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Feed nitrogen conversion in lactating dairy cows on pasture as affected by concentrate supplementation
Animal Feed Science and Technology 131 (2006) 25–41
Feed nitrogen conversion in lactating dairy cows on pasture as affected by concentrate supplementation H˚avard Steinshamn a,b,∗ , Mats H¨oglind c , Torstein H. Garmo d , Erling Thuen d , Ulrik Tutein Brenøe d a
d
Department of Plant and Environmental Sciences, Norwegian University of Life Sciences, ˚ Norway P.O. Box 5022, N-1432 As, b Bioforsk-Norwegian Institute for Agricultural and Environmental Research, Organic Food and Farming Division, N-6630 Tingvoll, Norway c Bioforsk-Norwegian Institute for Agricultural and Environmental Research, Horticulture and Urban Greening Division, Postveien 213, N-4353 Klepp St., Norway Department of Animal and Aquacultural Sciences, Norwegian University of Life Sciences, ˚ Norway P.O. Box 5025, N-1432 As,
Received 15 March 2005; received in revised form 27 January 2006; accepted 8 February 2006
Abstract The effect of concentrate supplementation on nitrogen (N) intake and excretion in grazing lactating dairy cows was determined in three herds in Norway. Grazing trials were conducted with each herd in June and August for two consecutive years. The average supplementation was 1.8 (S.D. 2.1) kg DM/day, and the concentrate was based on grain with a N content ranging from 18.7 to 24 g/kg DM. Herbage DM and N intake were reduced with increasing supplementation, but total DM and N intake increased. Milk yield and protein content increased by 1.1 kg milk and 0.28 g protein per kg milk for each kg extra concentrate. Milk N excretion increased with increasing supplementation (6.5 g N/kg DM), and N utilisation improved by 11.7 g N per kg N intake per kg extra concentrate. Excretion of urine N and its share of total excreta N decreased by 4.0 and 9.2 g/kg concentrate, respectively. The Abbreviations: AAT, amino acid absorbed in the intestine; aNDF, neutral detergent fibre; DM, dry matter; FEm, net energy lactation (6.9 MJ); IVDMD, in vitro dry matter digestibility; N, nitrogen; OM, organic matter; PBV, protein balance in the rumen; S.D., standard deviation; S.E., standard error of estimate ∗ Corresponding author. Tel.: +47 71532027; fax: + 47 71532001. E-mail address: [email protected] (H. Steinshamn). 0377-8401/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.anifeedsci.2006.02.004
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reducing effect on urine N excretion of supplementation was, however, numerically low compared with other studies, most likely due to a high crude protein content of the concentrate used. © 2006 Elsevier B.V. All rights reserved. Keywords: Dairy cattle; Concentrate; Nitrogen excretion; Nitrogen efficiency; Grazing; Milk production
1. Introduction The agricultural sector is the major contributor to anthropogenic reactive nitrogen (N) enrichment, and only a small part of the reactive N input to the sector is recovered in agricultural produce (Bleken and Bakken, 1997; Van der Hoek, 1998; Galloway et al., 2003). Nitrogen dissipates through volatilisation and leaching and perturbs the environment in various ways (Galloway et al., 2003). A large part of the losses is volatilisation of ammonia (NH3 ). Agricultural activity and livestock production, e.g. dairy cows, are the main sources of NH3 emissions to the atmosphere (Asman et al., 1998; Hutchings et al., 2001; SSB, 2005). Seventy percent or more of cows’ daily N intake is excreted in faeces and urine (Tamminga, 1992). Whereas faecal N excretion varies little with the dietary composition, urinary N varies with an excess of ingested protein or disequilibrium between protein and energy (V´erit´e and Delaby, 2000). Thus, N fed in excess of animal or rumen requirement will essentially be excreted in urine. Increased proportion of urine in the excreta may elevate NH3 losses as volatilisation from urinary N is several fold higher than from faecal N (Lockyer and Whitehead, 1990; Petersen et al., 1998). This is because most of the N in urine is in the form of urea that is rapidly hydrolyzed to NH3 in the soil, whilst faecal N is strongly bound in complex organic compounds. Pastures can provide cheap, high quality feed, enabling high milk yields with little or no supplementation (Ettala et al., 1986). This is partly due to the cow’s ability to select young plant parts, leaves and stems, which are high in energy and protein. However, high quality herbage may supply the cows with excessive amounts of N, which the animals cannot utilise and thus excrete in the urine. Volatilisation of NH3 -N is the most important route of N loss from grazed pastures (Jarvis, 1994). The utilisation of N by dairy cows can be improved in many ways, e.g. by reducing N intake, improving the output of N in milk and by improving N capture in the rumen (Tamminga, 1992; Bussink and Oenema, 1998). Partial replacement of pasture herbage with concentrated feeds low in protein and high in energy, like maize silage, has been shown to improve N utilisation and reduce N losses during grazing (Valk, 1994). However, in cool regions such as most of Scandinavia, maize can only be grown to a limited extent. Supplementation with grain based concentrates with low protein content might serve the same purpose and may be an alternative. The effects of supplementation and diet composition on feed intake, milk production and N utilisation in dairy cows fed during housing have been investigated in numerous studies (e.g. Dewhurst et al., 1996; Kebreab et al., 2000, 2001; Castillo et al., 2001; Frank and Swensson, 2002). Data on N intake, excretion and utilisation in grazing dairy cows, and the effect of domestically grown grain supplements on these parameters are scarce. Supplementation with concentrates with high starch proportion and very low N content (8.5 g N/kg DM) has improved the N utilisation to milk and strongly decreased the excretion of urine N and its proportion of excreta N in
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dairy cows on pasture (Berry et al., 2001). In Norway, the commercial concentrates used for dairy cows on pasture are, however, based on locally produced barley and oats with a N content of approximately 18 g N/kg DM (Institutt for husdyr og akvakulturvitenskap and Mattilsynet, 2005). The objective of the present study was to investigate the effects of concentrate level on N intake, the partition of N into milk, urine and faeces and the efficiency of N utilisation by rotationally grazing cows. 2. Materials and methods 2.1. Location The study was carried out in three different herds. Herds 1 and 2 were located at the Animal Production Centre, Norwegian University of Life Sciences (59◦ N, 10◦ E) and herd ˚ regional prison (58◦ N, 5◦ E). In all three herds, mea3 was located at the farm of the Ana surements were carried out in 1999 and 2000, and within each year in two periods: June and August. Herd 1 was managed according to organic standards, whilst herds 2 and 3 were conventionally managed. 2.2. Herds and supplementary feeding 2.2.1. Animals In total, 134 dairy cows of the Norwegian Red Cattle breed were used, of which 25 cows were used in all four periods, 23 cows in three periods, 52 cows in two periods and 34 cows in one period, altogether 307 observations. Of the 307 observations, 105 were from primiparous cows whilst 202 observations were from multiparous cows. The cows in herds 1 and 3 calved between January and May, and were thus in early and mid lactation during the experimental periods. The cows in herd 2 calved mainly between August and December and were thus in late lactation (Table 1). The cows in herds 2 and 3 consisted of two breeding lines with different genetic merit for milk production. Table 1 Number of cows, average number of lactations, days in milk production and concentrate supplementation (kg DM/day) in the sample collection periods of the three herds Herda
Grazing period
No. of cows
Lactations
Days in milk
Concentrate (kg DM/day)
1999 2000
1999
2000
1999
2000
1999
2000
1
June August
10 17
16 16
2.7 (1.6) 2.5 (1.4)
3.5 (1.8) 3.1 (1.5)
99 (32) 168 (46)
101 (41) 146 (51)
0.87 (1.46) 0.26 (0.42)
0.46 (0.85) 0.37 (0.73)
2
June August
36 22
50 39
2.5 (1.3) 2.3 (1.4)
2.6 (1.5) 2.4 (1.6)
220 (64) 253 (60)
208 (71) 246 (62)
2.90 (2.86) 1.82 (2.52)
1.96 (2.03) 1.53 (1.77)
3
June August
25 29
21 26
1.9 (1.2) 2.2 (1.2)
1.7 (1.1) 1.9 (1.0)
100 (30) 151 (33)
102 (34) 150 (36)
1.98 (1.85) 2.35 (1.89)
1.98 (1.63) 2.58 (1.79)
Standard deviation is in brackets. a See text for explanation of herd.
