Effects of Milk Urea Nitrogen and Other Factors on Probability of Conception of Dairy Cows

Abstract

The objective of this study was to evaluate the relationships between milk urea nitrogen (MUN) and other factors and the probability of conception in dairy cows. Data were retrieved from the Lancaster Dairy Herd Improvement Association (DHIA). A total of 713 dairy herds and 10,271 dairy cows were included in the study. Logistic regression was used to determine the within-herd effects of MUN, milk production, lactation number, and breeding season on the probability of conception for each of 3 services. Within herds, MUN displayed a slight negative association with probability of conception at first service. For example, there was a 2- to 4-percentage unit decrease in conception rate at first service with a 10-mg/dL increase in MUN. In among-herd regression analysis, there was no effect of MUN on probability of conception. These results suggest that MUN may be related to conditions affecting reproduction of individual cows within a herd. Diet formulation usually would affect MUN equally among all cows at a similar stage of lactation in a herd. Because there was no effect of MUN among herds, diet formulation did not appear to affect conception rate.

Full text

J. Dairy Sci. 87:1878–1885
 American Dairy Science Association, 2004.
Effects of Milk Urea Nitrogen and Other Factors
on Probability of Conception of Dairy Cows*
K. Guo, E. Russek-Cohen, M. A.Varner, and R. A. Kohn
Department of Animal and Avian Sciences,
University of Maryland, College Park 20742
ABSTRACT
The objective of this study was to evaluate the rela-
tionships between milk urea nitrogen (MUN) and other
factors and the probability of conception in dairy cows.
Data were retrieved from the Lancaster Dairy Herd
Improvement Association (DHIA). A total of 713 dairy
herds and 10,271 dairy cows were included in the study.
Logistic regression was used to determine the within-
herd effects of MUN, milk production, lactation num-
ber, and breeding season on the probability of concep-
tion for each of 3 services. Within herds, MUN displayed
a slight negative association with probability of concep-
tion at first service. For example, there was a 2- to
4-percentage unit decrease in conception rate at first
service with a 10-mg/dL increase in MUN. In among-
herd regression analysis, there was no effect of MUN
on probability of conception. These results suggest that
MUN may be related to conditions affecting reproduc-
tion of individual cows within a herd. Diet formulation
usually would affect MUN equally among all cows at a
similar stage of lactation in a herd. Because there was
no effect of MUN among herds, diet formulation did
not appear to affect conception rate.
(Key words: milk urea nitrogen, probability of concep-
tion, reproduction)
Abbreviation key: CR = conception rate.
INTRODUCTION
Milk urea nitrogen is a by-product of dairy cattle
protein metabolism and a reflection of urinary nitrogen
excretion (Jonker et al., 1998). It is highly correlated
with blood urea nitrogen and plasma urea nitrogen.
Excessive feeding of protein can lead to increased MUN
concentration (DePeters and Ferguson, 1992; Broderick
and Clayton, 1997). Milk urea nitrogen measurement
Received November 20, 2003.
Accepted February 3, 2004.
Corresponding author: R. A. Kohn; e-mail: rkohn@wam.umd.edu.
*A contribution from the Maryland Agricultural Experiment
Station.
1878
is convenient and noninvasive, and several DHIA labo-
ratories provide MUN as a regular analysis while sam-
pling milk.
Reproductive performance has a substantial impact
on economic profitability of dairy farms (Mourits et al.,
1997). Jorritsma et al. (2003) have reviewed the influ-
ences of metabolic changes during early lactation on
reproductive performance. Nutrition management may
be an important means to improve dairy cow reproduc-
tive performance (Ferguson and Chalupa, 1989). Sev-
eral studies reported the negative effects of blood urea
nitrogen or MUN on reproductive performance in dairy
cows and suggested that overfeeding CP caused repro-
ductive stress (Ferguson et al., 1993; Rajala-Schultz et
al., 2001). However, others did not find such negative
effects of high MUN on fertility of cows (Carroll et al.,
1988; Godden et al., 2001).
The previous studies were conducted with a small
number of animals (<200 to 300) and herds (<30); there-
fore, our objective was to evaluate the associations of
MUN and other factors on probability of conception of
dairy cows based on data from a large number of herds.
MATERIALS AND METHODS
Data Collection
Data were retrieved from the Lancaster DHIA (Man-
heim, PA) for herds in Pennsylvania. Cows that were
first bred between June 1, 2000 and May 31, 2001 were
included in the study. A total of 10,271 cows from 713
herds were selected (from a total of 44,090 cows and
1066 herds) when data on milk production, MUN, preg-
nancy status, and breeding date were available. Milk
urea nitrogen values were measured using Bentley
Chemspec Instrument, which is based on a modified
Berthelot reaction (Chaney and Marback, 1962) to de-
tect ammonia after urea hydrolysis (Bentley Instru-
ments, Chaska, MN).
The average days open to first service was 91 d, the
interval between first and second service was 60 d, and
the interval between second and third service was 49 d
(Table 1). Therefore, we used MUN and milk production
data from 60 to 90 d post calving for regression against

