were blocked at dry off (approximately 45 d prepartum) into 2 treatments of 4 goats each based on body weight (BW), previous milk production, and the number of detected embryos in utero. Treatments consisted of long-day (16 h light:8 h dark) and short-day (8 h light:16 h dark) photoperiods at normothermic ambient temperature (22 degrees C, 72% relative humidity). All goats were returned to ambient photoperiod after kidding, milked twice daily, and milk yield was automatically recorded. Dry matter intake was similar between treatments and averaged 980 g/d. Milk production was greater in the short-day than in the long-day treatment (2,932 vs. 2,320 g/d) during the 12-wk experimental period. Milk protein and lactose contents were similar in both treatments and averaged 3.61 and 4.88%, respectively, whereas milk fat was greater in the long-day treatment (4.80 vs. 4.22%). Plasma insulin-like growth factor 1 was greater in the long-day treatment (149 vs. 73 ng/mL) during the dry period than in the short-day treatment, but was similar postkidding, averaging 76 ng/mL. Concentrations of triiodothyronine in plasma were similar in both treatments during the dry period, but greater during lactation in the short-day treatment (122.1 vs. 94.1 ng/mL). Plasma prolactin was greater in the long-day than in the short-day treatment during the dry period (28.0 vs. 17.5 ng/mL), whereas it was similar throughout lactation (11.7 ng/mL). These data support the idea that greater milk production in goats exposed to short days during the dry period is not explained by differences in feed intake or increased secretion of insulin-like growth factor 1.

Full text

J. Dairy Sci. 90:699–705
©American Dairy Science Association, 2007.
Effect of Photoperiod in the Third Trimester of Gestation on Milk
Production and Circulating Hormones in Dairy Goats
S. J. Mabjeesh,*
1
O. Gal-Garber,* and A. Shamay†
*Department of Animal Sciences, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem,
P.O. Box 12, Rehovot 76100, Israel
†Agricultural Research Organization, Volcani Center, Institute of Animal Science, Bet Dagan 50250, Israel
ABSTRACT
Multiparous Israeli Saanen goats (n = 8) were blocked
at dry off (approximately 45 d prepartum) into 2 treat-
ments of 4 goats each based on body weight (BW), previ-
ous milk production, and the number of detected em-
bryos in utero. Treatments consisted of long-day (16
h light:8 h dark) and short-day (8 h light:16 h dark)
photoperiods at normothermic ambient temperature
(22°C, 72% relative humidity). All goats were returned
to ambient photoperiod after kidding, milked twice
daily, and milk yield was automatically recorded. Dry
matter intake was similar between treatments and av-
eraged 980 g/d. Milk production was greater in the
short-day than in the long-day treatment (2,932 vs.
2,320 g/d) during the 12-wk experimental period. Milk
protein and lactose contents were similar in both treat-
ments and averaged 3.61 and 4.88%, respectively,
whereas milk fat was greater in the long-day treatment
(4.80 vs. 4.22%). Plasma insulin-like growth factor 1
was greater in the long-day treatment (149 vs. 73 ng/
mL) during the dry period than in the short-day treat-
ment, but was similar postkidding, averaging 76 ng/
mL. Concentrations of triiodothyronine in plasma were
similar in both treatments during the dry period, but
greater during lactation in the short-day treatment
(122.1 vs. 94.1 ng/mL). Plasma prolactin was greater
in the long-day than in the short-day treatment during
the dry period (28.0 vs. 17.5 ng/mL), whereas it was
similar throughout lactation (11.7 ng/mL). These data
support the idea that greater milk production in goats
exposed to short days during the dry period is not ex-
plained by differences in feed intake or increased secre-
tion of insulin-like growth factor 1.
Key words: photoperiod, prolactin, dairy goat, dry
period
INTRODUCTION
Caprine milk production is gaining in importance
worldwide. Dairy goat farming for milk and cheese is
Received February 13, 2006.
Accepted August 23, 2006.
