The Effect of Enhanced Milk Yield of Dairy Cows by Frequent Milking or Suckling on Intake and Digestibility of the Diet

Abstract

Groups of 9 or 10 cows were assigned to one of three treatments 1) machine-milking three times daily, 2) machine-milking six times daily, and 3) suckling three times daily in addition to machine-milking three times daily. Treatments were conducted during the first 6 wk postpartum. During wk 5, digestibility of the diet was estimated by the indigestible neutral detergent fiber method. During wk 6, milk yield and dry matter intake (DMI) were recorded daily, and plasma concentrations of glucose, nonesterified fatty acids, urea, protein, growth hormone, insulin, insulin-like growth factor I, oxytocin, and prolactin were determined. Milk yields were 38.5, 46.8, and 52.7 kg/d, and DMI were 18.1, 21.2, and 17.2, for cows on treatments 1, 2, and 3, respectively. Plasma glucose concentrations decreased, and plasma nonesterified fatty acid concentrations increased, for cows on treatments 2 and 3 compared with cows on treatment 1. Digestibility of dry matter was 57.5, 60.5, and 60.6%; of organic matter was 62.6, 64.6, and 66.8%; and of crude protein was 59.3, 62.7, and 64.6% for cows on treatments 1, 2, and 3, respectively. Concentrations of all assayed hormones, except insulin, increased moderately for cows on treatment 2 compared with cows on treatment 1 and increased dramatically for cows on treatment 3. Insulin concentrations followed the opposite trend. The DMI were positively related to milk yields and negatively related to oxytocin concentrations. Digestibility was negatively related to plasma glucose concentrations in a nonlinear pattern. The possible involvement of hormones in improvement of digestibility is discussed.

Full text

1998 J Dairy Sci 81:1420–1427 1420
Received January 14, 1997.
Accepted December 3, 1997.
1Department of Animal Science.
2Corresponding author.
3Department of Statistics.
The Effect of Enhanced Milk Yield of Dairy Cows
by Frequent Milking or Suckling on Intake
and Digestibility of the Diet
U. BAR-PELED,*,†,1 Y. AHARONI,‡,2 B. ROBINZON,* I. BRUCKENTAL,†
R. LEHRER,†E. MALTZ,†C. KNIGHT,§J. KALI,†Y. FOLMAN,†
H. VOET,*,3 H. GACITUA,†and H. TAGARI*
*The Hebrew University of Jerusalem, Faculty of Agriculture,
PO Box 12, Rehovot, Israel 76100
†Agricultural Research Organization, The Volcani Center,
PO Box 6, Bet Dagan, Israel 50250
‡Agricultural Research Organization, Newe Ya’ar, North Research Center,
PO Box 1021, Ramat Yishay, Israel 30095
§Hannah Research Institute, Ayr, Scotland KA6 5HL
ABSTRACT
Groups of 9 or 10 cows were assigned to one of
three treatments 1) machine-milking three times
daily, 2) machine-milking six times daily, and 3)
suckling three times daily in addition to machine-
milking three times daily. Treatments were con-
ducted during the first 6 wk postpartum. During wk
5, digestibility of the diet was estimated by the in-
digestible neutral detergent fiber method. During wk
6, milk yield and dry matter intake (DMI) were
recorded daily, and plasma concentrations of glucose,
nonesterified fatty acids, urea, protein, growth hor-
mone, insulin, insulin-like growth factor I, oxytocin,
and prolactin were determined. Milk yields were 38.5,
46.8, and 52.7 kg/d, and DMI were 18.1, 21.2, and
17.2, for cows on treatments 1, 2, and 3, respectively.