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Table 2 Ingredient composition of concentrate supplements used Ingredients (g/kg fresh weight)
Herd 1a
Barley Oat Wheat bran Maize Sugar beet pulp Soya Rape seed Cane molasses Fish meal/silage Urea Fat Minerals and vitamins
1000 – – – – – – – – – – –
a b
Herd 2a
Herd 3a,b
1999
2000
403 219 158 – – 49 10 79 25 – 19 38
317 311 49 – 88 58 10 78 26 – 19 44
371 150 – 250 – – 50 70 60 6 10 33
See text for explanation of herd. The figures are from 2000. The composition was similar in 1999 but with some higher urea content.
2.2.2. Concentrate Each year at calving, the cows in herds 1 and 2 were divided into three and four treatment groups, respectively, balanced for calving date and parity. In herd 1, the groups were fed three different levels of concentrate: 2, 4 and 8 kg per cow per day, and the groups of herd 2 were fed four different levels of concentrate: 4, 8, 12 and 16 kg per cow per day for 30 days. Thereafter the level was reduced by 1 kg per cow every month in all treatments throughout the lactation, independent of the grazing period. The concentrate supplementation for each cow within herds 1 and 2 during the experimental periods is thus a function of treatment group and calving time. In herd 3, consisting of ca. 100 animals, a group of 30 cows was selected each year for the experiment. The experimental cows were allocated to 10 blocks according to milk yield, calving date and lactation number. The three animals in each block were randomly allocated to three different concentrate levels: 0, 3 or 6 kg per cow per day that were given throughout the experiment. The average and the variation of supplementations for each period by herd and year are presented in Table 1. In herd 1, rolled barley was used as a concentrate, whereas commercial grain based mixtures were used in herds 2 and 3 (Table 2). The commercial mixtures contained large shares of barley and oats (>0.50), and were developed for grazing dairy cows with low values for the protein balance in the rumen (PBV, Madsen et al., 1995). 2.3. Pasture management Within each herd the cows were grazed as a single group in fenced paddocks. The paddocks were rotationally grazed three to four times per season. The pasture for herd 1 has been organically managed since 1993, and showed high plant species diversity (Steinshamn et al., 2001). The sward had a high content of white clover (Trifolium repens); >0.25 proportion of the DM yields. The dominating grass species were Poa pratensis and Festuca pratensis. The pasture used for herd 2 was dominated by sown grasses, containing Festuca pratensis,
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Phleum pratense and Poa pratensis. The pasture used for herd 3 was also grass based with Lolium perenne (0.60–0.70) and Poa spp. (0.20–0.30) as the most important species. It had a very low content of herbs. The history of this sward is unknown, but according to the present manager it had not been reseeded for the last 30 years, except for occasional, partial surface seeding. The organically managed pasture for herd 1 received no fertilisation or manure except for the urine and dung that was excreted by the cows during grazing. The pasture for herd 2 was fertilised with 30 kg P and 100 kg K/ha at the beginning of each growing season. Nitrogen was applied to a level of 200 kg N/ha, dressed at three occasions. The pasture for herd 3 received 23 tonnes of cattle slurry (ca. 40 kg N/ha) at the beginning of each season, followed by one application of compound fertilizer (80 kg N, 5 kg P and 60 kg K/ha) and three applications of N fertilizer (25–40 kg N per ha per application), i.e. a yearly application of ca. 210 kg/ha. 2.4. Sample collection Two periods of 12 days each were chosen to represent mid (June) and late (August) grazing season in both years. The measuring periods were subdivided into 7 days of adaptation and 5 days of main data and sample collection. On collection days, milk yields were recorded daily and samples were aliquot pooled within period for each cow. Individual herbage intake was estimated in the same periods using the alkane double-indicator technique (Mayes et al., 1986). In herds 1, 2 and in the first year in herd 3, the animals were dosed twice daily after milking with alkane labelled cellulose bungs each containing 315 mg Dotriacontane (C32 ), giving a daily dose of 630 mg. In herd 3 in 2000, intra ruminal controlled release capsules (CRC, Captec® Ltd., New Zealand) releasing a constant daily amount of C32 and C36 at a rate of 400 mg/day were used. The devices were administered 7 days before the faecal collection periods. Over the 5 days of measurement, spot faecal samples were collected at dawn and afternoon from fresh dung pats of known individual animals or obtained by rectal sampling. During the faecal sampling periods, daily samples of the herbage were obtained by cutting to 3 cm stubble height with hand-held scissors from within sixteen 0.25 m2 quadrates. The herbage samples were pooled to one sample per day. After collection, faeces samples were frozen at −20 ◦ C and then thawed and pooled for each animal and period. Subsamples of faeces and the herbage samples were dried at 60 ◦ C for 48 h. 2.5. Sample analysis Dried feed and faecal samples were milled through a 1.00 mm screen. The feed DM and ash content were determined after drying overnight at 105 ◦ C and after 13 h at 550 ◦ C, respectively. Crude fibre was determined according to AOAC (1990, method 962.09) and fat after extraction with petroleum ether according to the EU directive 71/393/EEC (OJ L15, 1971). Neutral detergent fibre (aNDF) was determined according to Van Soest et al. (1991) with the use of ␣-amylase using the ANKOM Fibre Analyser (ANKOM Technology, Fairport, NY). In vitro DM digestibility (IVDMD) was determined according to Tilley and Terry (1963) for the samples collected from herds 1 and 2 and by 48-h incubation with
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rumen fluid in an ANKOM fibre apparatus (ANKOM Technology, Fairport, NY) for the samples taken from herd 3. Nitrogen contents in herbage, concentrate and faeces were determined by the Kjeldahl method on a Kjeltec Auto 1035 analyser (Tecator AB, H¨ogan¨as, Sweden). Milk protein content was determined using an infrared technique (Milcoscan 255 A/B; Foss Electric, Hillerød, Denmark) and milk N was estimated by assuming 15.67 g N/100 g in milk protein. The n-alkane extraction process of feed and faeces was based on the method described by Mayes et al. (1986) and the contents of n-alkane were determined on a gas chromatograph (Perkin-Elmer Autosystem) equipped with a capillary column (SGE 054657; 12 m long, 0.53 mm inner diameter). 2.6. Calculation and statistical analysis The herbage and concentrate energy concentration was calculated as net energy lactation according to Van Es (1978) and expressed as FEm (1 FEm = 6.9 MJ net energy lactation) using the chemical analysis and equations given in the standard feed tables (Institutt for husdyr og akvakulturvitenskap and Mattilsynet, 2005). Amino acids absorbed in the intestine (AAT) and PBV contents were calculated as described by Madsen et al. (1995), using the chemical analysis and degradability values from standard feed tables (Institutt for husdyr og akvakulturvitenskap and Mattilsynet, 2005). The NH3 volatilised during drying the faecal samples was corrected for by using the following regression equation: N content wet sample (g/kg DM) = 4.48 (S.E. = 1.15) + 0.947 (S.E. = 0.050)×N content dry sample (g/kg DM), r = 0.832, 2