1
Corresponding author: Mabjeesh@agri.huji.ac.il
699
increasing in Israel and the Mediterranean region. In
contrast to dairy cows, goats have seasonal reproduc-
tive cycles, but little information is available with re-
gard to the effects of photoperiod on milk production
and quality.
In the dairy cow industry, use of photoperiod manage-
ment is of great interest because it is a safe, noninva-
sive, and effective method of increasing milk production
that can be used throughout the lactation cycle. Dairy
cows exposed to a short-day photoperiod (SDPP) during
the dry period produce more milk than cows exposed
to long days. Long-day photoperiod (LDPP; 16 h light:8
h dark) during lactation increases daily milk yield by
an average of 2 kg/cow compared with cows held under
natural photoperiod (Dahl et al., 2000). Milk composi-
tion is generally unaffected by photoperiod, although
some studies have indicated a slight depression in milk
fat content when dairy cows are exposed to LDPP (Dahl
et al., 1997; Miller et al., 1999). Dry matter intake usu-
ally increases when dairy cattle are exposed to LDPP
to support the demands of the mammary gland, which
increases in response to this photoperiodic regimen
(Dahl et al., 2000).
Prolactin (PRL) emerged as the initial candidate re-
sponsible for the galactopoietic effects of photoperiod.
Indeed, long days increase circulating concentrations
of PRL in a number of species including cattle (Peters
and Tucker, 1978; Tucker et al., 1984). Prolactin has a
galactopoietic role in dairy cows similar to a number of
species, and is believed to be a mediator of the photope-
riodic effect (Dahl et al., 2000). This effect occurs in
dairy cows only during the periparturient period. Insu-
lin-like growth factor 1 is another hormone that shows
fluctuating concentrations in the circulation in re-
sponse to photoperiodic treatment, independent of
growth hormone, and could explain the effects of long
days (Dahl et al., 2000). Long days also increase mam-
mary parenchymal growth relative to short days (Pet-
itclerc et al., 1984), and IGF-I increases bovine mam-
mary growth in vitro (Shamay et al., 1988; McGrath et
al., 1991). In contrast, SDPP inhibits the effect stimu-
lated by LDPP, which could be related to IGF-I in cattle
(Dahl et al., 2000).

MABJEESH ET AL.700
Treating dairy cows during the dry period (i.e., the
third trimester) with LDPP increases PRL concentra-
tion, whereas SDPP (8 h light:16 h dark) decreases it
(Linzell, 1973; Auchtung et al., 2005). Because a robust
preparturient PRL surge is important to complete lacto-
genesis at calving (Tucker, 2000), LDPP was expected
to increase milk production. Results from the latter
study, however, revealed that SDPP increases milk pro-
duction in the subsequent lactation by much as 3.2
kg/d compared with cows held under LDPP when dry
(Linzell, 1973). Thus, appropriate manipulation of pho-
toperiod can dramatically improve the performance of
dairy cows.
Similar to dairy cattle, a greater milk yield response
to LDPP has been reported in ewes (Bocquier, 1985)
and goats (Linzell, 1973; Terqui et al., 1984). In fact,
the response was larger than that usually observed in
cattle (+20%). Little work, however, has focused on the
influence of environmental modulation during the dry
period, and its potential to affect subsequent milk yield
in dairy goats. Recently, it was reported that an SDPP
during the dry period is stimulatory to milk yield during
the subsequent lactation, and that the effect persists
throughout lactation (Aharoni et al., 2000). To the best
of our knowledge, however, only limited data are avail-
able regarding the influence of LDPP during an estab-
lished lactation, and none on the effect of SDPP during
the dry period. The purpose of this study was therefore
to examine the effect of a photoperiod treatment during
the dry period on subsequent milk production and
plasma hormonal profiles in dairy goats.
MATERIALS AND METHODS
Cows and Treatments
Multiparous Israeli Saanen goats (n = 8) in mid to
late lactation were synchronized and inseminated to
ensure their pregnancy. These goats were blocked at
dry off (∼45 d prepartum) into 2 treatments of 4 goats
each based on BW, previous milk production, and the
number of detected embryos in utero. Treatments were
LDPP (16 h light:8 h dark) and SDPP (8 h light:16 h
dark) at normothermic ambient temperature (22°C,
72% relative humidity). All goats were maintained un-
der ambient photoperiod until the onset of treatment.