MUN AND DAIRY COW FERTILITY 1879
Table 1. Distributions of variables.
1
Variables No. Mean SD
Observations per herd 713 14.4
Milk production at 60 to 90 d (kg/d) 10,271 38.1 10.5
MUN at 60 to 90 d (mg/dL) 10,271 13.4 3.9
Lactation number 10,271 2.41 1.5
Service number of current pregnancy 10,271 2.77 2.0
Days to first service (d) 10,271 90.6 53.7
Interval between first and second service (d) 6581 59.9 48.2
Interval between second and third service (d) 4138 49.0 34.3
Calving interval (d) 9174 451.3 99.9
Days open (d) 10,271 177 103.1
Average conception rate at first service 10,271 31%
Average conception rate at second service 6581 33%
Average conception rate at third service 4138 35%
305-d mature equivalent milk production (kg) 10,271 23,785 6654
1
Milk production and MUN values were test-day measurements at 60 to 90 d after calving (within 30 d
before the first service).
2
Standard deviation within herd.
the probability of conception at first service. By the
same token, we used MUN and milk production from
120 to 150 d and 170 to 200 d after calving for regression
against probability of conception at second service and
third service, respectively. We chose not to use the MUN
value at breeding because typically MUN value in-
creases and then declines during lactation (Jonker et
al., 1998). Thus, higher MUN at breeding may be con-
founded with fewer days in milk at breeding and there-
fore reduced probability on conception.
The time range for the study was divided into 4 sea-
sons: winter (December, January, and February),
spring (March, April, and May), summer (June, July,
and August), and fall (September, October, and No-
vember).
Statistical Analysis
Within-herd logistic regression analyses. The
probability of conception at each service was analyzed
separately using logistic regression of PROC GENMOD
of SAS (2000). The full model is listed as follows:
ln



p
1−p



= I + L + S + N + M + LS
+ LN + LM + SN + SM + NM + LSN + LSM
+ SMN + LSMN + N
2
+ M
2
+ LN
2
+ LM
2
+ H + e
where
p = probability of conception for a cow;
I = intercept of the model;
L = fixed effect of the lactation number;
S = fixed effect of season;
Journal of Dairy Science Vol. 87, No. 6, 2004
N = MUN at 60 to 90, 120 to 150, and 170 to 200 d
post calving for first, second, and third service,
respectively; and
M = daily milk production at 60 to 90, 120 to 150, and
170 to 200 d post calving for first, second, and
third service, respectively;
H = random effect of herd; and
e = error.
A GEE analysis in PROC GENMOD was used to
account for correlation among cows within a herd. All
quantitative variables were centered to reduce multi-
collinearity (SAS, 2000). Insignificant (P > 0.05) terms
were removed by stepwise elimination. Logistic regres-
sion fits the logit of the probability of conception to a
linear model of factors.
Among-herd analyses. To analyze the among-herd
effects, means for each herd were computed for concep-
tion rate (CR), MUN (60 to 90 d, 120 to 150, and 170
to 200 d), milk production (60 to 90, 120 to 150, and
170 to 200 d), and lactation number at first, second,
and third services. Medians were used for days open
because they were not normally distributed and be-
cause sometimes cows were culled prior to next service.
Herds with data from <3 cows were dropped. A total of
506 herds comprised of 9810 cows were included in
the analysis.
Conception rate and days open were analyzed using
multiple regression model of JMP (2000) according to
the following model:
Y = I + L + N + N
2
+ M + M
2
+ LN
+ LM + NM + LN
2
+ LM
2
+ LNM + e
where