Plasma glucose concentrations decreased, and plasma
nonesterified fatty acid concentrations increased, for
cows on treatments 2 and 3 compared with cows on
treatment 1. Digestibility of dry matter was 57.5,
60.5, and 60.6%; of organic matter was 62.6, 64.6, and
66.8%; and of crude protein was 59.3, 62.7, and 64.6%
for cows on treatments 1, 2, and 3, respectively. Con-
centrations of all assayed hormones, except insulin,
increased moderately for cows on treatment 2 com-
pared with cows on treatment 1 and increased dra-
matically for cows on treatment 3. Insulin concentra-
tions followed the opposite trend. The DMI were
positively related to milk yields and negatively
related to oxytocin concentrations. Digestibility was
negatively related to plasma glucose concentrations in
a nonlinear pattern. The possible involvement of hor-
mones in improvement of digestibility is discussed.
(Key words: frequent milking, milk yield, dry mat-
ter intake, digestibility)
Abbreviation key:GH = growth hormone, GIT =
gastrointestinal tract, M3 = milked 3×,M6 = milked
6×,MY = milk yield; PGC = plasma glucose concen-
tration, S= suckled 3×and milked 3×,×= times daily.
INTRODUCTION
The milk yield ( MY) of dairy cows is positively
related to milking frequency. When milking frequency
increased from 2 to 3 times daily (×), MY increased 6
to 25% (1, 12, 32). Further increases of 9 to 10% were
evident when cows were milked up to 6×(18, 43). In
a previous report (5), we recorded increases of 20.7
and 41.6% in MY during the first 6 wk of lactation
when cows were milked 6×(M6)or3×plus suckling
3×(S), respectively, compared with MY of cows that
were milked 3×(M3). However, although DMI of
cows in the M6 group increased by 15.5% compared
with that of cows in the M3 group, no increase oc-
curred for the S group. Nevertheless, the MY of cows
in the S group was the highest. As a result, cows in
the S group lost 59 kg of BW during the first 6 wk of
lactation compared with a loss of about 25 and 31 kg
for cows in the M3 and M6 groups, respectively. We
concluded that the difference in feeding response to
increased MY between cows in the M6 and S groups
should be a major area for further study. The goal of
the present study, based on the same experiment, was
to define the effects of frequent udder emptying,
either by milking or suckling, on the DMI and digesti-
bility of the diet as well as the possible involvement of
hormones with digestibility.

Journal of Dairy Science Vol. 81, No. 5, 1998
EFFECTS OF FREQUENT MILKING ON DIGESTIBILITY 1421
MATERIALS AND METHODS
Cows and Treatments
Twenty-nine Israeli Holstein cows in their second
lactation that calved between February 19 and May
2, 1991 were used. The cows were from the dairy herd
of Kibbutz Kefar Menahem (Israel). Cows were in-
dividually housed and fed and were assigned to three
milking treatments as described previously (5). The
milking treatments were 1) M3 every 8 h at 0400,
1200, and 2000 h (n = 10); 2) M6, first at the routine
milking times and again at the end of the routine
milkings at 0700, 1500, and 2300 h (n =9); and 3) S
(n = 10). Cows were suckled by two adopted calves.
Suckling was allowed for a controlled 15-min period
at 0700, 1500, and 2300 h. All treatments were ad-
ministered during the first 6 wk postpartum. All cows
were machine-milked 3×thereafter. All cows were fed
for ad libitum intake the same diet (1.71 Mcal of
NEL/kg of DM, 17% CP, 29% NDF, and 16% ADF) as
described previously (5).
Data Collection
Data for the present study were collected from
measurements taken during wk 5 and 6 of the trial.
During wk 6, MY was recorded from each milking.