During the experimental period, goats were housed in
metabolism cages equipped with automatic feeders in
2 separate but identical environmentally controlled
rooms (photophase light intensity = 350 lx at eye level
of the goats) in which photoperiod was adjusted ac-
cording to the treatment. Goats were fed a diet that
adequately met their nutritional demands for mainte-
nance and gestation stage. Feed was offered in 12 equal
meals by automatic feeder. The diet consisted of com-
Journal of Dairy Science Vol. 90 No. 2, 2007
mercial pellets containing 16% CP, 29.1% NDF, and
1.6 Mcal of NE
L
on a DM basis (Mixture 1471, Matmor
Ltd., Ashdod, Israel), and chopped clover and vetch hay
with 14.9% CP, 47.5% NDF, and 1.1 Mcal of NE
L
. The
hay supplied 60% of the daily DMI.
Dry matter intake was monitored while the goats
were in the metabolism cages, and was adjusted to allow
10% refusals. Apparent digestibility of DM, OM, and
CP was measured during a 5-d collection period after
4 wk in the metabolic rooms. At kidding, all goats were
returned to ambient photoperiodic conditions and a nor-
mal management regimen. Goats were milked twice
daily at 0700 and 1900 h, and daily milk production was
recorded by automatic milk meter (Free-flow, S.E.R.,
Netanya, Israel). Pellets were offered in the milking
parlor ad libitum and the hay was then offered in the
yards to supply 35% of the total daily DMI.
Blood samples were collected on several occasions
relative to kidding date (i.e., every 7 d from −45 to +45
d). Blood pH and Na concentration were measured in
fresh samples by the ABL70 system (Radiometer, Co-
penhagen, Denmark). Plasma was then separated and
stored at −20°C for later analysis of PRL, IGF-I, and
triiodothyronine (T3). Milk composition was quantified
weekly by a near infrared procedure (Milkoscan 605;
Foss Electric, Hillerød, Denmark). Postkidding data
were collected for 120 d postpartum, which was deemed
sufficient time to detect any treatment differences.
Hormone Analyses
Blood samples (10 mL) were collected from each goat
between 0830 and 1030 h via jugular venipuncture into
a sterile evacuated tube containing sodium heparin (Va-
cutainer, Becton Dickinson and Co., Franklin Lakes,
NJ), and immediately placed on ice. Plasma was har-
vested by centrifugation at 1,850 ×gfor 20 min at 4°C
and stored at −20°C until use.
The IGF-I RIA was performed as previously described
(Plaut et al., 1991). Briefly, 250 ␮L of plasma was ex-
tracted with 300 ␮L of glycyl-glycine HCl (0.1 M,pH
2) at 37°C for 24 h. To 275 ␮L of assay buffer (0.15 M
sodium phosphate, 0.02% protamine sulfate, 0.2% BSA,
0.02 sodium azide, pH 7.5) were added 25 ␮Lofthe
extracted plasma and 100 ␮L of anti-hIGF-I (NIH,
Bethesda, MD). After 1 h at room temperature, 100 ␮L
of
125
IGF-I (20,000 cpm) was added and the tubes were
incubated for 24 h at 4°C. Then, 100 ␮L of a second
antibody, goat antirabbit IgG in PBS containing 0.2%
normal rabbit serum was added and incubation contin-
ued for another 24 h at 4°C. The hormone-antibody
complex was precipitated by the addition of 1 mL of
12% polyethylene glycol 6000 and centrifugation for 30
min at 4°C at 3,000 ×g. The supernatant was decanted

SHORT DAYS INCREASE MILK PRODUCTION IN GOATS 701
and the assay precipitates were counted in a Kontron
gamma counter with a log-logit program. Intra- and
interassay coefficients of variation were 5.7 and 8.7%,
respectively.
