Effects of silage quality, protein evaluation systems and milk urea content on milk yield and reproduction in dairy cows

Effects of silage quality, protein evaluation systems and milk urea content on milk yield and reproduction in dairy cows

Livestock Production Science, 37 ( 1993) 91-105 91 Elsevier Science Publishers B.V., Amsterdam Effects of silage quality, protein evaluation system...

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Livestock Production Science, 37 ( 1993) 91-105


Elsevier Science Publishers B.V., Amsterdam

Effects of silage quality, protein evaluation systems and milk urea content on milk yield and reproduction in dairy cows Anders H. Gustafsson a and Jonas Carlsson b aSwedish Association for Livestock Breeding and Production, Eskilstuna, Sweden bExperimental Station, Veterinary Institute, Skara, Sweden (Accepted 23 March 1993 )

ABSTRACT A field study with 29 dairy herds fed wilted silage was conducted to determine the effects of silage quality, protein evaluation systems and milk urea content on milk yield and reproductive efficiency. Sampling and data on rations were collected weekly and evaluated on herd level. The herds were fed more than their requirements of energy (17%) and protein (6 to 15%). The effect of variations in amount of amino acids absorbed in the small intestine (AAT) on milk yield was small. The concentration of urea in bulk milk (mean 4.00 mM) was positively correlated with digestible crude protein (DCP), DCP/energy and protein balance in the rumen (PBV), with partial correlations of 0.41,0.43 and 0.40 respectively, but not with AAT or milk yield. The interval between calving and first (mean 82 days) and last (mean 108 days) service was longer in herds with either low or high milk urea concentrations; the shortest interval occurred between 4.5 and 5.0 mM. The interval to last service was lengthened by 2.2 days/percentage unit of higher ammonia-N content (% of total N) in the silage. It is concluded that for optimal reproductive efficiency the milk urea concentration may need to be higher than the present average value and that deterioration of the protein quality in silage may reduce the reproductive efficiency of a herd. Key words: Dairy cow; Silage quality; Milk yield; Reproduction


For many years the apparently digestible crude protein (DCP) content of the diet has been used to assess the dietary supply and requirements of protein for ruminants. The DCP system is easy to use but has clear limitations because, for example, it does not distinguish between the absorption of amino acids and other nitrogenous compounds (Madsen, 1985 ). A high proportion of rumen degradable protein may result in the absorption of significant Correspondence to: A.H. Gustafsson, Swedish Association for Livestock Breeding and Production, S-631 84 Eskilstuna, Sweden.

0301-6226/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.



amounts of ammonia from the rumen and an inadequate supply of amino acids (AA) to the small intestine, even if the requirement for DCP is met. Increasing the proportion of rumen undegradable protein in the diet has been reported to increase the absorption of AA from the small intestine (Stern et al., 1985 ). In order to overcome the shortcomings of the DCP system and to predict more accurately the amount of AA absorbed from the small intestine (AAT) and the protein balance in the rumen (PBV), a new protein evaluation system was proposed by Madsen ( 1985 ), and became official in Sweden in 1991. PBV is the balance between degraded feed protein and nitrogen used in the microbial protein synthesis. AAT is the estimated sum of AA from microbial protein and AA from ruminally undegraded feed protein. However, the possible interactions between silage protein quality and tureen degradability, and thereby influence on AAT/PBV and milk urea content, have not been elucidated. Reduced silage quality and development of heat in tower silos are common problems in Swedish dairy farming, but so far not studied. Increasing the intake of DCP, or D C P / M J of metabolizable energy (ME) increases the urea content of blood and milk (Oltner and Wiktorsson, 1983; Grings et al., 1991 ) because of the increased conversion of ammonia to urea in the liver. The u r e a content of milk is closely correlated not only with blood urea content (Eckart, 1980) but also with rumen ammonia concentration (Ropstad et al., 1989; Gustafsson and Palmquist, 1993 ). The ammonia content of silage would also therefore be expected to influence the urea content of milk. As a result, milk urea concentration is potentially an indicator of the balance of protein and energy in the diet of dairy cows, as suggested by Lewis (1957). Because the protein/energy balance of the diet can affect the milk yield and fertility of dairy cows (Folman et al., 1981 ) the milk urea content may be a useful indicator of the performance of a herd of cows in these respects. The purposes of the present study were to determine the effects of silage quality, protein evaluation systems and milk urea content on milk yield and reproductive efficiency in dairy cows. Identifying relationships between protein/energy in the diet and milk urea content also was an objective. The study took place while the DCP system was still the official system in Sweden. Measurements also were made for a more extensive investigation of the quality of the silage fed, which will be presented elsewhere (Carlsson et al., 1993 ). MATERIALSAND METHODS The study took place between September 1987 and February 1988 and used 29 Swedish dairy herds; 12 consisted of Swedish Red and White cattle, 12 of Swedish Friesians and five herds had both breeds. The average herd size was 38 cows, ranging from 19 to 85. All the herds were included in the official milk recording scheme and used artificial insemination. Sampling and data