GUO ET AL.1880
Table 2. Within herd logistic regression for probability of conception
at first service in dairy cows (713 herds; 10,271 cows).
Factor Estimate SE P <
Lactation no. 0.053 0.03 0.06
MUN
1
−0.020 0.008 0.01
Milk production (milk)
1
−0.021 0.005 0.0001
Season
2
Winter 0 0
Spring 0.66 0.06 0.0001
Summer −0.28 0.1 0.02
Fall −0.36 0.1 0.001
Lactation no. × season
Lactation no. × winter 0 0
Lactation no. × spring −0.056 0.03 0.05
Lactation no. × summer 0.018 0.07 0.1
Lactation no. × fall −0.091 0.05 0.8
Lactation no. × milk −0.0002 0.003 0.9
Season × milk
Winter × milk 0 0
Spring × milk 0.012 0.001 0.03
Summer × 0.016 0.01 0.2
Fall × milk −0.010 0.01 0.3
Lactation no. × season × milk
Lactation no. × winter × milk 0 0
Lactation no. × spring × milk −0.002 0.004 0.6
Lactation no. × summer × milk −0.028 0.006 0.0001
Lactation no. × fall × milk −0.0008 0.005 0.2
1
Milk production and milk urea nitrogen (MUN) were the test-day
measurements at 60 to 90 d after calving.
2
Seasonal effects were estimated by using winter as the baseline.
Y = CR or median days open to first, second, or third
service;
I = intercept of the model;
L = fixed effect of the lactation number;
N = fixed effect of MUN at 60 to 90, 120 to 150, and
170 to 200 d post calving for first, second, and
third service, respectively;
M = fixed effect of daily milk production at 60 to 90,
120 to 150, and 170 to 200 d post calving for first,
second, and third service, respectively; and
e = error term.
Partial F tests were used to select the variables that
were significantly related to conception rate and days
open at first, second, and third service, respectively.
RESULTS
Among all cows included in the study, a mean of 2.8
services were required for pregnancy. The overall CR
were 31, 33, and 35% for first, second, and third service,
respectively (Table 1).
Within-Herd Analysis
Among cows within herds, the effects on probability
of conception at first to third service are shown in Tables
2 to 4, respectively. There was a negative association
Journal of Dairy Science Vol. 87, No. 6, 2004
Table 3. Within-herd logistic regression for probability of conception
at second service in dairy cows (496 herds; 6358 cows).
Factor Estimate SE P <
Lactation no. −0.007 0.03 0.8
Milk production (milk)
1
−0.022 0.008 0.005
Milk × milk 0.003 0.0002 0.3
Season
2
Winter 0 0
Spring 0.668 0.08 0.0001
Summer 0.434 0.09 0.0001
Fall −0.040 0.1 0.7
Milk × season
Milk × winter 0 0
Milk × spring 0.013 0.009 0.1
Milk × summer 0.005 0.01 0.6
Milk × fall 0.029 0.01 0.02
Lactation no. × milk 0.0002 0.002 0.9
Lactation no. × milk × milk −0.0003 0.0002 0.04
1
Milk production was the test-day measurement at 120 to 150 d
after calving.
2
Seasonal effects were estimated by using winter as the baseline.
of MUN with CR at first service (Figure 1) but not in
subsequent services. For example, a change in MUN
from 9 to 18 mg/dL resulted in a 2.2- or 4.4-percentage
unit change in conception rate at first service for low-
producing cows bred in spring and high-producing cows
bred in fall, respectively (Figure 1). Within herds, there
was a negative association of milk production with prob-
ability of conception at all three services (Tables 2 to
4; Figure 2). Seasonal effects were significant for all
three services, with higher CR in the spring (Tables 2
to 4; Figure 1). There was an interaction of lactation
number by season by milk production for first service
(Figure 3).
Among-Herd Analyses
Among herds, the main linear effects on probability
of conception at first service were not significant (P >
0.05), but there was a positive quadratic effect of milk
production. Lactation number by milk production and
lactation number by MUN interactions were also sig-
Table 4. Within-herd logistic regression for probability of conception
at third service in dairy cows (460 herds; 4138 cows).
Factor Estimates SE P <
Milk production
1
−0.017 0.004 0.001
Season
2
Winter 0 0
Spring 0.559 0.09 0.001
Summer 0.334 0.1 0.001
Fall 0.653 0.1 0.001
1
Milk production was the test-day measurement at 170 to 200 d
after calving.
2
Seasonal effects were estimated by using winter as the baseline.