Milk intake by the calves was measured as described
previously (5). Feed intake during wk 6 was deter-
mined daily for each cow, and the DM content and
chemical composition of the feed and orts were deter-
mined (5). Blood samples were collected once a week,
beginning at 2 wk prepartum, and continuing through
wk 6 postpartum. The samples from wk 6 postpartum
were used to determine plasma concentrations of
NEFA, glucose, urea, and protein. In addition, on a
single day during wk 6, blood samples were collected
at intervals of 30 min from 0600 to 1300 h and at one
extra sampling time at 0715 h. Plasma samples were
separated immediately as described by Bar-Peled et
al. (5). Samples were kept at –20°C and were used
for hormone analyses. Digestibility measurements
were made during wk 5, and not during wk 6, to avoid
excessive stress on the cows. Fecal grab samples were
taken on 4 consecutive d during the second part of wk
5 for digestibility determinations. The grab samples
were collected twice daily at 0500 and 1700 h, 0900
and 2100 h, 1100 and 1900 h, and 1400 and 2400 h,
respectively, from d 1 to 4. These samples and sam-
ples of feed and orts from wk 5 were composited for
each cow on an equal DM basis and were spared for
the digestibility determinations. Because feed intake,
MY, and environmental conditions during wk 5 and 6
were similar, the digestibility during wk 6 was as-
sumed to be similar to that during wk 5. Therefore,
these digestibility estimations were related to the
other traits measured during wk 6.
Chemical Analyses
Specific double-antibody radioimmunoassays were
used to measure growth hormone ( GH), prolactin,
and insulin as described by Vernon et al. (44); IGF-I
after acid ethanol extraction as described by Daugha-
day et al. (11); and oxytocin after extraction as
described by Stock and Uvans-Moberg (40). Plasma
glucose concentrations (PGC) and concentrations of
NEFA, urea, and protein in plasma were determined
as described previously (5). Digestibility was deter-
mined by the indigestible NDF method, as described
by Lippke et al. (21), after suspending three repli-
cates of feces, feed, and orts samples in dacron bags
for 8 d in the rumens of fistulated cows. Those cows
were kept in metabolic units in Bet Dagan and were
fed diets similar to the experimental diet. Determina-
tions of DM, OM, and CP in samples and orts in the
dacron bags were according to methods of the AOAC
(3). The percentage of digestibility was calculated by
the equation
digestibility (percentage) = 100
[1 – (INDFI×XF)/(INDFF×XI)] [1]
where INDFIand INDFF= intake and fecal concen-
trations, respectively, of the indigestible NDF in the
DM, and XIand XF= intake and fecal concentrations,
respectively, of the fraction in question in the DM.
Statistical Analyses
Analyses of variance for differences among treat-
ments and correlations among variables were carried
out using the general linear models procedure of SAS
(37). Regression analyses, either single and multiple
linear regressions or single quadratic regressions, to
define relationships among variables, were carried
out using the stepwise multiple regression procedure
of SAS (37).
RESULTS
Treatment means for MY (kilograms per day),
DMI, and plasma hormones and metabolites during
wk 6 of lactation and digestibilities of DM, OM, and
CP during wk 5 of lactation for cows in groups M3,
M6, and S are presented in Table 1. Cows in group
M6 had increased MY compared with cows in group

Journal of Dairy Science Vol. 81, No. 5, 1998
BAR-PELED ET AL.
1422
TABLE 1. Treatment means of the digestibilities of DM, OM, and CP in the 5th wk postpartum and of
milk yield (MY), DMI, and plasma hormone1and metabolite concentrations in the 6th wk postpartum.
a,b,cMeans within rows with different superscripts differ (P< 0.05).
1Hormone concentrations were represented by the area under the curve in the 7-h sampling period.
2M3 = Milked three times daily, M6 =milked six times daily, and S = milked three times daily and
suckled three times daily.
3P> 0.05.
4Growth hormone.
*P< 0.05.