were collected approximately once a week; by technicians at four herd visits per farm, once every third week, and by the farmers between these visits. Most visits took place from October through January. The available weighing scales were calibrated during the first visit to each farm. The mean + sd milk yield of the herds in the year before the study was 7893 _+696 kg fat corrected milk ( F C M ) . The mean_+ sd bodyweight of the cows, estimated from the heart girth measured once, on the first farm visit, was 613 _+32 kg. Data on fertility were obtained from the official AI-service reports. The mean _+sd interval from calving to first service (CFS) and to last service (CLS) were 8 2 + 9 and 108 + 13 days, respectively. CLS was used synonymous with calving to conception due to lack of effective service, as pregnancy was not confirmed by rectal palpation in all cows. If only services with confirmed pregnancies had been used, a loss of data would have occurred. It was assumed that CLS was not influenced by differences between herds in frequency of confirmed pregnancies. The major source of forage in the cows' ration was wilted silage, fed at an average rate of 7.8 _+2.2 kg/day. The silage was stored in vertical steel silos and unloaded from the top. Small amounts of hay, on average 1.9 _+ 1.1 kg dry matter ( D M ) / d a y , were fed to all the herds but one. Oats, barley and commercial high-protein concentrates were fed according to the Swedish recommendations for rations based on the feeding standards 5.0 MJ M E / k g ECM. Ten of the herds received small amounts of molassed sugarbeet pulp and seven received small amounts of molasses.

Milk analysis, feed evaluation and nutrient supply Bulk milk samples were preserved with bronopol solution and stored at - 18 °C or lower, until they were analyzed for urea by flow injection analysis (Andersson et al., 1986). The feed samples were analyzed as recommended by the National Laboratory for Agricultural Chemistry, Uppsala (Table 1 ). The m e a n + sd dry matter content of 117 silage samples was 35 + 9%; this value and its crude protein content and to some extent its metabolizable energy content were lower than in previous years because the season had been wet and the silage was harvested late. The mean contents of ammonium-nitrogen (NH3-N) and acid detergent fibre nitrogen (ADF-N) of the silage were 11.0 + 3.5% and 6.8 + 1.7% of the total nitrogen respectively. Nutrient supply was calculated for the lactating cows based on the feed analysis and the weekly observations on amounts fed. The data were used as actual intakes as feeds to dry cows and major refusals had been considered while registrations were made. This resulted in a total of 207 observations on nutrient balance, i.e. estimated nutrient supply compared with requirements. These emanated from both the technicians' herd visits and the farmers registrations. The average y i e l d / c o w / d a y was calculated from the a m o u n t delivered to the dairy (plus 5% as an estimated average for the milk used on the



TABLE 1 Nutrient content (~ = mean, SD = standard deviation ) of feed samples Per kg dry matter Per kg dry matter



Silage 110 Hay 30 Grain (oats, barley) 29 Commercial concentrates CP, % of dry matter <21 10 21to30 21 >30 15