MUN AND DAIRY COW FERTILITY 1881
Figure 1. The within-herd milk urea nitrogen (MUN) effect and
interaction of milk production and season (spring and fall) on concep-
tion rate at first service. Lactation = 2, low milk production = 25.5
kg, moderate milk production = 37.7 kg, and high milk production =
51.4 kg. Low, moderate, and high milk production are reflections of
the lower 10%, median, and upper 10% of all observations, respec-
tively. Probability of conception was calculated from the logistic re-
gression model; non-parallel lines do not necessarily represent inter-
actions.
nificant (Table 5). For the second service, only milk
production had a negative impact on CR (Table 6).
There was a quadratic effect of milk production on CR
at third service (Table 7).
In regression analysis on days open to first service
(Table 8), lactation number and MUN had no effect (P
> 0.05); however, milk production had a negative linear
effect on days open and a positive quadratic effect.
There were lactation number × MUN and milk produc-
tion × MUN interactions (Figure 4). For the second
service, milk production had a positive quadratic effect
on days open (Table 9). No significant effects were found
at third service.
DISCUSSION
MUN Effect
Jorritsma et al. (2003) have reviewed the influences
of urea and ammonia on reproduction during early lac-
tation. Larson et al. (1997) found that non-pregnant
cows with low progesterone post breeding were often
associated with high MUN. Elrod and Butler (1993)
suggested that high MUN may be associated with a
Journal of Dairy Science Vol. 87, No. 6, 2004
Figure 2. The within-herd milk urea nitrogen (MUN) effect and
the interaction of milk production and season (spring and fall) on
conception rate at first service. Lactation = 2, low MUN = 9 mg/dL,
moderate MUN = 13.5 mg/dL, and high MUN = 18 mg/dL. Low,
moderate, and high MUN are reflections of the lower 10%, median,
and upper 10% of all observations, respectively.
Figure 3. Within-herd milk production by lactation number by
season (spring and fall) interaction on conception rate at first service.