**P< 0.01.
***P< 0.001.
Treatment2
Variable M3 M6 S SEM P
MY, kg/d 38.45b46.81a52.73a3.22 ***
DMI, kg/d 18.10b21.23a17.16b0.52 ***
Digestibility, %
DM 57.53b60.54a60.60a1.42 *
OM 62.62b64.60ab 66.81a1.19 **
CP 59.29b62.68a64.64a1.23 ***
Plasma metabolite
Glucose, mg/100 ml 67.52a62.52b61.19b1.41 ***
NEFA, meq/L 385.0c448.4b539.0a23.5 ***
Urea, mM5.24 5.48 5.41 0.23 NS3
Plasma protein, % 3.75 3.75 3.88 0.12 NS
Hormone
GH,4ng/ml 17.75c20.03b28.56a1.41 ***
IGF-I, ng/ml 319.0b513.2b865.5a121.1 ***
Insulin, ng/ml 11.35a8.56b5.38c1.23 ***
Oxytocin, pg/ml 87.7c106.0b169.1a6.9 ***
Prolactin, ng/ml 597.8c672.9b822.7a52.1 ***
M3, and cows in group S had the highest MY. This
difference between cows in the M6 and S groups of
approximately 6 kg/d during wk 6 of lactation was not
significant, but was significant for the entire 6-wk
period (5). Cows in group M6 had higher DMI than
did cows in group M3, and cows in group S had the
lowest DMI. During the first 6 wk postpartum, cows
in groups M3, M6, and S lost 25, 31, and 59 kg of BW,
respectively (5). During this period, the decrease in
body condition score for cows in groups M6 and S was
greater than that for cows in group M3 (5). The
differences in both BW and body condition score
among cows in the treatment groups indicate that
cows in group M6 were in a moderate negative energy
balance compared with cows in the M3 group and that
cows in the S group were in a severe negative energy
balance. Cows in the M6 group had increased digesti-
bilities of DM, OM, and CP compared with cows in
the M3 group, and digestibilities of OM and CP in-
creased, although nonsignificantly, for cows in the S
group compared with cows in the M6 group. The PGC
were higher in cows in the M3 group than in cows in
the M6 and S groups. The NEFA concentration was
lowest for cows in the M3 group and was highest for
cows in the S group. Plasma concentrations of all of
the assayed hormones, except insulin, showed a simi-
lar trend for increase; the concentrations were higher
for cows in the M6 group than for cows in the M3
group and were highest for cows in the S group.
However, plasma insulin concentrations were highest
in cows in the M3 group and lowest in cows in the S
group.
Correlations among all 14 assayed variables are
presented in Table 2. Digestibility variables were
positively correlated with MY and oxytocin and
prolactin concentrations and were negatively cor-
related with PGC and insulin concentrations. Milk
yield was also highly related, except for its relation-
ship with digestibility, to PGC (negative) and NEFA
(positive) concentrations, as well as to insulin (nega-
tive) and oxytocin and prolactin (positive) concentra-
tions. The PGC were negatively related to NEFA
concentrations and positively related to insulin, oxy-
tocin, and prolactin concentrations. The NEFA were
highly related to concentrations of all hormones,
either negatively (insulin) or positively (all of the

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EFFECTS OF FREQUENT MILKING ON DIGESTIBILITY 1423
TABLE 2. Correlation matrix (n = 29) among variables for performance, digestibility, and composition.
1MY = Milk yield, DMD = DM digestibility, OMD = OM digestibility, CPD =CP digestibility, PGC = plasma glucose concentration, and
GH = growth hormone.
2Correlations among digestibility coefficients.
3Correlations among plasma hormone concentrations.
*P< 0.05.
Variable
Variable11234567891011121314
1 DMI 10.13 0.48 0.16 0.24 –0.12 –0.19 0.22 –0.16 –0.37 –0.26 0.11 –0.31 –0.16
2MY 10.55* 0.71* 0.70* –0.77* 0.59* 0.12 0.18 0.49 0.46 –0.57* 0.78* 0.66*
3 DMD 10.8520.922–0.49 0.26 0.22 0.22 0.16 0.09 –0.44 0.40 0.32
4 OMD 10.922–0.59* 0.46 0.18 0.20 0.38 0.29 –0.52 0.66* 0.52
5 CPD 1–0.63* 0.45 0.17 0.30 0.45 0.30 –0.59* 0.64* 0.48
6 PGC 1–0.62* –0.33 –0.22 –0.45 –0.37 0.47 –0.67* –0.62*
7 NEFA 10.05 0.04 0.70* 0.69* –0.67* 0.81* 0.73*
8 Urea 1–0.14 0.08 0.10 0.20 0.17 0.04
9 Plasma protein 10.27 0.08 –0.29 0.17 0.06
10 GH 10.673–0.5930.7630.523
11 IGF-I 1–0.4230.7230.693
12 Insulin 1–0.663–0.563
13 Oxytocin 10.823
14 Prolactin 1
Figure 1. Relationship between DMI (kilograms per day) and
milk yield (kilograms per day) within treatment groups [milked
three times daily (M3; ⁄), milked six times daily (M6; ♦), and
milked three times daily and suckled three times daily (S; ÿ)]. The
regressions within treatment groups are indicated by a solid line
(M3), a dotted line (M6), and a broken line (S).