CP, g

DCP, g

AAT, g

PBV, g








10.0 9.5 12.5

0.5 0.4 0.6

126 104 111

17 15 16

87 66 86

16 13 14

65 65 76

3 2 10

13.2 14.0 13.2

0.4 0.1 0.4

173 266 323

14 5 32

141 225 280

14 2 25

91 113 121

6 11 18






16 19 - 6 15 - 1 3 20

90 88 87


28 11 91 16 139 28

75 68 67

4 5 6

ME = Metabolizable energy; CP = Crude protein; DCP = Digestible crude protein; AAT = Amino acids absorbed from the small intestine; P B V = P r o t e i n balance in the rumen; EPD=Effective protein degradability in the rumen.

farm) and number of cows in milk. The milk's fat and protein contents as analyzed by the dairies were used to calculate the yield of energy corrected milk (ECM). The Swedish feeding standards (Eriksson et al., 1976) were used except for the AAT/PBV system which was adapted from Madsen (1985) with two modifications. First, 40 g AAT/kg ECM were used instead of the suggested 45, and secondly, an efficient protein degradability (EPD) value, as described by Madsen ( 1985 ), of 90% was used throughout for silage, and a value of 88% for hay. Digestible carbohydrates in forage were estimated as described by SLU-Info ( 1991 ). The AAT and PBV values for the commercial concentrates were calculated on the basis of their constituents provided by the feed manufacturers, and from feed-table values (Lindberg, 1986).

Statistical analysis The effects of independent variables on the urea content of milk and milk yield were analyzed on the basis of the weekly measurements, and reproductive efficiency was analyzed on a herd basis. Four herds with limited data on amounts of feed, two herds with few analyses of milk urea concentration, and one herd without fertility data were omitted from some of the statistical analyses. This reduced the number of observations (e.g. from 207 to 200 in model 1 ). The statistical models were developed in a stepwise manner by backward elimination of nonsignificant ( P > 0.05 ) linear independent variables using PROC GLM in SAS (SAS Institute Inc., 1985 ). For reproductive traits, quadratic effects were also tested ( P < 0.05). The daily intake of metabolizable energy (IME), digestible crude protein (IDCP) and amino acids absorbed



from the small intestine (IAAT) were used when their effects on the concentration of urea in milk were evaluated. Milk yield was regressed on the daily energy intake minus the maintenance requirement (IME-M), the daily intake of DCP minus the maintenance requirement (IDCP-M), and the daily intake of AAT minus the maintenance requirement (IAAT-M) as independent variables. The final statistical models and variables were as described below. The average daily milk yield of 200 observations was analyzed in models 1 and 2. Model 1: Yijk = # + h i +wj + b l (fk) +eijk where Yijk = kth observation of daily milk yield # = overall mean hi = ith herd (i = 1,... 25 ) wj = j t h ordinal n u m b e r of feed-weighings (j = 1,... 11 ) b l = the linear regression coefficient of daily milk yield on IDCP-M (fk) eijk= r a n d o m residual with mean = 0 and variance = ~e2 Model 2 was the same as model 1 except that for the intake of protein IDCPM was replaced by IAAT-M and PBV. Model 3 (n = 200) was used in order to estimate partial correlations between the dependent variables IDCP-M, IAAT-M and the ratio of D C P / M J ME by employing PROC MANOVA in SAS (SAS Institute Inc., 1985 ). The model was identical to model 1 except for b~ which was replaced by the linear regression coefficient of the Yijk variable on IME-M. Model 4 was used for evaluating the urea content of milk. The model was identical to model 1, except that for the intake of protein IDCP-M was replaced by either IDCP, PBV or the ratio of digestible C P / M J ME, one at a time. Due to missing observations for milk urea on some days of feed-weighings, slightly fewer data were used in model 4 (n = 154 ) than the total number of urea analyses. Model 5. Partial correlations between milk urea and other parameters ( n = 154) were analyzed by using PROC MANOVA in SAS (SAS Institute Inc., 1985). Model 5 was identical to model 3 except that IME-M was replaced by IME, and data from only 23 herds were used. Milk urea content was also analyzed by using the data collected on the four visits to each herd, because these data were considered to be the most reliable. The mean values for each herd were analyzed separately for each visit. As the estimates were similar, the results of only the third visit (model 6), which had the highest R 2 value, are presented (n = 10). Model 6:



Yij = #+b~ (mi) +b2 (oj) +eij where Yij = k t h observation of milk urea content as a herd mean /t = overall mean bx = t h e linear regression coefficient of milk urea content on herd mean of PBV (mi) b2 = t h e linear regression coefficient of milk urea content on herd mean of body weight (oj) eij = random residual with mean = 0 and variance = ~2. The effects of milk urea concentration, milk yield and silage quality on the interval from calving to last service (CLS) and to first service (CFS) were evaluated in models 7, 8 and 9 at the herd level ( n = 2 4 ) . The reproduction data were taken from the AI service reports for the 12 month period September 1987 to September 1988. Despite the shorter data collecting period for feed intake and milk urea, the correlations are reasonable to evaluate, because September to January was the period when most of the cows were bred. Model 7: Yijk =/Aq-bl (hi) q-b2 (pj) +b3 (qk) +eijk

where Yijk= kth observation of the interval from calving to last service bl --the linear regression coefficient of CLS on herd mean of urea content in milk (ni) b2 = the linear regression coefficient of CLS on herd mean of 12 months' milk yield prior to the trial (pj) b3--the linear regression coefficient of CLS on herd mean of ammonia-N in silage (qk) and the other effects are as defined in model 1. Model 8 was also used for evaluating the CLS interval, excluding the significant effect of pj in model 7. Model 9 was used for evaluating the CFS interval. The final model consisted of one explanatory variable; urea content in milk. RESULTS

In comparison with the accepted Swedish feeding standards the cows received on average a surplus of dietary energy of 17%, a surplus of DCP of 15% and a surplus of AAT of 6% (Table 2 ). The cows in high-yielding herds consumed less energy and protein per kg ECM than the cows in herds with



TABLE2 Average (~) daily milk yield and nutrient intake in the 29 herds, with a total of 207 observations. SD = standard deviation SD

Milk yield, kg E C M / d Feed intake, kg DM/100 kg BW Intake metabolizable energy, M J / d digestible crude protein, g/d AAT, g/d Intake minus maintenance requirement energy, M J / d digestible crude protein, g/d AAT, g/d PBV (protein balance in the rumen ), g

22.8 3.0

3.5 0.4

207 2010 1383

24 341 167

144 1623 988 375

24 338 165 219

AAT = amino acids absorbed from the small intestine. TABLE3 Energy and protein intake (minus maintenance requirement) per kg energy corrected milk in low-, medium- and high-yielding groups of cows. ~ = mean and SD = standard deviation Per kg ECM

Yield group <20 kg ECM 20 to 25 kg ECM >25 kg ECM

(n=54) (n=93) (n=53)

Yield, kg ECM

Energy, ME, MJ

DCP, g

AAT, g

PBV, g/d











18.7 22.5 27.4

0.9 1.4 2.2

6.9 6.4 5.9

1.1 0.9 0.9

75 72 68

17 13 11

47 44 40

8 6 6

283 362 491

250 197 170

ME = Metabolizable energy; DCP = Digestible crude protein; AAT = Amino acids abosrbed from the small intestine; PBV = Protein balance in the rumen.

lower yields (Table 3). The ratios of AAT-M/ECM and MJ ME-M/ECM were visibly linearly correlated (Fig. 1 ). The estimated effects of DCP and AAT-PBV on milk yield are shown in Table 4. The daily milk yield was not correlated with the cow's bodyweight, or with their intake of metabolizable energy, or with the concentration of urea in bulk milk and these factors were therefore omitted from the models. Models 1 and 2 both had R 2 values of 88%. The estimates corresponded to 0.12 kg ECM per 40 g AAT, and 0.14 kg ECM per 60 g DCP. The partial correlations (r) between milk yield and D C P / M J , DCP, AAT and PBV were 0.21, 0.20, non-significant and 0.18 respectively (model 3). The correlation between DCP and PBV was 0.99.