GUO ET AL.1882
Table 5. Among-herd regression on conception rate at first service
(506 herds; 9810 cows).
Term Estimate SE P <
Intercept 0.40 0.01 0.0001
Milk production (milk)
1
−0.001 0.002 0.5
MUN
1
0.005 0.005 0.3
Lactation no. 0.02 0.02 0.3
Milk × milk 0.0005 0.0001 0.004
Lactation no. × milk 0.007 0.003 0.006
Lactation no. × MUN −0.013 0.005 0.02
1
Milk production and milk urea nitrogen (MUN) were the test-day
measurements at 60 to 90 d after calving.
decrease in uterine pH, which could make the environ-
ment within the uterus unsuitable for early embryo
development. Previous research has also shown that
cows within herds with high MUN were associated with
reduced probability of conception at first service, but
not at subsequent services (Ferguson et al., 1993). In
this study, we saw a negative effect of MUN on concep-
tion rate at first service among cows within herds, but
no such effects were found at second and third service.
In among-herd analyses, MUN had minimal effect on
conception rate, but was associated with greater days
open among high-producing herds. These results agree
with the hypothesis that urea affects cleavage and blas-
tocyst formation but not necessarily early oocyte devel-
opment (Jorritsma et al., 2003).
High MUN may be caused by many factors. Excessive
protein intake is a common nutritional factor (Jonker et
al., 1998). Blood urea nitrogen or plasma urea nitrogen,
which is the origin of MUN, may also be affected by
diseases or medicines from treatments (Vestweber et
al., 1989). Any disease or body condition that reduces
glomerular filtration such as dehydration, heart dis-
ease, and renal disease or any condition that increases
protein catabolism can result in increased blood urea
nitrogen level (Fraser, 1991). In this geographic region,
most herds (75%) are fed a single diet (Jonker et al.,
2002), and certainly most cows within a herd are fed
the same diet during the same stage of lactation (e.g.,
60 to 90 d postpartum). Therefore, ration formulation
is likely to affect MUN equally among all cows in the
herd at a similar stage of lactation. Conversely, any
number of factors including health or energy balance
Table 6. Among-herd regression on conception rate at second service
(305 herds; 5737 cows).
Term Estimate SE P <
Intercept 0.504 0.01 0.0001
Milk production
1
−0.005 0.002 0.02
1
Milk production was the test-day measurement at 120 to 150 d
after calving.
Journal of Dairy Science Vol. 87, No. 6, 2004
Table 7. Among-herd regression on conception rate at third service
(207 herds; 2840 cows).
Term Estimate SE P <
Intercept 0.577 0.02 0.0001
Milk production (milk)
1
−0.004 0.003 0.1
Milk × milk 0.0006 0.0003 0.03
1
Milk production was the test-day measurement at 170 to 200 d
after calving.
can affect MUN among individual cows within a herd
(Collard et al., 2000; Stockham and Scott, 2002).
In the present study, we detected a negative associa-
tion of MUN with CR at first service among cows within
herds. This implies that the within-herd negative asso-
ciation of MUN with probability of conception during
early lactation could relate to the status or condition
of individual cows. Negative energy balance and ill-
nesses are common during 60 to 90 d postcalving (Wal-
tner et al., 1993; Collard et al., 2000). Some illnesses
may result in higher MUN or BUN as well as reproduc-
tive problems (Finco et al., 1997; Stockham and Scott,
2002). When cows are at second or third service, in-
creased energy supply may reduce the stress from milk
production. As body condition improves, the incidence of
illness may be less likely, and the relationship between
MUN and CR would disappear.
Diet formulation is usually similar for all animals in
a similar stage of lactation within a herd. Therefore,
among-herd effects are likely to reflect diet differences.
Among herds, CR was largely unaffected by MUN, al-
though there was a significant negative interaction of
MUN and lactation number for CR at first service. The
magnitude of this effect was negligible. Thus, diet for-
mulation appeared to have a minimal effect on CR.
Although diet formulation did not appear to affect
CR, it may have been associated with days open at
first service. Figure 4 shows that herds with high milk
production (>40 kg) from 60 to 90 d postpartum had
increased days open at first service, especially when
those herds also had high MUN. These high-producing
Table 8. Among-herd regression on days open to the first service
(506 herds; 9810 cows).
Term Estimate SE P <
Intercept 84.0 1.4 0.0001
Milk production (milk)
1
−0.410 0.2 0.05
MUN
1
0.218 0.5 0.7
Lactation # −0.145 0.7 0.9
Milk × milk 0.057 0.02 0.003
Milk × MUN 0.153 0.07 0.02
Lactation no. × MUN −1.191 0.5 0.03
1
Milk production and milk urea nitrogen (MUN) were the test-day
measurements at 60 to 90 d after calving.