rest). The DMI was not related to any of the other
assayed variables, except for a positive ( P= 0.08)
relationship with DM digestibility.
When DMI correlations with MY were tested with-
in treatment groups, no significant relationship was
found for cows in the M3 group (P> 0.25), but
positive dependencies of DMI on MY were found for
cows in the M6 and S groups (P< 0.05). The slopes of
regressions of these treatments were similar (0.103
and 0.092 for M6 and S, respectively), but the inter-
cepts differed between them (16.4 and 12.3 for M6
and S, respectively; Figure 1). Therefore, a multiple
regression was exercised to relate DMI to several
variables in addition to MY. Of all possible X2varia-
bles in the regression
DMI = MY + X2,
only the function that included oxytocin concentration
yielded high R values with very significant effects of
both X variables. The coefficient values for this func-
tion were DMI (kilograms per day) = 0.215(MY) –
0.0556(oxytocin) + 15.64; R2= 0.442; P= 0.0004;
P(MY) = 0.0014; P( oxytocin) = 0.0001.
When a third variable was added to this function,
there was no combination that resulted in a solution
in which all effects were significant. When, however,
the relationship between PGC and DMI was calcu-
lated within each treatment group, R = 0.69 (P<
0.03), –0.55 ( P< 0.12; NS), and –0.89 ( P< 0.001),
respectively, were evident for cows in groups M3, M6,
and S.
The PGC was negatively related to all digestibility
variables. However, the distribution of points (Figure
2a) did not follow a linear pattern. When this regres-
sion was tested within treatment groups, the digesti-
bility coefficients were significantly related to PGC;
slopes were close to –1.0 (–0.98, –1.00, and –0.90 for

Journal of Dairy Science Vol. 81, No. 5, 1998
BAR-PELED ET AL.
1424
Figure 2. Relationships (A, linear regression; B, quadratic
regression) between the digestibilities of DM (⁄), OM(♦), and CP
(ÿ) and plasma glucose concentration. The regressions are indi-
cated by a solid line (DM digestibility), a broken line (OM digesti-
bility), and a dotted line (CP digestibility).
DM, OM, and CP digestibility, respectively) only for
cows in the S group. For cows in the M6 group, the
slopes tended to be negative, and, for cows in the M3
group, the slopes tended to be positive. Quadratic
relationships of PGC and digestibility across treat-
ment groups (Figure 2b) improved R2values from
0.24 to 0.40, from 0.35 to 0.57, and from 0.40 to 0.51
for digestibilities of DM, OM, and CP, respectively,
compared with linear regressions.
DISCUSSION
Enhanced MY was associated with increased DMI
in dairy cows (35). Increased GH and prolactin con-
centrations, which were frequently reported to be as-
sociated with increased MY, were also shown to in-
crease feed intake of male reindeer (36) and dairy
cows (6). Furthermore, glucose deprivation from
body tissues enhanced DMI in goats (16), an effect
that was shown to be directly related to PGC rather
than mediated via an insulin control mechanism.
Conversely, high concentrations of oxytocin depressed
DMI in rats (2, 30) and cattle (41). The results of
the previously mentioned reports imply that DMI is
affected by many stimuli that are evoked by altera-
tions in plasma concentrations of hormones and
metabolites, and these concentrations affect one
another. Thus, we suggest that MY be considered a
representative of all of the factors that enhance DMI
and oxytocin be considered a representative of all of
the factors that depress DMI.
