This study was conducted on a sample of 2237 Polish Holstein-Friesian cows. The aim was to estimate the effect of selected environmental factors on the level of urea in cow's milk and on its genetic parameters, i.e. the heritability coefficients, and genetic correlation with other selected traits of milk production. The present study has revealed the existence of a highly significant influence of herd, year of calving, parity, lactation phase, and milk performance level on the urea content in cow's milk. A high urea level in milk was detected in samples collected from older animals, both during the winter season and the middle phase of lactation (101–200 days).
The heritability estimates were generally at a low level, particularly in
terms of milk yield (0.183) and urea content (0.152–0.159), which may
indicate the dominant role of the environment in shaping them. Relatively
low values of genetic correlation (
An appropriate cow feeding programme is a major factor in high milk
production. Many authors (Aguilar et al., 2012; Biswajit et al., 2011; Geerts et al.,
2004;
Hojman et al., 2004; Oltner et al., 1985) are of the opinion that, for dairy cows, the
level of urea in cow's milk (MU, milk urea) may be the indicator of whether
the energy balance and protein ratio are proper. In general, in properly
balanced feed, the MU ratio is at 15–30 mg dL
Milk urea nitrogen (MUN), which is commonly used for monitoring feeding programmes implemented in order to achieve environmental goals, is correlated with nitrogen balance, intake, and dietary content (Aguilar et al., 2012; Jonker et al., 1999; Křížová et al., 2013). Urea is the end product of protein metabolism. Excess protein is not excreted by the cow and is usually broken down into ammonia. Ammonia is toxic to animal tissues, and is therefore rapidly converted to urea in the liver. Urea is measurable in both blood and milk (Rajala-Schultz et al., 2001). A low level of urea may be due to protein deficiency; on the other hand, too high a content may be the result of excess protein delivered by feed. Monitoring the level of urea in milk can provide valuable information on the cow's nutritional state and health (Geerts et al., 2004; Rajala-Schultz et al., 2001). Hojman et al. (2004) reported that production and environmental factors account for 37 % of the variation in MU. Arunvipas et al. (2003) stated that factors such as breed, parity, days in milk, milk production, milk quality, and milk components were responsible for 13.3 % of the variation in MUN concentration. Rajala-Schultz et al. (2001) indicate that increased MUN levels appeared to be negatively related to dairy cow fertility.
The aim of the present study was to evaluate the effect of the selected environmental factors on the level of urea in cow's milk and its genetic parameters, i.e. the heritability coefficients and genetic correlation with other selected traits of milk production.
The study was conducted on 2237 cows of the Polish Holstein-Friesian (PHF) breed, born between 2004 and 2009, and reared for their milk in three high-performance herds localized in Poland. Cows calved between 2006 and 2012. The housing, nutrition, and milking conditions were different across the herds. Cows were kept in free-stall barns, on litter. In herds A and B, the buildings were new, whereas the building for herd C is a free-stall barn built in the 1970s (upgraded from a tie-stall barn). On all farms, the production cycle was closed, and production groups were determined based on production volume. In herds A and B, milking is done in a herringbone parlour manufactured by Afimilk; herd C is milked in a rotary parlour. The total mixed ration (TMR) feeding method was used for all herds. In herd A, animal feed was produced on the farm based on own silage made of lucerne, straw, hay, or purchased feed, which is soya, brewer's grains, post-extraction soya meal, and pea meal. In herd B, feeding was based on wilted silage, maize silage, and dried brewer's grains, as well as rapeseed and soya meals. In herd C, animals were fed with wilted silages or maize silage, and the ration also included straw and wet sugar beet pulp, as well as soya, rapeseed, and grain meal.
The levels of urea in milk, milk yield in test milking, content (%) of
fat, protein, lactose, and dry matter in 36 902 test milkings were all
controlled. The variation in the traits was examined based on the following
factors: herd (A–C), parity (1, 2,
Statistical calculations were conducted with the use of results from test milkings, which took place between lactation days 5 and 305.