TABLE 4 Estimated effects (j~) of dietary protein evaluated as digestible crude protein and AAT/PBV on average daily milk yield and the estimated effects on the interval from calving to last service (CLS) of production level, milk urea content and ammonia-N in silage. SEE = standard error of estimate Variable*




Effect on milk yield, kg ECM/d DCP-M, g/d (model 1) AAT-M, g/d (model 2) PBV, g/d (model 2)

0.0024 0.0031 0.0019

0.0006 0.0011 0.0007

<0.001 0.005 0.013

Effect on CLS (model 7), days HYld, kg FCM Urea content in milk, mM Ammonia-N, % of total-N

- 0.0079 - 16.49 2.53

0.0033 4.95 1.02

0.026 0.003 0.022

aDCP-M = intake of digestible crude protein minus maintenance requirement. AAT-M = intake of amino acids absorbed from the small intestine minus maintenance requirement. PBV = protein balance in the rumen. HYld = herd mean milk yield the year before the experiment.

O 000


• •

O 0 ° 0000 ~55-'

O0 °


qU~lJ~u~ •

• • ~,ab4jr ~ t

~452 L


's352 ~30i ~254

e 0



,. . . . . . .









I n t a k e of ME-M p e r k~ ECM, MJ Fig. 1. The relationship between the intake of amino acids absorbed from the small intestine minus requirement (AAT-M) per kg energy corrected milk (ECM) and the intake of metabolizable energy minus requirement (ME-M) per kg ECM.

The mean+_sd concentration o f urea in 588 bulk milk samples was 4.00 + 0.74 raM, and the concentrations ranged from 1.70 to 6.10 raM. The mean values were 3.7 mM in September, 3.8 mM in October, 4.1 mM in November, 4.2 mM in December and 4.1 mM in January, with a standard error



o f 0.1 mM. The lowest and highest herd means were 2.70 m M and 5.30 m M with a standard error o f 0.1 mM. A higher intake o f protein increased the urea content o f the bulk milk; in model 4 the regression coefficients (~) were 0.350_+ 0.066 for D C P / M J ( P < 0.001 ), 0.0008 _+0.0002 for DCP ( P < 0.001 ) and 0.0014_+0.0003 for PBV ( P < 0 . 0 0 1 ) . A similar tendency was also observed when the values were corrected for energy intake. The partial correlation coefficients for urea vs. D C P / M J , D C P and PBV were 0.43, 0.41 and

Body weight;

--=580 kg ~5.0 -~

. . . . 610 kg . . . . . . 640 k ~

() 4 . 0




....-" ""




" ..-**

y = O.O03*PBV

"~ 3.0

- O.013*BW



R8= 87~ I

















PBV in t o t a l r a t i o n , g Fig. 2. The relationships between protein balance in the rumen (PBV) and urea concentration in bulk milk at different bodyweights. Bodyweightsare exemplified at mean2_one standard deviation.

~" o 130 >


", ", " ", .



NHaN i n silage, % of t o t a l N. . . . . . 14.5 . . . 11.0


x "". ,,

= 7.5 "-..


-P i 0 0 b/)






y = 3 1 9 . 9 + 2 . 2 0 NH3N-103.0 UREA+t0.8 UREg

// ..........................


3.5 4.0 4.5 5.0 5.5 Milk u r e a c o n t e n t , mM Fig. 3. The relationship between urea concentration in bulk milk and calving to last service interval (CLS) at different levels of ammonia nitrogen in the silage (NH3-N). NH3-N is exemplified at mean _ one standard deviation.



0.40 respectively, but they were not significant for urea vs. the intake of AAT or milk yield (model 5). In an additional analysis only the data derived from the farm visits were used; the estimates were similar for each visit and the highest R 2 value (87%) occurred on the third visit (model 6), the results of which are shown in Fig. 2. The CLS means for individual herds were correlated with the mean herd milk yield, the urea concentration in bulk milk and the ammonia-N in silage (Table 4); the ADF-N in the silage and the DCP, AAT and PBV values of the rations had no significant effect on the interval. When only milk urea and ammonia-N in silage were considered as independent variables, there was a significant curvilinear relationship between the urea content of milk and the interval between calving and conception (model 8, Fig. 3). The CFS means for individual herds were correlated with the urea concentration in bulk milk; y = 2 4 7 - 73.9x + 8.02x 2 where y = CFS, days, and x = milk urea content, mM. According to this equation, the shortest CFS was found close to 4.6 mM of urea. DISCUSSION