In the present study, cows in the M6 group had
higher (25%) plasma concentrations of oxytocin and
higher (17.3%) DMI than did cows in the M3 group.
For cows in the S group, MY increased 37%, and
plasma concentrations of oxytocin increased 93%, but
these cows had lower DMI than did cows in the M6
group. This huge rise in plasma oxytocin was proba-
bly the result of the unique stimulus of suckling, and
might have also depressed the DMI of cows in the S
group to DMI values similar to those recorded for
cows in the M3 group. Nevertheless, despite the sup-
pressing effect of oxytocin on the DMI of the cows in S
group, the positive relationship of DMI and MY with-
in this group was highly significant ( P< 0.001),
revalidating the positive effect of MY on DMI. We
assume that this positive effect caused the increase in
DMI for cows in the M6 group, an increase that was
not depressed by the relatively small increase in oxy-
tocin (Table 1).
Exceptionally high MY drain PGC. Glucose depri-
vation of body tissues (15) enhanced feed intake in
goats, an effect that was shown to be related directly
to glucose rather than mediated via the insulin con-
trol mechanism. In the present experiment, such a
negative relationship between PGC and DMI was
observed only for the treatment groups consisting of
cows with the highest MY (M6 and S) and a negative
energy balance. Conversely, the relationship between
PGC and DMI was positive for cows in the M3 group.
This group of cows had the smallest MY and was
already in a positive energy balance at wk 6 postpar-
tum, as was deduced from the increase in BW (5).
Therefore, it is plausible to assume that the DMI of

Journal of Dairy Science Vol. 81, No. 5, 1998
EFFECTS OF FREQUENT MILKING ON DIGESTIBILITY 1425
cows in the M3 group was the independent variable,
and PGC was the dependent variable.
Eriksson et al. (13) studied the interrelationships
of glucose, GH, and suckling. Those researchers were
able to disconnect some of these relationships by va-
gotomy. In rats treated with sham operations, suck-
ling increased MY and plasma GH, but, in
vagotomized rats, suckling depressed GH and PGC,
which suggests that GH helps to maintain PGC in
intact suckling rats despite enhanced glucose require-
ments for MY. In the present study, the increased MY
of cows in the M6 group was accompanied by a
decrease in PGC with only a minor increase in plasma
GH concentrations compared with results for cows in
the M3 group. However, the MY of cows in the S
group was further increased, although DMI was
decreased, compared with those of cows in the M6
group (Table 1). However, these changes resulted in
only a slight further decrease in PGC of cows in the S
group compared with that of cows in the M6 group.
The maintenance of PGC in cows in the S group could
have been a result of the extreme increase in GH
secretion by cows in the S group compared the GH
secretion by cows in the two other groups (Table 1).
Increased feed intake results in decreased digesti-
bility (29). However, in the present experiment, DM
digestibility of cows in the M6 group was significantly
increased despite the significant increase in DMI for
this group of cows. Also, during lactation, glucose
helps to supply the energy required for MY (4). Glu-
cose concentration in blood is both mediated by
homeostasis control and also serves as a mediator. In
addition to its well-known effect on sympathetic ac-
tivity and on the secretion of insulin and glucagon,
decreased PGC has been shown to enhance feed in-
take (16) and to increase secretion of several
glycemic hormones, such as vasopressin (38) and GH
(13). The possible role of a glycemic state in func-
tions of the gastrointestinal tract (GIT) is of special
interest. If decreased concentrations of glucose result
in increased digestibility, this increased digestibility
should be mediated through neuroendocrine effects.
Peptides common to anterior and posterior pituitary,
as well as IGF, were found either to be produced in
the GIT or to have receptors there. Thus, these hor-
mones may affect digestion and feeding.
Arginine, vasotocin, and mesotocin, the avian ana-
logs of vasopressin and oxytocin, are present at
several sites along the GIT of chickens (34). Further-
more, oxytocin inhibits peristaltic contractions along
the GIT of dogs and guinea pigs (26, 27). This inhibi-
tion may suppress feed intake, but, as the time that
digesta are present in the GIT is prolonged, digestion
and absorption may be better.