The number of cows included in the output data set used in the genetic
analysis was reduced from 2237 to 1825, due to several of the applied
criteria. The only data taken into consideration were those from cows for
which the number of test milkings in a lactation was at least six, and the
last test milking took place after lactation day 200. At least 10 cows per
herd
The pedigree information of the studied animal population was, where
possible, gathered up to the third generation. Using the INBREED
procedure of the SAS package (SAS, 2013), we examined the population
inbreeding level. The resultant rate in reference to 13 inbred animals was
22.92 % (standard deviation
In order to statistically evaluate the impact of the above-mentioned factors on the analysed features, the multifactor analysis of variance was used. The linear model describing the variability of milk production traits, in addition to the impact of the main factors – herd, milking season, lactation phase, daily yield of milk, and year of cow's birth – takes into consideration all first-order interactions between the main factors. The significance of differences between groups was examined by using the Bonferroni test. Calculations were performed with the SAS statistical package.
We evaluated indicators of heritability, as well as the genetic correlation between the urea content and both milk yield during test milking and milk composition, i.e. the content (%) of fat, protein, lactose, and dry matter.
Estimation of parameters was performed with the use of the restricted maximum likelihood (REML) method and
the REMLF90 software (Misztal, 2007). While estimating genetic parameters
with the REML method, the same convergence index, equal to
10
These are model assumptions, as well as variance and co-variance components necessary for estimating heritability coefficients and genetic correlations for particular lactation days (from day 5 to 305), calculated based on formulas proposed by Jamrozik and Schaeffer (1997).
Table 1 shows arithmetic means and standard deviations for the analysed
traits in the data set. The results of our study indicate that the average
level of urea in milk is approximately 20.64 mg dL
On the basis of the calculated standard deviation (7.92 mg dL
Descriptive statistics of the analysed traits (
The significance of the impact of the tested factors on the level of the urea in cow's milk.
**
Level of urea in milk with respect to selected factors.
AA – means marked with identical capital letters differ
(
Estimates of (co)variance components, heritability and repeatability of milk yield, milk composition calculated based on the bivariate linear–linear models.
By using the multifactor analysis of variance it was proven that factors
which highly significantly and significantly differentiated the level of
urea in cow's milk, milk yield, and its composition were herd, parity,
lactation phase, season of test milking, and year of birth (Table 2). In
addition, the factor that statistically affected the level of urea in milk
and milk content traits was the level of milk yield. Furthermore, a
statistical effect of the majority of interactions included in the
classification model for the analysis of interaction variance was shown.
Jonkus and Paura (2011) reported that milk productivity traits were
affected by factors such as season, parity, and lactation phase
(
Due to the initial aim of the study, the next stage of description included an interpretation of the results for the analysis of variance and Bonferroni test, relating to the urea content in cow's milk.
The level of urea in milk varied depending on the herd. In herd C, the
average level of this component was the highest and amounted to 22.63 mg dL
Taking parity into account, it was proven that the level of urea was the highest during the first lactation and the lowest during the third one (Table 3). According to Fatehi et al. (2012) and Jílek et al. (2006), the mean MUN concentration, reported for Holstein cows in the third and later lactations, was lower than in the first or second lactation. Opposite results were obtained by Bendelja et al. (2011), who noted that the lowest urea concentration in milk was during the first lactation. Also, according to Carlsson et al. (1995) and Godden et al. (2001), MUN concentrations were lowest during the first lactation.
Studies reporting lower MUN values in the first lactation suggested that primiparas were still growing and therefore might have used amino acids more effectively (Arunvipas et al., 2003; Canfield et al., 1990; Carlsson et al., 1995; Ferguson et al., 1997; Ng-Kwai-Hang et al., 1985; Oltner et al., 1985). According to Doska et al. (2012), primiparas showed higher MUN values compared to multiparas, which may be explained by the lower milk production of younger cows. Moreover, Hojman et al. (2004) reported the association between high MU levels and high milk yields.