The new protein evaluation system, AAT/PBV, was no better correlated with the herd milk yield than the old DCP system. Furthermore, the response of milk yield to variations in AAT was small compared with the results of recent production experiments (Bertilsson, 1990; Volden, 1990). The low response to AAT was probably partly due to the high energy intake - about 17% higher than the cows' calculated requirements - and also to the surplus of AAT (Table 2 ). The calculated intake of AAT would have been even higher, by approximately 4%, if the present standard value for effective protein degradability in forages in Sweden of 80% had been used instead of 88 to 90%. Volden (1990) suggested that an excessive supply of energy might reduce the response of milk yield to AAT, possibly because the abundant supply of carbohydrates in the rumen would increase the rate of synthesis of microbial protein and the rate of production of propionate, thus reducing the requirement for amino acids for gluconeogenesis. However, Aaes et al. ( 1991 ) found no interaction between the intake of energy and AAT. These conflicting resuits may be due to differences between the energy evaluation systems. The Swedish feeding standard for intake of metabolizable energy (5 MJ ME/kg ECM), which was used in this study, has been shown (Berg and Thuen, 1991 ) to underestimate the requirement for cows yielding more than about 20 kg ECM. Berg and Thuen ( 1991 ) reported energy consumptions of 5.6 and 5.9 MJ ME/kg ECM at yields of 25 and 35 kg ECM respectively. The present results in the high yielding group (Table 3) agree quite well with these higher estimates, and also with a recent field study (Olsson et al., 1991 ). The mean bulk milk urea concentration of 4.0 mM recorded in the present



study was slightly lower than the values recorded in earlier studies (Refsdal et al., 1985; Ropstad and Refsdal, 1987; Gustafsson et al., 1987; Carlsson, 1989). The difference may have been due to variations from year to year (Gustafsson et al., 1987; Carlsson, 1989) or to the fact that only herds fed wilted silage stored in steel silos were used in the present study. The effect of D C P / M J ME on milk urea was slightly less than that reported by Emanuelson (1989) for cows in parities 2 and 3, but similar to her results for cows of parity 1. The effect of the daily intake of DCP on milk urea was also small; Refsdal et al. ( 1985 ) reported three times larger regression coefficients. Previous reports of the effect of PBV on milk urea content (Kristensen et al., 1987; Ropstad et al., 1989) indicated twice as large an effect as was observed in the present study. The partial correlation coefficients between milk urea and D C P / M J ME, DCP and PBV were similar and may therefore be used as an alternative method for the prediction of milk urea if the intake of energy is taken into account. The partial correlation between milk urea and D C P / MJ was, however, slightly higher than the value reported by Emanuelson (1989). The results of the linear regression in model 7 (Table 4) suggest that herds with higher milk urea contents also had shorter CLS intervals, whereas previous reports have often found that high urea contents in blood or milk are associated with lower reproductive efficiency (Ropstad and Refsdal, 1987; Ferguson et al., 1988; Canfield et al., 1990). However, Carlsson (1989) reported the longest CLS intervals in herds with a very low bulk milk urea content. In a study by Pehrson et al. (1992) lower fertility was reported in herds with either high or low milk urea content, and Miettinen ( 1991 ) observed a tendency towards longer calving to conception intervals in herds with a low milk urea content. These rather conflicting results and our data on both CFS and CLS suggest that there may be an optimal milk urea content, possibly higher than previously proposed and probably dependent on the energy balance and amino acid supply of the herd's diet. The negative effect of high ammonia-N levels in the silage on reproductive efficiency (Fig. 3 ) may be correlated with an adverse effect of ammonia on the palatability of the silage or on the rumen microflora, and hence on the cows' energy balance. It is well documented that a negative energy balance during early lactation is associated with reduced fertility (Butler and Smith, 1989 ). However, a possible toxic effect of silage with high content of amines has been suggested (Tveit et al., 1992 ), which may interact with reproduction and contribute to explaining our results as their (Tveit et al., 1992) silage also was high in butyric acid and ammonia nitrogen content. High protein diets have been reported to increase not only plasma urea but also the concentration of ammonia in the uterus (Jordan et al., 1983 ), and changes in the uterine environment have been suggested as the most likely route by which such diets may depress fertility (Canfield et al, 1990). In rats,



the highest rates of embryonic loss have been observed in animals fed very high protein diets (Saitoh and Takahashi, 1977) and in vitro studies have suggested that urea may inhibit the mobility of sperm (Dasgupta et al., 1971 ). The present results suggest that factors associated with the quality of the protein in the diet may influence calving to conception rather than the CFS interval (i.e. onset of estrus and ovulations ), and be at least as important as the crude protein content of the diet for the reproduction efficiency of dairy cows. CONCLUSION