Concentrations of T3 were determined by using a RIA
kit from Diagnostic Products Corporation (Los Angeles,
CA) according to the manufacturer’s protocol. Intra-
and interassay coefficients of variation were 5.8 and
9.2%, respectively.
The PRL RIA was performed as previously described
(Miller et al., 1999). The assay was carried out with a
primary antibody to PRL (AFPC35810691R; provided
by A. F. Parlow, National Hormone and Peptide Pro-
gram, Torrance, CA) diluted 1:50,000 in working solu-
tion and then to a final tube dilution of 1:200,000. In-
traassay coefficient of variation was 6.1% and in-
terassay coefficient of variation averaged 15.5%. Assay
sensitivity averaged 0.26 ng/mL.
Statistical Analyses
Statistical analysis was performed using the mixed
models procedures of SAS (Version 8.2, SAS Inst. Inc.,
Cary, NC). The model included treatment (LDPP vs.
SDPP) and time as fixed variables, goat as the random
variable, and their interaction. All dependent variables
were included as repeated measures. No interaction
effects were detected, so the model was reduced to in-
clude the main effects and the error term. Comparisons
of means between treatments at various times (weeks of
lactation) were performed using Student’s t-test. Means
are presented as least squares means.
RESULTS
Goat Performance
All goats were healthy throughout the experimental
period and all of the kids were healthy. Dry matter,
OM, and CP intakes and digestibilities did not differ
between treatments, averaging 980, 941, and 138 g/d
and 64, 69, and 60%, respectively (Table 1).
Milk yield, monitored for 12 wk in lactation, was
greater in goats exposed to SDPP than in goats exposed
to LDPP (Figure 1). Milk yields began to differ during
the second week of lactation, being greater (P<0.001)
in the SDPP treatment. Averaged over the monitored
lactation period, milk yield in the SDPP goats was 612
g/d greater than in the LDPP goats (Table 2). Milk fat
concentration was affected by the treatments. It was
greater in the LDPP than in the SDPP goats (Table 2).
Milk protein concentration, on the other hand, did not
differ between treatments and averaged 3.61%. Milk,
milk fat, and milk protein concentrations decreased
with time. Milk lactose was similar for both treatments,
Journal of Dairy Science Vol. 90 No. 2, 2007
Table 1. Dry matter, OM, CP intake, and diet digestibilities during
the dry period until parturition of dairy goats exposed to long-day
photoperiod (LDPP; 16L:8D) or short-day photoperiod (SDPP;
8D:16L)
Photoperiod
LDPP SDPP SEM P-value
Intake, g/d
DM 935 1,024 204 0.767
OM 902 980 166 0.614
CP 135 141 25.6 0.688
Digestibility, %
DM 66.6 61.4 3.37 0.098
OM 70.3 67.3 2.94 0.496
CP 62.2 57.8 2.61 0.376
averaging 4.88%, and was not influenced by time after
kidding (Table 2).
Plasma Hormones, pH, and Na
Plasma hormones were measured from 7 wk before
to 8 wk after kidding. Plasma T3 concentrations were
similar between treatments at wk 7 prepartum, averag-
ing 94.3 ng/mL (Figure 2). At kidding and up to 8 wk
thereafter, plasma T3 was greater in the SDPP goats,
averaging 122.1 ng/mL vs. 94.1 ng/mL in the LDPP
goats.
Plasma IGF-I was greater in the LDPP treatment
than in the SDPP treatment from wk −7 to kidding,
but did not differ at wk −4 and −6 (Figure 3). Concentra-
tion of IGF-I averaged 149 and 73 ng/mL for LDPP and
SDPP treatments, respectively, before parturition, and
did not differ between treatments during wk 1 to 8
postpartum, averaging 76 ng/mL.
Concentration of PRL in plasma was greater in the
LDPP animals than in their SDPP counterparts. Differ-
Figure 1. Milk yield of goats exposed to long-day photoperiod
(LDPP) or short-day photoperiod (SDPP) during the third trimester
of gestation. Average milk production differed (P<0.001) for LDPP
(2,320 g/d) and SDPP (2,932 g/d) treatments (SEM = 158); weeks of
lactation also differed (P<0.001).