MUN AND DAIRY COW FERTILITY 1883
Figure 4. Among-herd interactions of milk production and milk
urea nitrogen (MUN) on days open at first service. Low MUN = 10.7
mg/dL, moderate MUN = 13.7 mg/dL, and high MUN = 17 mg/dL.
Low, moderate, and high MUN are reflections of the lower 10%,
median, and upper 10% of all observations, respectively.
herds might have delayed estrus because of negative
energy balance caused by high milk production. The
high MUN associated with this effect might have re-
sulted from high-protein diets, which exacerbated the
negative energy balance due to the energy required to
excrete nitrogen (Tyrrell et al., 1970). This leaves open
the possibility that feeding high-protein diets can affect
reproduction by increasing the days open at first
service.
Milk Production Effect
The negative effect of milk production on conception
rate has long been recognized (Spalding et al., 1975).
In the present study, high milk production of individual
cows within herds was associated with reduced proba-
bility of conception at all three services (Figure 2; Ta-
Table 9. Among-herd regression on days open to the second service
(305 herds; 5737 cows).
Term Estimate SE P <
Intercept 135.2 2.6 0.0001
Milk production (milk)
1
−0.369 0.4 0.3
Milk × milk 0.087 0.03 0.01
1
Milk production was the test-day measurement at 120 to 150 d
after calving.
Journal of Dairy Science Vol. 87, No. 6, 2004
bles 2 to 4). During early lactation, dietary energy in-
take does not meet energy requirements for increasing
milk production. As a result, body fat is mobilized. High
producing dairy cows have more severe negative energy
balance, which was shown to reduce progesterone secre-
tion and the luteal support for the uterus during preg-
nancy, thereby lowering the CR (Villa-Godoy et al.,
1988; Spicer et al., 1990).
Herds that are well managed can maintain reproduc-
tion even in the face of high milk production. Among
herds, the positive quadratic association of milk produc-
tion with CR at first and third service might have been
due to the fact that herds with effective reproduction
programs can have higher culling rates and shorter
calving intervals. The contrasting negative association
of milk production with CR at second service might
have been due to the reduced CR from the stress of
higher milk production.
This interplay of milk production and reproduction
among herds is also apparent for days open. In this
study, number of days open was positively associated
with milk production among herds with cows averaging
>45 kg/d during first and second service. Negative en-
ergy balance that results from high milk production
can delay the estrous cycle, therefore prolonging days
open (Butler and Smith, 1989). Producers with high-
producing cows may choose to delay breeding compared
with other producers. However, at lower levels of milk
production, number of days open was negatively associ-
ated with milk production at first and second service.
This latter observation is consistent with Laben et al.
(1982), who indicated that, on average, the highest-
yielding herds had 21 fewer days open than the low-
producing herds. Better reproduction enables greater
culling rates and thus higher milk production.
Lactation Number Effect
Gwazdauskas et al. (1975) indicated that the repro-
ductive performance (CR) decreased as cows grew older.
Ray et al. (1992) found that first and sixth lactation
cows had the highest number of services per conception,
and second to fifth lactation cows had better reproduc-
tive performance. In the current study, lactation num-
ber had a near significant positive effect and strong
interaction with breeding season and milk production
on CR among cows within herds at first service. Concep-
tion rate was lowest among first lactation cows and
increased as cows were more mature at second lacta-
tion. However, after second lactation, the effect of lacta-
tion number varied greatly under the influences of milk
production and breeding season (Figure 3). Similar in-
teraction with milk production can also be found in
within-herd analysis at second service and among-herd

GUO ET AL.1884
analyses. This implies that lactation number, as an
indicator of maturity, might not be the primary factor
affecting reproductive performance of dairy cows; how-
ever, it might be important to consider while analyzing
other factors, such as milk production and breeding
season.
Seasonal Effect
The reproductive performance of dairy cows fluctu-
ated throughout the year. Logistic regression indicated
that cows first bred in winter and spring had much
higher CR than cows bred in summer and fall. Previous
research (Thatcher, 1974; Ray et al., 1992) reported
similar results. Rajala-Schultz et al. (2001) also re-
ported that cows calving in summer were least likely
to conceive.
Climatic temperature change is associated with fer-
tility (Thatcher, 1974). High temperature in the sum-
mer above the thermoneutral zone could significantly
reduce CR in dairy cows (Cavestany et al., 1985). Monty
and Wolf (1974) indicated that cows calving in cool
weather had fewer services per conception than cows
that calved in hot weather. Cool weather 12 d prior to
breeding or 4 to 6 d after AI was beneficial for reproduc-
tive performance (Monty and Wolf, 1974).
The results of this study agree with previous studies
(Thatcher, 1974; Cavestany et al., 1985) that showed
that cows in early lactation (first service) that had been
bred in relatively hot weather (summer and fall) had
lower CR than did those bred in cooler weather (winter
and spring). However, cows at second and third service
during summer and fall did not show the same negative
effects on CR as at first service. At second or third
service, cows might have been less influenced by the
stress of negative energy balance as they gained suffi-
cient energy to cope with the environmental changes.
Therefore, high temperature in summer or fall did not
have the same negative effect on probability of con-
ception.
CONCLUSION
Within herd, milk production and MUN (60 to 90 d
after calving) had negative effects on conception rate
of dairy cows at first service. Among herds, MUN inter-
acted with milk production and lactation number on
days open to first service. The results suggest that the
negative effect of MUN on reproduction relates to the
status or condition of individual cows. High MUN
among herds, which might result from diet formulation,
was not associated with reduced conception rates but
was associated with a slight increase in days open to
first service in high-producing herds.
Journal of Dairy Science Vol. 87, No. 6, 2004
ACKNOWLEDGMENTS
The research data for this study were provided by
Lancaster DHIA.
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