RMSE = 1.07,
n = 74
The equation is based on analysis of samples collected from N balance studies carried out at the Department of Animal and Aquacultural Sciences, Norwegian University of Life Sciences (Harald Volden, Personal communication). Herbage intake was estimated according to the equations presented by Mayes et al. (1986), using C32 as the external marker to provide an estimate of faecal output and C31 as a digestibility marker. Alkane C32 (dosed) and C33 (natural) have similar recovery rates and are preferred for estimating intake. However, in our herbage samples C33 concentration was rather low; C31 concentrations of herbage (means ± S.D. = 209 ± 8 mg/kg DM) were three-fold that of C33 (means ± S.D. = 75 ± 3 mg/kg DM). Penning (2004) recommends to use the C31 :C32 pair under such conditions, and Estermann et al. (2001) found a higher accuracy of estimation of known DM intake with C31 :C32 than with C33 :C32 and a similar recovery rate for C31 , C32 and C33 . Faecal output was calculated according to the following equation: Faecal output (kg DM per day) =
rate of dosed marker (C32 ) × recovery rate (0.87) concentration of marker (C32 ) in faeces
Quantities of N ingested and N excreted in the faeces were calculated from the herbage and concentrate DM intake and faecal DM output, respectively, and from herbage, concentrate and faecal N content. Animal body weight (BW) was recorded twice in each
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experimental period by a mobile scale. Daily BW change was calculated by fitting curves to the weight measurements taken during the three last weeks indoors prior to grazing, measurements taken during the grazing season and the first three weeks indoors after grazing. Body N retention was estimated by assuming 26 g N/kg of BW change (Gibb et al., 1992), and urine N excretion was estimated from N balance calculations. The effect of concentrate (C) was evaluated by treating it as a regression effect in the statistical analysis. Statistical analyses were carried out using MIXED procedures (method = reml) of SAS (SAS, 1999). The model was: Yijklm = μ + hi + tj (hi ) + Lk + B1 C + B2 C2 + B3 D + V (al(i) ) + cov(eijklm ) where Yijklm is a response parameter (DM intake, milk yield, protein content in milk, N intake and excretion and N utilisation); μ is the overall mean; h is the random effect of herd (i = 1, 2, 3); t(h) = within herd period random effect (j = 1, 2, 3, 4) where periods 1 and 2 are June and August in 1999, respectively, and period 3 is June and 4 August in 2000; L is the fixed class effect of lactation number 1 or older (≥second lactation), k = 1, 2; B1 , B2 and B3 are the regression coefficients for the fixed linear and quadratic effects of concentrate level (C, kg DM/day) and days in milk (D, days), respectively; a is the random effect of animal within herd. As most cows were present in more than one grazing period, the covariation within animals V(al(i) ) was accounted for in an analysis of repeated measures. The optimal covariance structure (cov(eijklm )) was assessed for each parameter with attention to the Schwartz Baysian information criterion (Littell et al., 1998). Determination of the significance of fixed terms was through Wald tests with denominator degrees of freedom estimated by Satterhwaite’s approximation. Days in milk were included as a covariate in order to adjust for the effect of stage of lactation. The interactions between the fixed effects were included when the significant level exceeded the 5% level, which sometimes occurred for L × D, but never for the L × C and C × D interactions. The quadratic effect of concentrate was never significant and was therefore not included in the final model The random effect of h and t(h) on the regression coefficient of Y on C (random slope effect), judged by the likelihood ratio test, was not significant and was therefore not included as a random term. Thus, the individual regressions within h and t(h) are all parallel lines with different random intercepts. The random effect of h did not improve the model, as judged by the likelihood ratio test, and was therefore omitted in the final model. The relationship between total N intake and faecal N excretion, urinary N excretion and milk N excretion was tested for linear and curvilinear effects accounting for the covariation within animal and using h and t(h) as a random effect as described above. Information on using the mixed model methodology in analysis of data from multiple studies is given by St-Pierre (2001). Relationships between predicted and observed parameters were estimated with linear regression. The effect of breeding line, in herds 2 and 3, was not accounted for in the statistical analysis, which consequently increased the error.
3. Results Animal and concentrate characteristics are presented in Tables 1 and 2, respectively. The concentrate supplementation varied from 0 to 10 kg, but the overall average was only
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1.8 kg DM per cow and day with a standard deviation of 2.05. The herbage quality, Table 3, varied, but for most periods it can be characterised as fair with medium digestibility of organic matter (OM) and FEm (0.77–0.93 FEm/kg DM) and medium to low contents of aNDF (371–559 g/kg DM). The concentrates were all grain based and had a negative PBV (Table 3). The estimated herbage DM intake, milk yield and milk protein content are presented in Table 4. The mean intake and excretion of N are presented in Table 5. There was a large variation in herbage N intake and excretion both between herds and within herds, and within grazing periods. However, overall mean and median values were very similar, indicating that the data were normally distributed (figures not shown). For total N intake, the mean value was higher than the median, indicating that the distribution was biased towards low intake. This was due to the fact that the concentrate supplementation was biased towards low levels. 