The high intake of energy by the cows in this study may in part explain the small response observed in their milk yield to variations in AAT. Both low and high concentrations of urea in bulk milk were associated with lower fertility. We concluded that the optimal milk urea concentration for the maintenance of fertility may be slightly higher than as previously been proposed, and that the quality of the dietary protein appears to be an important modifier of the relationship between milk urea and fertility. These results indicate that milk urea content may be a useful indicator of nutritional status, and used together with an evaluation system like AAT/PBV, it may be a good tool for improving reproduction efficiency. ACKNOWLEDGEMENTS

We thank all the dairy farmers and technicians who made this field study possible and also feed manufacturers, cooperative and private, which gave us information on the composition of commercial concentrates. Appreciation is also expressed to Ulf Emanuelson for valuable advice on the statistical evaluations.

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Oltner, R. and Wiktorsson, H., 1983. Urea concentrations in milk and blood as influenced by feeding varying amounts of protein and energy to dairy cows. Livest. Prod. Sci., 10: 457467. Pehrson, B., Plym Forshell, K. and Carlsson, J., 1992. The effect of additional feeding on the fertility of high-yielding dairy cows. J. Vet. Med., A 39:187-192. Refsdal, A.O., Baevre, L. and Bruflot, R., 1985. Urea concentration in bulk milk as an indicator of the protein supply at the herd level. Acta Vet. Scand., 26:153-163. Ropstad, E. and Refsdal, A.O., 1987. Herd reproductive performance related to urea concentration in bulk milk. Acta Vet. Scand., 28: 55-63. Ropstad, E., Vik-Mo, L. and Refsdal, A.O., 1989. Levels of milk urea, plasma constituents and rumen liquid ammonia in relation to the feeding of dairy cows during early lactation. Acta Vet. Scand., 30: 199-208. Saitoh, M. and Takahashi, S., 1977. Embryonic loss and progesterone metabolism in rats fed a high energy diet. J. Nutr., 107: 230-234. SAS Institute Inc., 1985. SAS User's Guide: Statistics, 1985 Edition, Cary, NC: SAS Institute Inc. SLU-Info (Arbetsgruppen f'6r foderv~irdering och n~iringstillf6rsel till idisslare), 1991. Fodertabeller f6r idisslare (Feed tables for ruminants). Swed. Univ. Agric. Sci., Uppsala, Sweden. Stern, M.D., Santos, K.A. and Satter, L.D., 1985. Protein degradation in rumen and amino acid absorption in small intestine of lactating dairy cattle fed heat-treated whole soybeans. J. Dairy Sci., 68: 45-56. Tveit, B., Lingaas, F., Svendsen, M. and Skaastad. O.V., 1992. Etiology of acetonemia in Norwegian Cattle. 1. Effect of ketogenic silage, season, energy level, and genetic factors. J. Dairy Sci., 75: 2421-2432. Volden, H., 1990. Response of dietary AAT density on milk yield - review of production trials in Norway. In: Gustafsson, A.H. (ed.), Fat and protein feeding to the dairy cow. Proc., Swed. Ass. Livest. Breed. Prod., Eskilstuna, Sweden, pp. 81 e-89. RESUME Gustafsson, A.H. et Carlsson, J., 1993. Relations entre la charge GnergGtique et protGique de la ration alimentaire, la qualitG de l'ensilage et la teneur du lair en urge, et l'effet de ces facteurs sur la production de lair et les performances de reproduction des vaches laiti~res. Livest. Prod. Sci., 37:91-105 (en anglais). Un essai sur le terrain comprenant 29 troupeaux laitiers nourris avec de l'ensilage a Gt6 effectug afin de dGterrniner les effets de la qualitG de l'ensilage, des mGthodes d'estimation des protGines et de la quantitG d'urGe dans le lair sur les performances laitiGre et de reproduction. Le prGl6vement d'Gchantillons et le rassemblement des donnGes sur les rations alimentaires ont GtG faits chaque semaine et GvaluGs au niveau du troupeau. Les niveaux d'Gnergie et de protGines alimentaires ont GtGsupGrieurs aux besoins d'entretien ( 17% et 6-15% respectivement). L'effet d'une variation de la quantitG d'acides aminGs absorbGs dans l'intestin gr61e (AAT) sur la production de lait a GtG faible. La concentration de l'urGe dans le lait (moyenne 4,00 mM) a GtG positivement corrGIGe/~ la digestibilitG protGique brute (DCP), au rapport DCP/Gnergie et au bilan protGique dans le rumen (PBV), avec des corrGlations partielles respectives de 0,41, 0,43 et 0,4, mais n'Gtait pas corrGIGe h I'AAT et h la production de lait. L'intervalle v61age - premi6re (moyenne 82 jours) et derniGre (moyenne 108 jours) insGmination a GtG plus long dans les troupeaux/l faible ou forte concentration d'urGe dans le lair; l'intervalle le plus court a GtG observG entre 4,5 et 5 raM. L'intervalle v~lage - derni6re insGmination est prolongG de 2,2 jours pour chaque augmentation de 1% de la teneur en N ammoniac de l'ensilage (% d'N total). Ou