MABJEESH ET AL.702
Table 2. Postpartum yield and composition of milk of dairy goats previously exposed to long-day photoperiod
(LDPP) or short-day photoperiod (SDPP) during the third trimester of gestation
Photoperiod (PP) P-value
LDPP SDPP SEM PP Week
1
Milk, g/d 2,320 2,932 158 0.001 0.001
Milk composition, %
Fat 4.80 4.22 0.29 0.001 0.001
Protein 3.56 3.65 0.21 0.261 0.001
Lactose 4.91 4.85 0.08 0.036 0.31
1
Week = Week of lactation.
ences differed (P<0.01) from −5 to +1 wk relative to
kidding (Figure 4). Both treatments exhibited a PRL
surge at kidding, averaging 28 and 17.5 ng/mL for the
LDPP and SDPP treatments, respectively. After kid-
ding, during the second week of lactation, PRL concen-
tration dropped to 11.7 ng/mL in both treatments.
Blood pH was similar between treatments and aver-
aged 7.47 from wk −8 until parturition, and then de-
creased (P<0.001) to 7.39. Although this decrease in
pH was significant, it did not reach or cause acidosis
(Figure 5A). Concentration of Na in plasma did not
differ between treatments (Figure 5B). It averaged 141
mMbefore kidding, and then decreased (P<0.001) to
137 mMduring wk 2; from wk 4 to 7, Na averaged
138 mM.
DISCUSSION
The purpose of this study was to test the hypothesis
that exposing dairy goats to SDPP during the dry period
would affect subsequent lactational performance, as ob-
Figure 2. Plasma triiodothyronine (T3) concentration in goats
exposed to long-day photoperiod (LDPP) or short-day photoperiod
(SDPP) during the third trimester of gestation (SEM = 9.51). Plasma
T3 differed (P<0.014) between photoperiod treatments and among
weeks of lactation (P<0.062). *Indicates differences between photope-
riod treatments.
Journal of Dairy Science Vol. 90 No. 2, 2007
served in dairy cows. Indeed, our results mirror those
from dairy cows (Miller et al., 2000) in that LDPP dur-
ing the dry period did not increase milk production in
the following lactation compared with SDPP. In con-
trast, we found that exposure to SDPP during the dry
period increased subsequent milk production, perhaps
because hormonal changes influenced the mammary
gland, rather than a general effect on feed intake.
Because DMI and nutrient digestibility during the
dry period did not differ between treatments, the effect
of photoperiod is likely mediated hormonally. In fact,
exposing dairy cows to LDPP during lactation increases
milk yield (Dahl et al., 1997), which supports the con-
cept that long days are galactopoietic in cattle. Effects
of long days on DMI in lactating cows are not always
observed, but, generally in long-term studies, DMI in-
creases to meet the increased demand for energy output
from the mammary gland (Dahl et al., 2000). Similar
photoperiodic effects have been reported for milk pro-
Figure 3. Plasma IGF-I concentration in goats exposed to long-
day photoperiod (LDPP) or short-day photoperiod (SDPP) during the
third trimester of gestation. Average plasma IGF-I concentration was
0.110 and 0.082 ng/mL in LDPP and SDPP treatments, respectively
(SEM = 0.22). Treatments differed (P<0.01) and weeks of lactation
differed (P<0.001). *Indicates differences between photoperiod
treatments.

SHORT DAYS INCREASE MILK PRODUCTION IN GOATS 703
Figure 4. Plasma prolactin concentration of goats exposed to long-
day photoperiod (LDPP) or short-day photoperiod (SDPP) during the
third trimester of gestation. Average plasma prolactin concentration
was 15.6 and 11.8 ng/mL in LDPP and SDPP treatments, respectively
(SEM = 2.27). Treatments differed (P<0.01), but weeks of lactation
did not differ. *Indicates differences between photoperiod treatments.
duction in ewes (Gootwine and Pollott, 2000) and goats
(Linzell, 1973; Terqui et al., 1984). Milk yield in the
present study, however, was 26% greater in the SDPP
goats than in their LDPP counterparts.