3.1. Effect of concentrate on DM intake and animal performance Corrected for days in milk, each additional kg of concentrate increased the total DM intake by 0.31 kg DM/day, milk production by 1.1 kg/day and the milk protein concentration by 0.28 g kg−1 (Table 6). The reduction in herbage DM intake was 0.69 kg herbage DM per kg concentrate DM intake. First parity cows had lower dry matter intake and milk production than older cows. The difference became less in late lactation than in early lactation as indicated by the significant positive interaction (P<0.05) between parity and days in lactation (Table 6). The effect of concentrate was, however, not influenced by parity or days in milk. 3.2. Nitrogen intake and excretion Significant positive linear relationships were observed between N intake and milk, faecal and urine N excretion (Fig. 1). Milk, faecal and urinary N output increased by 13%, 17% and 71%, respectively, per unit increase in N intake. 3.3. Effect of concentrate on herbage nitrogen intake and excretion Concentrate supplementation was significant (P<0.05) for all parameters regarding N intake and excretion (Table 6). Each additional kg of concentrate reduced herbage N intake by 17.0 g/day, but total N intake increased by 6.2 g/day. The amount of N secreted to milk increased by 6.5 g per day per kg concentrate. The excretion of N in urine was reduced by 4.0 g/day whilst the faecal N losses increased by 2.8 g/day for each extra kg concentrate. Thus, the total loss of N in excreta (urine + faeces) was unaffected (b1 = −1.1 g N/kg DM, P=0.57) by concentrate supplementation (data not shown). As for DM intake, first parity cows had lower N intake and less N excretion in milk, faeces and urine than older cows. Except for urinary N excretion, this effect of parity diminished with increasing number of lactating days (Table 6). 3.4. Effect of concentrate on nitrogen utilisation The utilisation of feed N to milk N increased by 11.7 g N/kg N intake for each additional kg of concentrate (Table 6). Supplementation reduced the proportion of the N intake that
Table 3 Chemical composition of the pastures and the concentrate supplements fed in the sample collection periods of the three herds Herd 1
Herd 2
1999
2000
Herd 3
1999
2000
1999
2000
Augusta
Junea
Augusta
Junea
Augusta
Junea
Augusta
Junea
Augusta
Junea
Augusta
Pasture n DM (g/kg) Nb (g/kg DM) OMc (g/kg DM) aNDFd (g/kg DM) OM digestibility FEme (kg−1 DM) PBVf (g/kg DM) AATg (g/kg DM)
8 179 22.1 922 453 0.767 0.89 −14 88
8 194 28.5 900 371 0.767 0.91 23 89
6 188 25.9 912 446 0.783 0.92 5 90
7 156 29.7 901 452 0.783 0.93 30 90
8 181 23.1 922 482 0.761 0.89 −8 88
8 250 20.0 914 475 0.751 0.86 −21 85
8 198 32.6 913 493 0.750 0.91 44 90
7 199 23.4 914 438 0.742 0.86 −2 86
3 170 28.2 894 477 0.799 0.93 17 91
3 257 20.3 930 559 0.700 0.81 −16 83
4 188 30.1 914 549 0.675 0.83 40 85
6 170 22.4 919 536 0.653 0.77 3 79
Concentrate (n = 1) DM (g/kg) Nb (g/kg DM) OMc (g/kg DM) aNDFd (g/kg DM) OM digestibility FEme (kg−1 DM) PBVf (g/kg DM) AATg (g/kg DM)
868 19.6 972 183 0.821 1.09 −37 100
868 19.6 972 183 0.821 1.09 −37 100
847 18.7 972 263 0.832 1.11 −43 99
847 19.7 971 311 0.818 1.08 −36 99
863 21.0 933 162 0.769 1.11 −15 98
863 21.0 933 162 0.798 1.09 −15 98
867 22.1 917 311 0.808 1.03 −15 94
886 21.7 925 351 0.769 1.00 −14 89
872 24.0 – – – 1.11 −10 105
872 24.0 – – – 1.11 −10 105
865 23.2 – – – 1.16 −15 105
865 23.2 – – – 1.16 −15 105
33
See text for explanation of herd. a Period. b Nitrogen. c Organic matter. d Neutral detergent fiber including residual ash (Van Soest et al., 1991). e Net energy of lactation, 1 FEm = 6.9 MJ. f Protein balance in the rumen. g Amino acids absorbed from the intestine.
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Junea
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Table 4 Mean herbage DM intake (kg/day), milk yield (kg/day) and milk protein content (g/kg) in the sample collection periods of the three herds Herda
Grazing period
Herbage DM intake (kg/day)
Milk yield (kg/day)
Milk protein content (g/kg)
1999
2000
1999
2000
1999
2000
1
June August
13.0 (2.7) 14.5 (2.1)
12.7 (2.5) 13.5 (2.8)
19.1 (3.8) 17.6 (3.6)
21.8 (4.2) 18.9 (4.6)
31.5 (1.9) 31.0 (1.9)
29.6 (2.1) 31.4 (1.7)
2
June August
10.9 (2.6) 15.6 (3.1)
12.3 (2.7) 15.4 (3.2)
14.8 (5.6) 14.4 (5.5)
17.5 (5.0) 13.2 (4.6)
33.1 (2.3) 34.7 (2.4)
33.1 (2.7) 36.0 (3.5)
3
June August
16.2 (3.2) 12.6 (2.6)
8.6 (2.6) 15.8 (4.8)
21.4 (3.3) 19.2 (3.7)
20.8 (4.2) 22.3 (4.0)
31.6 (2.2) 31.5 (3.1)
30.8 (3.2) 32.6 (2.1)
Standard deviation is in brackets. a See text for explanation of herd.
Fig. 1. Urinary, faecal and milk N excretion plotted over the N intake range observed for the cows in the study (milk N (g/day) = 42.5 (S.E. ± 7.33) + 0.129 (S.E. ± 0.0156)×N intake (g/day); R2 = 0.43; P<0.0001, Faecal N (g/day) = 41.2 (S.E. ± 5.07) + 0.174 (±S.E.0.0167)×N intake (g/day); R2 = 0.90; P<0.0001, urine N (g/day) = −97.3 (S.E. ± 9.41) + 0.710 (S.E. ± 0.0215)×N intake (g/day); R2 = 0.92; P<0.0001).
was excreted as N in excreta (urine + faeces) by 12.2 g N/kg N intake and changed the partitioning of the N excretion between urine and faeces; the urine N proportion of N intake decreased by 13.8 g, whilst the faecal N proportion increased by 2.6 g N/kg N intake. Thus, the proportion of urine N to total excreta N excretion decreased by 9.2 g N/kg N intake for each kg DM supplementary increase.
4. Discussion Substitution rate (SR), or the reduction in pasture DM intake per kg concentrate DM intake, is a factor which may explain the variation in milk response (MR) to supplementation. In general, a negative relationship exists between SR and MR (Leaver, 1985), but there are several pasture, animal and supplement factors that will influence this relationship (Bargo et al., 2003). According to Bargo et al. (2003), there is an inconsistent relationship between
Herd 1
Herd 2
1999
N (g/day) n Total intake Herbage intake Faecal excretion Urine excretion Milk excretion Retention
2000
Herd 3
1999
2000
1999
2000
June
August
June
August
June
August
June
August
June
August
June
August
10 304 (58) 287 (60) 110 (22) 101 (29) 94 (16) −1 (7)
17 419 (61) 414 (61) 122 (17) 204 (42) 85 (16) 7 (6)
16 337 (57) 328 (65) 116 (12) 115 (44) 101 (18) 6 (7)
16 409 (79) 401 (83) 120 (14) 189 (62) 92 (20) 7 (7)
36 318 (54) 250 (61) 101 (19) 130 (32) 76 (28) 12 (9)
22 355 (49) 313 (62) 119 (17) 145 (36) 77 (27) 14 (6)
50 444 (69) 400 (87) 110 (18) 233 (57) 90 (24) 11 (7)
39 397 (55) 361 (69) 107 (15) 203 (35) 73 (23) 14 (5)
25 506 (76) 458 (90) 107 (19) 285 (49) 106 (18) 8 (7)
29 313 (45) 257 (53) 105 (14) 101 (32) 95 (22) 12 (9)
21 304 (71) 259 (79) 76 (11) 115 (54) 100 (21) 13 (9)
26 414 (106) 353 (108) 86 (14) 200 (82) 114 (24) 13 (8)
Standard deviation in brackets. See text for explanation of herd.