It was proven that the season of the test milking differentiated urea level (Table 2). The highest urea content was noted in milk of cows milked in winter and spring and the lowest in cows milked in summer (Table 3).
The increased level of urea in milk may be caused by excess of protein in
feed, too small an amount of easily fermentable carbohydrates in rumen,
excess protein which degrades in rumen too rapidly, too low an intake of
water, detoxification processes that excessively burden the liver, and
energy losses that arise from the rearing conditions. On the other hand, too
low a level of urea in cow's milk may arise from an insufficient level of
protein in feed, the excess of easily fermentable carbohydrates (feed
concentrates), incorrectly balanced feed ratio, and malnutritio that not necessarily inhibit the potential productivity of dairy cows (Jonker et al., 1999). Doska et al.
(2012) reported that the concentrations of milk urea nitrogen were found to
be higher (
Another factor that was found to affect the level of urea in cow's milk was the phase of lactation. In the present research the highest level of urea was found in samples collected between 101 and 200 days of lactation, while the lowest was noted during the first stage of lactation, which results from the natural physiology of cows after calving (Table 3). At the beginning of lactation, the increase in the milk yield is accompanied by a lack of appetite, which in turn leads to a reduced intake of feed. In later periods of lactation, appetite as well as the milk yield and the level of urea in milk increases (Godden et al., 2001).
Daily heritability values for MU.
Fatehi et al. (2012) reported that the concentration of MUN in milk was the lowest during the first 30 days. Godden et al. (2001) obtained the lowest MU during the first 60 days of lactation, while MU in milk was higher between 60 and 150 days. After approximately 150 days of lactation, MU decreased. Also, in the study of Doska et al. (2012), the impact of the lactation phase on MUN was found to be significant, as the highest MUN values were observed in the sixth month of lactation.
In the present study it was noted that while the daily performance increased, the level of urea increased as well. The differences between the tested samples were highly significant (Table 3). The results are in accordance with the tendency presented by Carlsson et al. (1995) and Rajala-Schultz and Saville (2003).
Calculated averages suggest that the urea content in the milk of cows born between 2004 and 2007 was similar. Differences that were proven to be statistically significant were found between the above-mentioned years and the year 2009 and 2008. This may suggest that some changes in feeding programmes used in those years were made (Table 3). Monitoring the urea content in milk may allow the status of nutrition and cow's health to be controlled. It would also allow protein efficiency in dairy cows to be evaluated in order to optimize the efficiency of utilization of the dietary protein (Guo, 2004).
Heritability of the tested milk traits was at a low or moderate level: milk
yield – 0.183; MU – 0.152–0.159; fat content – 0.271; protein content
– 0.327; lactose content – 0.340; and dry matter – 0.298. Genetic correlation
indices (
The estimated MU heritability values during a 305-day lactation fluctuated between 0.139 and 0.194 (Fig. 1). The highest values were found in the first 30 and the last 20 days of lactation. In the remaining lactation time, the MU heritability was clearly lower, at 0.133 to 0.157. The heritability values shown in Fig. 1 confirm that there is a changing effect of genetic factors on MU during lactation. The resultant curve is similar to that presented by other authors in relation to milk yield and milk content (Nixon et al., 2009; Strabel and Misztal, 1999).
In conclusion, the present study shows highly significant influence of herd, year of calving, parity, lactation phase, and the level of milk performance on the urea content in cow's milk. It was noted that the high urea level in milk was detected in samples collected from older animals, during the winter season, and during the middle phase of lactation (101–200 days).
The heritability estimates were generally at a low or moderate level, particularly in terms of milk yield and urea content, which may indicate the dominant role of the environment in shaping them. Relatively low values of genetic correlation between the urea content and other traits suggest that improvement in milk yield and its composition only modify the urea level in milk to a small degree. Edited by: A.-E. Freifrau von Tiele-Winckler Reviewed by: three anonymous referees