Growth hormone receptors are expressed along the
GIT of humans, rabbits, and rats (28). Furthermore,
GH enhances proliferation of mucosal cells in the
ileum of rats (15), increases the transport of water
and electrolytes across the intestinal wall (24), and
enhances amino acid uptake from the lumen of the
gut in humans (17). A combined treatment with GH,
glutamine, and a modified diet improves the absorp-
tion of protein by 39% and reduces stool output by
33% in patients with short bowl syndrome (9). Thus,
the rise in plasma GH along with the increase in MY
of cows in the present study might be a contributing
factor to the observed improvement of digestibility.
Similarly, Buyse et al. (8) suggested that, in chick-
ens selected for feed efficiency, the improved effi-
ciency was contributed by a higher GH concentration
in plasma.
Prolactin receptors are present in the GIT of hu-
mans (14), rabbits (22), rats (28), and snakes (10).
Prolactin induces development of both endocrine and
exocrine components of the pancreas (7, 25); en-
hances water, sodium, potassium, and chloride ab-
sorption from the GIT of rats (24); and directly en-
hances calcium absorption in the intestine of normal
and lactating rats (19) even under vitamin D defi-
ciency (31). Thus, the rise in plasma prolactin and
the significant positive correlation between prolactin
and two of the three variables of digestibility might
indicate that prolactin is also involved in the improve-
ment of digestibility.
The peptides IGF-I and IGF-II stimulate cell
proliferation of intestinal crypts in pigs (45), rats
(39), and mice (33); enhance absorptive functions in
rats (20); and increase concentrations of sucrase,
maltase, and leucine aminopeptidase activities in the
ileum of rats (20, 42). Furthermore, in the small
intestine of rats, the concentration of IGF-I and IGF-
II receptors is affected by nutritional state (46) and
is increased following a massive small bowel resection
(23), which suggests that IGF-I and IGF-II are in-
volved in adaptation of the GIT to changes in nutri-
tional needs.
All of these hormones are both mediators of PGC,
and are mediated by PGC. Thus, although there is no
direct evidence for a pathway through which digesti-
bility is affected by the glycemic state, the previously
mentioned results suggest the existence of such a
pathway.
In the present study, decreased PGC was as-
sociated with increased digestibility (Table 2), espe-
cially for cows in the S group (Figure 2). This associ-
ation of decreased PGC with increased digestibility,
however, was not linear. At the highest PGC, typical

Journal of Dairy Science Vol. 81, No. 5, 1998
BAR-PELED ET AL.
1426
of cows in the M3 group, no response of digestibility to
PGC could be detected; however, response of digesti-
bility to PGC in the lower range, typical of cows in the
S group, was more profound and significant than that
for cows with PGC in the middle range (cows in the
M6 group). By exercising quadratic relationships be-
tween PGC and digestibility, the significance of the
regressions was improved. Therefore, we suggest that
PGC affects digestibility only when it drops below a
certain limit and is enhanced in a nonlinear pattern
as the concentration continues to drop.
CONCLUSIONS
Cows in the M6 group had increased MY by 22%,
and their DMI increased by 17%, to compensate
partly the rise in nutrient requirements. Dry matter
digestibility was also increased despite the increase
in DMI. When MY was enhanced even more by suck-
ling, no further increase occurred in DMI above that
observed for cows in the M3 group. This discrepancy
was probably the result of the anorexic effect of the
dramatic rise in oxytocin concentrations in cows in
the S group. As a result, cows in the M6 group were in
a moderate negative energy balance compared with
cows in the M3 group; cows in the S group were in a
severe negative energy balance, which was expressed
by a heavy loss of BW, elevated NEFA concentrations,
and decreased glucose concentrations in blood. The
decreased glucose concentrations, via its endocrine
effects, increased digestibility in a nonlinear manner.
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