H. Steinshamn et al. / Animal Feed Science and Technology 131 (2006) 25–41
Table 5 Mean nitrogen (N) intake and excretion (g/day) in each grazing period of each year and herd
35
36
S.E.a
C
S.E.a
P
Lb
S.E.a
P
D
S.E.a
DM intake (kg/day) Total 15.9 Herbage 15.9
0.94 0.94
0.31 −0.69
0.09 0.09
<0.001 <0.001
−3.5 −3.5
0.82 0.82
<0.001 <0.001
−0.005 −0.005
0.003 0.003
Milk production Milk (kg/day) Protein (g/kg)
0.91 0.59
1.09 0.28
0.12 0.081
<0.001 <0.001
−7.4 −0.4
0.99 0.31
<0.001 0.186
−0.046 0.026
Y
N intake (g/day) Total Herbage
a
24.9 27.8
L*Db
S.E.a
0.933 0.898
0.010 0.010
0.004 0.004
0.004 0.002
<0.001 <0.001
0.030 ns
0.005
P
400 402
26 26
6.2 −17.0
2.3 2.3
0.009 −95 <0.001 −95
21 21
<0.001 <0.001
−0.11 −0.11
0.08 0.08
0.726 0.758
0.27 0.27
0.11 0.11
N excretion (g/day) Milk 114 Faeces 111 Urine 185
5 5 18
6.5 2.8 −4.0
0.6 0.6 1.6
<0.001 −40 <0.001 −19 0.012 −32
5 5 6
<0.001 <0.001 <0.001
−0.17 −0.05 ns
0.02 0.02
<0.001 0.599
0.16 0.07 ns
0.03 0.03
N output/input (g/kg) Milk 300 Faeces 279 Urine 419 Faeces + urine 703
16 16 29 18
11.7 2.6 −13.8 −12.2
1.6 1.1 2.3 1.5
<0.001 −46 0.025 25 <0.001 −22 <0.001 31
15 4 9 15
0.002 <0.001 0.014 0.035
−0.40 ns 0.25 0.22
0.06
<0.001
0.08
0.08 0.06
0.003 0.018
0.24 ns ns −0.18
U/(U + F)
30
−9.2
2.3
<0.001 −31
9
<0.001
0.20
0.08
0.015
ns
582
0.08
RMSE
R2
0.028 0.028
1.7 1.7
0.749 0.789
<0.001
1.7 1.2
0.906 0.841
P
0.015 41 0.014 41 <0.001 0.004
0.796 0.833
8 8 39
0.911 0.847 0.725
0.003 32 25 51 0.028 33
0.802 0.812 0.776 0.792
52
0.753
P, probability value of P>|t|: estimate significant different from zero or estimate of the difference 1. Lactation vs. ≥2 lactation significant different from zero. RMSE, root mean square error. a S.E., standard error of estimate or mean standard error of difference among the effects of 1. Lactation cows and cows ≥ 2 lactations. b Estimates for L refers to cows in first lactation, cows ≥ 2 lactations have the value 0.
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Table 6 Estimates for the relationship between concentrate supplementation (C, kg DM/day), parity (L, 1 or ≥2) and days in milk (D) on total and herbage DM intake (kg/day), milk yield (kg/day), milk protein content (g/kg), total and herbage N intake (g/day), N excretion (g/day), N excretion as the proportion of N intake (g/kg) and urine N excretion as the proportion of total urine N and faecal N excretion (U/(U + F) g/kg)
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37
the amount of supplement and the MR and SR in grazing studies with highyielding dairy cows, and when using data from four experiments they could not find any relationship between MR and SR. The simultaneously high SR (0.69 kg pasture/kg concentrate) and MR (1.1 kg milk/kg concentrate) estimated in the present study contrast findings in older reviews (Leaver, 1985). However, recent studies and reviews report similar SR and MR to supplementation by dairy cows at milk yield levels, days in lactation and level of concentrate feeding as in the present study (Bargo et al., 2003; Delaby et al., 2003). Using the estimated herbage and concentrate net energy content (FEm) in Table 3, it was calculated that the total net energy intake increased by 0.51 FEm (3.51 MJ net energy lactation) per kg concentrate DM. The marginal response was therefore 0.46 FEm (3.17 MJ) per kg milk, which is, when adjusting for milk fat and protein content, close to the standard energy requirement for milk production (Van Es, 1978). The estimated SR of 0.69 kg pasture per kg concentrate indicates a relatively high pasture quality and pasture allowance (Dixon and Stockdale, 1999; Bargo et al., 2003), although the chemical analysis of the herbage samples indicates medium and even for some periods rather low quality (Table 3). However, as the cows are selective, particularly at high pasture allowances, the quality of the pasture actually consumed might have been better than indicated by the analysis, as the herbage sampling method did not account for this fact. The SR is also reported to increase with the amount of concentrate (Leaver, 1985), but the present data do not indicate such a relationship. Although, the range in concentrate feeding was wide in the present study, most of the supplementation was in the lower range (average 1.8 kg DM/day S.D. 2.05). The marginal response of 0.28 g protein per kg milk per extra kg concentrate is similar to the 0.22 g/kg reported by Delaby et al. (2003), but higher than the 0.10 g/kg presented by Bargo et al. (2003). This positive response of milk protein content was likely due to the additional energy input with supplementation that stimulated the production of glucogenic nutrients in the rumen, which in turn stimulated protein synthesis (Coulon and Remond, 1991; Rigout et al., 2003). Ruminal fractional passage rate is high at pasture, and starch dominated concentrate supplementation may increase the ruminal escape of dietary starch. This has shown to increase both milk protein production and decrease urinary N excretion (Reynolds et al., 2001). Thus, the increased secretion of milk N per unit N intake was likely due to both increased milk production and increased protein synthesis. The linear increase of urinary N output by 71% per unit increase in N intake (Fig. 1) is very similar to the 72% found by Mulligan et al. (2004). Others have found an exponential increase in urinary N excretion as N intake increased (Castillo et al., 2000; Kebreab et al., 2001). Albeit discrepancy in the functional form, it is evident that urine appears to be the main route of N excretion in dairy cows. Outputs of faecal and milk N increased by less than 20% per unit increase in N intake, and the rates found in the present study agree with other observations (Castillo et al., 2000; Kebreab et al., 2001; Mulligan et al., 2004). Linear regression between total N intake (Ni , g/day) and the efficiencies of conversion of dietary N to milk (Ncm , g/g) and faecal N (Ncf , g/g) resulted in the following equations: Ncm = −0.00028 × Ni + 0.36, R2 = 0.48 Ncf = −0.00029 × Ni + 0.40,
R2 = 0.88
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The coefficients are almost identical to those reported by Castillo et al. (2000), who used data from N balance experiments published in the literature; −0.0002/g for milk and −0.0003/g for faecal N excretion. The reduction in urinary N excretion by supplementation was probably due to incorporation of NH3 -N, which was produced from the highly degradable herbage protein, into microbial protein in the rumen (Van Vuuren et al., 1993; Reis and Combs, 2000; Bargo et al., 2002, 2003). With increased concentrate supplementation, more fermentable OM was available for microbial growth. Consequently, more dietary N was incorporated into microbial protein, even though the total N supply also increased with supplementation. A reduction of NH3 -N by supplementation is either associated with improved utilisation of NH3 -N or with a reduction in total N intake due to low N content in the supplement. Rumen NH3 -N concentration was not measured in the present study, but presumably it was reduced by supplementation and, most likely, the reduction was caused by an increased microbial synthesis as the total N intake increased. As the rates of urinary and faecal N excretion with increasing supplementation were inversely related, but of the same order of magnitude, the total N excretion was not affected. The amount of N excreted in dairy cow faeces is relatively constant and consists mainly of metabolic faecal N and undigested material and is thus closely linked to DM intake (Peyraud et al., 1995). However, the proportion of urine N of total N excreted in faeces and urine was reduced. The proportion averaged 0.60 of the total excreta return, which is close to the typical 0.57 on pasture (Jarvis, 1994). Reduced proportion of urinary N of total N excreted is important, even when total N excretion remains the same, as it presumably reduces the NH3 volatilisation rate (Lockyer and Whitehead, 1990; Petersen et al., 1998). Although statistically significant, the effect of concentrate supplementation on herbage N intake and N excretion and utilisation was numerically small. Supplementation reduced the herbage N intake as anticipated, but the total N intake, from herbage and concentrate, increased. Nitrogen contents in the supplements was probably too high, although the commercial feed used trials 2 and 3 were designed for pasture conditions. Berry et al. (2001) used concentrate with a N content of 8.5 g/kg DM to dairy cows at similar N production and N intake as in the present study. The N content ranged between 19.6 and 24.0 g/kg DM in our study, approximately the same as the herbage N content (Table 3). Berry et al. (2001) found that supplementation with the low protein concentrate improved N utilisation (N in milk relative to N intake) by 13% kg−1 , whilst in the present study the improvement was only 5% kg−1 concentrate. Excretion of urine N and its proportion of excreta (urine + faeces) N decreased in the present study by approximately 2.4% and 1.5% kg−1 supplementation, respectively, whilst Berry et al. (2001) found 11% and 4% reductions. This illustrates that it should be possible to achieve higher transfer efficiencies than found in our study by using concentrate with a lower N content. The commercial mixtures currently used in Norway are too high in protein if the aim is to reduce urinary N and the proportion of urine N of the total N excretion and thereby significantly reducing the potential N losses to the wider environment. On the other hand, as V´erit´e and Delaby (2000) argue, an improvement of N utilisation at the individual cow level does not imply improvement on a higher system level. Supplementation makes it possible to have a higher stocking rate or to extend the grazing season, and therefore, the amount of N excreted per area grazed may remain the same or even increase. Supplementation also transfers N from grain production areas to pasture
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areas via animal excreta, either within the farm itself or at a larger (regional, national) scale. An analysis of dairy farm N balance studies across Europe showed that the amount of N that dissipates from the farm system in order to produce 1 kg of N in milk + meat increased linearly with feed N purchase, especially when taking into account the dissipation related to the production of the purchased feed (Bleken et al., 2005).