Milk fat in the present study was affected by treat-
ment, being greater (13.7%) in LDPP than in SDPP
Figure 5. A) Blood pH in goats exposed to long-day photoperiod
(LDPP) or short-day photoperiod (SDPP) during the third trimester
of gestation differed (P<0.0001) between treatments (SEM = 0.02).
B) Blood Na concentration in goats exposed to LDPP or SDPP during
the third trimester of gestation did not differ between treatments
(SEM = 1.48), but differed (P<0.001) among weeks of lactation.
Journal of Dairy Science Vol. 90 No. 2, 2007
treatments, whereas milk protein and lactose were not
affected. Milk composition is generally unaffected by
photoperiod, although results of some studies indicate
that a slight depression in milk fat percentage may
occur during exposure to long days (Stanisiewski et al.,
1985). Although the mechanism governing this phe-
nomenon was not determined in the present study, the
increased milk production, which caused apparent
greater DMI, especially the concentrate part of the diet,
might explain the reduced fat in the SDPP treatment.
Exposing the goats to different photoperiodic regi-
mens (i.e., LDPP vs. SDPP) resulted in changes in hor-
monal profiles. Hormones that were measured in the
current study (T3, IGF-I, and PRL) were relevant to,
and could explain, the differences in subsequent milk
production. Concentration of T3 was greater in the
SDPP treatment throughout lactation likely as a conse-
quence of the greater metabolic demand. This profile
(i.e., greater plasma T3 concentration postpartum in
the SDPP treatment) parallels that of high-producing
dairy cows, which have enhanced metabolic priority and
nutrient partitioning to the mammary gland (Tucker,
2000). Similarly, this change has been observed in dairy
cows that experience differences in postpartum meta-
bolic adaptations upon onset of lactation (Reist et al.,
2003). Moreover, plasma T3 concentration is lower in
dairy cows that have reduced energy intake (Ronge et
al., 1988). In the prepartum period, however, no differ-
ence in the metabolic demands occurred, as shown by
equal DMI and T3 concentrations in plasma of goats
in both treatments, Hence, T3 was not affected by pho-
toperiod during the dry period.
Plasma IGF-I concentration was affected by photope-
riod and was greater in goats exposed to LDPP com-
pared with SDPP during the dry period. Nonetheless,
IGF-I concentration was similar after kidding and dur-
ing lactation in both treatments. There is strong evi-
dence to support the hypothesis that long days stimu-
late IGF-I secretion in ruminants, providing a possible
endocrine mechanism by which photoperiod effects on
growth might be explained (Dahl et al., 2000). Photope-
riod effects on growth have been shown in prepubertal
heifers exposed to LDPP for 4 mo (Spicer et al., 1994).
Those heifers consistently had greater circulating IGF-
I concentrations relative to those held under SDPP.
Long days also increase mammary parenchymal growth
relative to short days (Petitclerc et al., 1984). Concen-
trations of IGF-I further increase bovine mammary
growth in vitro (Shamay et al., 1988; McGrath et al.,
1991). Furthermore, IGF-I increased concomitantly
with PRL when dairy cows were exposed to LDPP com-
pared with SDPP during an established lactation (Dahl
et al., 1997). It has also been reported that LDPP in-
creases secretion of IGF-I rather than by altering clear-

MABJEESH ET AL.704
ance or via growth hormone-induced IGF-I stimulation.
Thus, the galactopoietic effect of LDPP during an estab-
lished lactation might be driven by the increased con-
centration of IGF-I in plasma (Dahl et al., 1997). In
contrast, exposing dairy cows to LDPP during the dry
period did not affect circulating IGF-I in plasma, which
was similar to that in cows under SDPP (Miller et al.,
2000). Nonetheless, milk yield was greater in the SDPP
cows (+3.2 kg/d) compared with LDPP cows. Taking
these results together with the results of the current
study, we suggest that the effect of manipulating photo-
period during the dry period is not mediated by changes
in IGF-I in dairy goats (reduced in SDPP) or dairy cows
(no difference between SDPP and LDPP).