en d6duit que pour obtenir des performances de reproduction optimales, la concentration d'ur6e dans le lait doit ~tre sup6rieure ~ la moyenne actuellement observ6e, et qu'une d6t6rioration de la qualit6 prot6ique de l'ensilage peut r6duire les performances de reproduction. KURZFASSUNG Gustafsson, A.H. und Carlsson, J., 1993. Beziehungen zwischen dem Energie-und Proteingehalt des Futters, der Silagequalit~it und Milchharnstoffgehalt, und der Effekt dieser Faktoren auf die Milchleistung und Fortpflanzungseffizienz bei Milchkiihen. Livest. Prod. Sci., 3 7 : 9 1 105 (auf englisch). Ein Feldversuch wurde in 29 mit Vorwelksilage gef'titterten Milchherden ausgef'tihrt, um die Effekte von Silofutterqualit~it, Proteinbewertungssysteme und Milchhamstoffkonzentration auf Milchleistung und Fortpflanzungseffizienz festzustellen. Stichproben und Daten beziiglich Rationen wurden w6chentlich eingesammelt und auf Herdenniveau verwertet. Die Fiitterung der Herden lag bei Energie und Protein fiber dem Normbedarf ( 17% bezw. 6-15%). Schwankungen des Gehalts von Aminos~iuren absorbiert im Diinndarm (AAT) beeinflussten die Milchleistung nur wenig. Die Harnstoftkonzentration im Herdengemelk (Durchschnitt 4,00 mM) korrelierte positiv mit verdaulichem Rohprotein (DCP), DCP/Energie und Proteinbalance im Pansen (PBV), mit partiellen Korrelationen von 0,41, 0,43 bezw. 0,40, aber nicht mit AAT oder der Milchleistung. Das Intervall zwischen Kalbung und erster (Durchschnitt 82 Tage) und letzter (Durchschnitt 108 Tage) Besamung war l~inger bei sowohl niedrigen als auch hohen Milchharnstoffwerten und am kiirzesten bei Harnstoffwerten zwischen 4,5 bis 5,0 mM. Das Intervall von Kalben bis zur letzten Besamung verl~ingerte sich um 2,2 Tage/Prozentpunkt h6heres Ammoniak-N-Gehalt (% totaler N) in der Silage. Daraus wird geschlossen, dass ftir eine optimale Fortpflanzungsrate der Harnstoffgehalt im Herdengemelk h6her sein sollte als der vorhandene Herdenmittelwert und dass Verschlechterungen der Proteinqualit~it der Silage die Fortpflanzungseffizienz einer Herde reduzieren kann.