5. Conclusions The present study provides data on the potential to improve N utilisation in dairy cows on pasture by supplementation with grain based concentrates. It was demonstrated that increasing concentrate supplementation resulted in a higher proportion of N in milk, less total urinary N excreted, and a lower proportion of urinary N of the total N excreted. This reduced the potential NH3 -N loss from the pasture. However, the improved utilisation of N by increasing concentrate supply was rather small compared to similar studies. This was mainly due to a high N content in the concentrate used.
Acknowledgements We thank the staff of the Animal Production Centre, Norwegian University of Life ˚ regional prison farm for grazing management. The authors Sciences and the staff at the Ana are grateful to Henrik Stryhn, Atlantic Veterinary College, University of Prince Edward Island and Normand St-Pierre, Department of Animal Sciences, Ohio State University for their statistical assistance, to Harald Volden, Department of Animal and Aquacultural Sciences, Norwegian University of Life Sciences, for critical review of the manuscript and to Karl N. Kerner for improving the English of this manuscript. Financial support from the Research Council of Norway is acknowledged.
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Feed nitrogen conversion in lactating dairy cows on pasture as affected by concentrate supplementation H˚avard Steinshamn a,b,∗ , Mats H¨oglind c , Torstein H. Garmo d , Erling Thuen d , Ulrik Tutein Brenøe d a
d
Department of Plant and Environmental Sciences, Norwegian University of Life Sciences, ˚ Norway P.O. Box 5022, N-1432 As, b Bioforsk-Norwegian Institute for Agricultural and Environmental Research, Organic Food and Farming Division, N-6630 Tingvoll, Norway c Bioforsk-Norwegian Institute for Agricultural and Environmental Research, Horticulture and Urban Greening Division, Postveien 213, N-4353 Klepp St., Norway Department of Animal and Aquacultural Sciences, Norwegian University of Life Sciences, ˚ Norway P.O. Box 5025, N-1432 As,
Received 15 March 2005; received in revised form 27 January 2006; accepted 8 February 2006
Abstract The effect of concentrate supplementation on nitrogen (N) intake and excretion in grazing lactating dairy cows was determined in three herds in Norway. Grazing trials were conducted with each herd in June and August for two consecutive years. The average supplementation was 1.8 (S.D. 2.1) kg DM/day, and the concentrate was based on grain with a N content ranging from 18.7 to 24 g/kg DM. Herbage DM and N intake were reduced with increasing supplementation, but total DM and N intake increased. Milk yield and protein content increased by 1.1 kg milk and 0.28 g protein per kg milk for each kg extra concentrate. Milk N excretion increased with increasing supplementation (6.5 g N/kg DM), and N utilisation improved by 11.7 g N per kg N intake per kg extra concentrate. Excretion of urine N and its share of total excreta N decreased by 4.0 and 9.2 g/kg concentrate, respectively. The Abbreviations: AAT, amino acid absorbed in the intestine; aNDF, neutral detergent fibre; DM, dry matter; FEm, net energy lactation (6.9 MJ); IVDMD, in vitro dry matter digestibility; N, nitrogen; OM, organic matter; PBV, protein balance in the rumen; S.D., standard deviation; S.E., standard error of estimate ∗ Corresponding author. Tel.: +47 71532027; fax: + 47 71532001. E-mail address: [email protected] (H. Steinshamn). 0377-8401/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.anifeedsci.2006.02.004
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reducing effect on urine N excretion of supplementation was, however, numerically low compared with other studies, most likely due to a high crude protein content of the concentrate used. © 2006 Elsevier B.V. All rights reserved. Keywords: Dairy cattle; Concentrate; Nitrogen excretion; Nitrogen efficiency; Grazing; Milk production
1. Introduction The agricultural sector is the major contributor to anthropogenic reactive nitrogen (N) enrichment, and only a small part of the reactive N input to the sector is recovered in agricultural produce (Bleken and Bakken, 1997; Van der Hoek, 1998; Galloway et al., 2003). Nitrogen dissipates through volatilisation and leaching and perturbs the environment in various ways (Galloway et al., 2003). A large part of the losses is volatilisation of ammonia (NH3 ). Agricultural activity and livestock production, e.g. dairy cows, are the main sources of NH3 emissions to the atmosphere (Asman et al., 1998; Hutchings et al., 2001; SSB, 2005). Seventy percent or more of cows’ daily N intake is excreted in faeces and urine (Tamminga, 1992). Whereas faecal N excretion varies little with the dietary composition, urinary N varies with an excess of ingested protein or disequilibrium between protein and energy (V´erit´e and Delaby, 2000). Thus, N fed in excess of animal or rumen requirement will essentially be excreted in urine. Increased proportion of urine in the excreta may elevate NH3 losses as volatilisation from urinary N is several fold higher than from faecal N (Lockyer and Whitehead, 1990; Petersen et al., 1998). This is because most of the N in urine is in the form of urea that is rapidly hydrolyzed to NH3 in the soil, whilst faecal N is strongly bound in complex organic compounds. Pastures can provide cheap, high quality feed, enabling high milk yields with little or no supplementation (Ettala et al., 1986). This is partly due to the cow’s ability to select young plant parts, leaves and stems, which are high in energy and protein. However, high quality herbage may supply the cows with excessive amounts of N, which the animals cannot utilise and thus excrete in the urine. Volatilisation of NH3 -N is the most important route of N loss from grazed pastures (Jarvis, 1994). The utilisation of N by dairy cows can be improved in many ways, e.g. by reducing N intake, improving the output of N in milk and by improving N capture in the rumen (Tamminga, 1992; Bussink and Oenema, 1998). Partial replacement of pasture herbage with concentrated feeds low in protein and high in energy, like maize silage, has been shown to improve N utilisation and reduce N losses during grazing (Valk, 1994). However, in cool regions such as most of Scandinavia, maize can only be grown to a limited extent. Supplementation with grain based concentrates with low protein content might serve the same purpose and may be an alternative. The effects of supplementation and diet composition on feed intake, milk production and N utilisation in dairy cows fed during housing have been investigated in numerous studies (e.g. Dewhurst et al., 1996; Kebreab et al., 2000, 2001; Castillo et al., 2001; Frank and Swensson, 2002). Data on N intake, excretion and utilisation in grazing dairy cows, and the effect of domestically grown grain supplements on these parameters are scarce. Supplementation with concentrates with high starch proportion and very low N content (8.5 g N/kg DM) has improved the N utilisation to milk and strongly decreased the excretion of urine N and its proportion of excreta N in