Prolactin is critically important to the secondary
stage of lactogenesis, which occurs at parturition (Ak-
ers, 1985). Prolactin is also necessary for complete dif-
ferentiation of the bovine mammary secretory epithelial
cells at the time of parturition (Akers et al., 1981a,b).
Inhibition of the preparturient PRL surge by treatment
with bromocriptine (CB154) resulted in a greater per-
centage of undifferentiated mammary epithelial cells
compared with untreated controls (Akers et al., 1981b).
Indeed, in this study goats exposed to SDPP had less
plasma PRL than LDPP goats from wk 5 before to wk
1 postpartum and a parturient surge was observed.
Hence, both treatments experienced the same pattern
of PRL surge, although it was greater in the LDPP
treatment; yet the subsequent milk production was
greater in the SDPP treatment.
Prolactin secretion does not drive mammary growth
during pregnancy (Tucker, 2000). Some of the mecha-
nisms by which PRL induces lactogenesis in laboratory
species involve binding to a specific receptor (R)on
the surface membrane of the mammary epithelial cell
(Frantz et al., 1974; Tucker, 2000). Following R binding,
a cascade of events is initiated that eventually induces
transcription of genes that regulate the secretion of
milk proteins (Tucker, 2000). Bovine tissues express at
least 2 forms of PRL-R, the long and short forms (Schu-
ler et al., 1997). It has been shown that photoperiod
affects the concentrations of PRL and its receptors in
different tissues in steers exposed to SDPP or LDPP
(Auchtung et al., 2003). Steers exposed to LDPP had
greater PRL concentrations and reduced PRL-R mRNA
expression in the liver, mammary gland, and lympho-
cytes, whereas the opposite effect was observed in ani-
mals under SDPP. Recently, this inverse relationship
between PRL concentration and PRL-R expression in
the mammary gland was confirmed in dairy cows ex-
posed to photoperiod treatments during the dry period
(Auchtung et al., 2005). Furthermore, it was suggested
that because PRL and PRL-R influence the mammo-
genic ability of mammary cells to produce milk compo-
Journal of Dairy Science Vol. 90 No. 2, 2007
nents, it is likely that shifts in PRL sensitivity alter the
extent of cellular differentiation during the postpartum
period. Because all goats in the present study were
moved to ambient photoperiod after kidding, it is possi-
ble that goats under SDPP expressed more PRL-R in
the mammary gland, and the parturient PRL surge
together with the shift in photoperiod caused dramatic
stimulation of the parenchymal cells that led to a
greater lactogenic response. Indeed, milk production
began to differ between treatments at wk 3 postkidding,
implying an effect of PRL binding to its receptors, which
would initiate the observed lactogenic response.
Blood-electrolyte homeostasis is very tightly con-
trolled and balanced in pregnancy and postpartum by
hormones such as rennin, angiotensin, aldosterone, and
glucocorticoids, as well as by nutritional status and
filtration rate of the kidney (Robb et al., 1970). Hence,
a plausible suggestion is that the slight decrease in
plasma Na
+
concentration between pre- and postkid-
ding is a reflux of the enlarged Na distribution pool,
including the milk pool in the udder. The pH drop in
the plasma postkidding compared with gestation is a
direct effect of the increased consumption of the concen-
trate portion of the diet.
CONCLUSIONS
Exposure of dairy goats to SDPP during the dry pe-
riod increased milk production during the subsequent
lactation compared with LDPP exposure. Sensitivity to
PRL, particularly during the transition to lactation,
may be the mechanism underlying the greater milk
yield observed throughout the subsequent lactation.
ACKNOWLEDGMENTS
This research was funded by the US-Israel Binational
Agricultural Research and Development Fund (Award
#US-3201-01).
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