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dairy cows on pasture (Berry et al., 2001). In Norway, the commercial concentrates used for dairy cows on pasture are, however, based on locally produced barley and oats with a N content of approximately 18 g N/kg DM (Institutt for husdyr og akvakulturvitenskap and Mattilsynet, 2005). The objective of the present study was to investigate the effects of concentrate level on N intake, the partition of N into milk, urine and faeces and the efficiency of N utilisation by rotationally grazing cows. 2. Materials and methods 2.1. Location The study was carried out in three different herds. Herds 1 and 2 were located at the Animal Production Centre, Norwegian University of Life Sciences (59◦ N, 10◦ E) and herd ˚ regional prison (58◦ N, 5◦ E). In all three herds, mea3 was located at the farm of the Ana surements were carried out in 1999 and 2000, and within each year in two periods: June and August. Herd 1 was managed according to organic standards, whilst herds 2 and 3 were conventionally managed. 2.2. Herds and supplementary feeding 2.2.1. Animals In total, 134 dairy cows of the Norwegian Red Cattle breed were used, of which 25 cows were used in all four periods, 23 cows in three periods, 52 cows in two periods and 34 cows in one period, altogether 307 observations. Of the 307 observations, 105 were from primiparous cows whilst 202 observations were from multiparous cows. The cows in herds 1 and 3 calved between January and May, and were thus in early and mid lactation during the experimental periods. The cows in herd 2 calved mainly between August and December and were thus in late lactation (Table 1). The cows in herds 2 and 3 consisted of two breeding lines with different genetic merit for milk production. Table 1 Number of cows, average number of lactations, days in milk production and concentrate supplementation (kg DM/day) in the sample collection periods of the three herds Herda
Grazing period
No. of cows
Lactations
Days in milk
Concentrate (kg DM/day)
1999 2000
1999
2000
1999
2000
1999
2000
1
June August
10 17
16 16
2.7 (1.6) 2.5 (1.4)
3.5 (1.8) 3.1 (1.5)
99 (32) 168 (46)
101 (41) 146 (51)
0.87 (1.46) 0.26 (0.42)
0.46 (0.85) 0.37 (0.73)
2
June August
36 22
50 39
2.5 (1.3) 2.3 (1.4)
2.6 (1.5) 2.4 (1.6)
220 (64) 253 (60)
208 (71) 246 (62)
2.90 (2.86) 1.82 (2.52)
1.96 (2.03) 1.53 (1.77)
3
June August
25 29
21 26
1.9 (1.2) 2.2 (1.2)
1.7 (1.1) 1.9 (1.0)
100 (30) 151 (33)
102 (34) 150 (36)
1.98 (1.85) 2.35 (1.89)
1.98 (1.63) 2.58 (1.79)
Standard deviation is in brackets. a See text for explanation of herd.
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H. Steinshamn et al. / Animal Feed Science and Technology 131 (2006) 25–41
Table 2 Ingredient composition of concentrate supplements used Ingredients (g/kg fresh weight)
Herd 1a
Barley Oat Wheat bran Maize Sugar beet pulp Soya Rape seed Cane molasses Fish meal/silage Urea Fat Minerals and vitamins
1000 – – – – – – – – – – –
a b
Herd 2a
Herd 3a,b
1999
2000
403 219 158 – – 49 10 79 25 – 19 38
317 311 49 – 88 58 10 78 26 – 19 44
371 150 – 250 – – 50 70 60 6 10 33
See text for explanation of herd. The figures are from 2000. The composition was similar in 1999 but with some higher urea content.
2.2.2. Concentrate Each year at calving, the cows in herds 1 and 2 were divided into three and four treatment groups, respectively, balanced for calving date and parity. In herd 1, the groups were fed three different levels of concentrate: 2, 4 and 8 kg per cow per day, and the groups of herd 2 were fed four different levels of concentrate: 4, 8, 12 and 16 kg per cow per day for 30 days. Thereafter the level was reduced by 1 kg per cow every month in all treatments throughout the lactation, independent of the grazing period. The concentrate supplementation for each cow within herds 1 and 2 during the experimental periods is thus a function of treatment group and calving time. In herd 3, consisting of ca. 100 animals, a group of 30 cows was selected each year for the experiment. The experimental cows were allocated to 10 blocks according to milk yield, calving date and lactation number. The three animals in each block were randomly allocated to three different concentrate levels: 0, 3 or 6 kg per cow per day that were given throughout the experiment. The average and the variation of supplementations for each period by herd and year are presented in Table 1. In herd 1, rolled barley was used as a concentrate, whereas commercial grain based mixtures were used in herds 2 and 3 (Table 2). The commercial mixtures contained large shares of barley and oats (>0.50), and were developed for grazing dairy cows with low values for the protein balance in the rumen (PBV, Madsen et al., 1995). 2.3. Pasture management Within each herd the cows were grazed as a single group in fenced paddocks. The paddocks were rotationally grazed three to four times per season. The pasture for herd 1 has been organically managed since 1993, and showed high plant species diversity (Steinshamn et al., 2001). The sward had a high content of white clover (Trifolium repens); >0.25 proportion of the DM yields. The dominating grass species were Poa pratensis and Festuca pratensis. The pasture used for herd 2 was dominated by sown grasses, containing Festuca pratensis,
H. Steinshamn et al. / Animal Feed Science and Technology 131 (2006) 25–41
29
Phleum pratense and Poa pratensis. The pasture used for herd 3 was also grass based with Lolium perenne (0.60–0.70) and Poa spp. (0.20–0.30) as the most important species. It had a very low content of herbs. The history of this sward is unknown, but according to the present manager it had not been reseeded for the last 30 years, except for occasional, partial surface seeding. The organically managed pasture for herd 1 received no fertilisation or manure except for the urine and dung that was excreted by the cows during grazing. The pasture for herd 2 was fertilised with 30 kg P and 100 kg K/ha at the beginning of each growing season. Nitrogen was applied to a level of 200 kg N/ha, dressed at three occasions. The pasture for herd 3 received 23 tonnes of cattle slurry (ca. 40 kg N/ha) at the beginning of each season, followed by one application of compound fertilizer (80 kg N, 5 kg P and 60 kg K/ha) and three applications of N fertilizer (25–40 kg N per ha per application), i.e. a yearly application of ca. 210 kg/ha. 2.4. Sample collection Two periods of 12 days each were chosen to represent mid (June) and late (August) grazing season in both years. The measuring periods were subdivided into 7 days of adaptation and 5 days of main data and sample collection. On collection days, milk yields were recorded daily and samples were aliquot pooled within period for each cow. Individual herbage intake was estimated in the same periods using the alkane double-indicator technique (Mayes et al., 1986). In herds 1, 2 and in the first year in herd 3, the animals were dosed twice daily after milking with alkane labelled cellulose bungs each containing 315 mg Dotriacontane (C32 ), giving a daily dose of 630 mg. In herd 3 in 2000, intra ruminal controlled release capsules (CRC, Captec® Ltd., New Zealand) releasing a constant daily amount of C32 and C36 at a rate of 400 mg/day were used. The devices were administered 7 days before the faecal collection periods. Over the 5 days of measurement, spot faecal samples were collected at dawn and afternoon from fresh dung pats of known individual animals or obtained by rectal sampling. During the faecal sampling periods, daily samples of the herbage were obtained by cutting to 3 cm stubble height with hand-held scissors from within sixteen 0.25 m2 quadrates. The herbage samples were pooled to one sample per day. After collection, faeces samples were frozen at −20 ◦ C and then thawed and pooled for each animal and period. Subsamples of faeces and the herbage samples were dried at 60 ◦ C for 48 h. 2.5. Sample analysis Dried feed and faecal samples were milled through a 1.00 mm screen. The feed DM and ash content were determined after drying overnight at 105 ◦ C and after 13 h at 550 ◦ C, respectively. Crude fibre was determined according to AOAC (1990, method 962.09) and fat after extraction with petroleum ether according to the EU directive 71/393/EEC (OJ L15, 1971). Neutral detergent fibre (aNDF) was determined according to Van Soest et al. (1991) with the use of ␣-amylase using the ANKOM Fibre Analyser (ANKOM Technology, Fairport, NY). In vitro DM digestibility (IVDMD) was determined according to Tilley and Terry (1963) for the samples collected from herds 1 and 2 and by 48-h incubation with
30
H. Steinshamn et al. / Animal Feed Science and Technology 131 (2006) 25–41
rumen fluid in an ANKOM fibre apparatus (ANKOM Technology, Fairport, NY) for the samples taken from herd 3. Nitrogen contents in herbage, concentrate and faeces were determined by the Kjeldahl method on a Kjeltec Auto 1035 analyser (Tecator AB, H¨ogan¨as, Sweden). Milk protein content was determined using an infrared technique (Milcoscan 255 A/B; Foss Electric, Hillerød, Denmark) and milk N was estimated by assuming 15.67 g N/100 g in milk protein. The n-alkane extraction process of feed and faeces was based on the method described by Mayes et al. (1986) and the contents of n-alkane were determined on a gas chromatograph (Perkin-Elmer Autosystem) equipped with a capillary column (SGE 054657; 12 m long, 0.53 mm inner diameter). 2.6. Calculation and statistical analysis The herbage and concentrate energy concentration was calculated as net energy lactation according to Van Es (1978) and expressed as FEm (1 FEm = 6.9 MJ net energy lactation) using the chemical analysis and equations given in the standard feed tables (Institutt for husdyr og akvakulturvitenskap and Mattilsynet, 2005). Amino acids absorbed in the intestine (AAT) and PBV contents were calculated as described by Madsen et al. (1995), using the chemical analysis and degradability values from standard feed tables (Institutt for husdyr og akvakulturvitenskap and Mattilsynet, 2005). The NH3 volatilised during drying the faecal samples was corrected for by using the following regression equation: N content wet sample (g/kg DM) = 4.48 (S.E. = 1.15) + 0.947 (S.E. = 0.050)×N content dry sample (g/kg DM), r = 0.832, 2
RMSE = 1.07,
n = 74
The equation is based on analysis of samples collected from N balance studies carried out at the Department of Animal and Aquacultural Sciences, Norwegian University of Life Sciences (Harald Volden, Personal communication). Herbage intake was estimated according to the equations presented by Mayes et al. (1986), using C32 as the external marker to provide an estimate of faecal output and C31 as a digestibility marker. Alkane C32 (dosed) and C33 (natural) have similar recovery rates and are preferred for estimating intake. However, in our herbage samples C33 concentration was rather low; C31 concentrations of herbage (means ± S.D. = 209 ± 8 mg/kg DM) were three-fold that of C33 (means ± S.D. = 75 ± 3 mg/kg DM). Penning (2004) recommends to use the C31 :C32 pair under such conditions, and Estermann et al. (2001) found a higher accuracy of estimation of known DM intake with C31 :C32 than with C33 :C32 and a similar recovery rate for C31 , C32 and C33 . Faecal output was calculated according to the following equation: Faecal output (kg DM per day) =
rate of dosed marker (C32 ) × recovery rate (0.87) concentration of marker (C32 ) in faeces
Quantities of N ingested and N excreted in the faeces were calculated from the herbage and concentrate DM intake and faecal DM output, respectively, and from herbage, concentrate and faecal N content. Animal body weight (BW) was recorded twice in each
H. Steinshamn et al. / Animal Feed Science and Technology 131 (2006) 25–41
31
experimental period by a mobile scale. Daily BW change was calculated by fitting curves to the weight measurements taken during the three last weeks indoors prior to grazing, measurements taken during the grazing season and the first three weeks indoors after grazing. Body N retention was estimated by assuming 26 g N/kg of BW change (Gibb et al., 1992), and urine N excretion was estimated from N balance calculations. The effect of concentrate (C) was evaluated by treating it as a regression effect in the statistical analysis. Statistical analyses were carried out using MIXED procedures (method = reml) of SAS (SAS, 1999). The model was: Yijklm = μ + hi + tj (hi ) + Lk + B1 C + B2 C2 + B3 D + V (al(i) ) + cov(eijklm ) where Yijklm is a response parameter (DM intake, milk yield, protein content in milk, N intake and excretion and N utilisation); μ is the overall mean; h is the random effect of herd (i = 1, 2, 3); t(h) = within herd period random effect (j = 1, 2, 3, 4) where periods 1 and 2 are June and August in 1999, respectively, and period 3 is June and 4 August in 2000; L is the fixed class effect of lactation number 1 or older (≥second lactation), k = 1, 2; B1 , B2 and B3 are the regression coefficients for the fixed linear and quadratic effects of concentrate level (C, kg DM/day) and days in milk (D, days), respectively; a is the random effect of animal within herd. As most cows were present in more than one grazing period, the covariation within animals V(al(i) ) was accounted for in an analysis of repeated measures. The optimal covariance structure (cov(eijklm )) was assessed for each parameter with attention to the Schwartz Baysian information criterion (Littell et al., 1998). Determination of the significance of fixed terms was through Wald tests with denominator degrees of freedom estimated by Satterhwaite’s approximation. Days in milk were included as a covariate in order to adjust for the effect of stage of lactation. The interactions between the fixed effects were included when the significant level exceeded the 5% level, which sometimes occurred for L × D, but never for the L × C and C × D interactions. The quadratic effect of concentrate was never significant and was therefore not included in the final model The random effect of h and t(h) on the regression coefficient of Y on C (random slope effect), judged by the likelihood ratio test, was not significant and was therefore not included as a random term. Thus, the individual regressions within h and t(h) are all parallel lines with different random intercepts. The random effect of h did not improve the model, as judged by the likelihood ratio test, and was therefore omitted in the final model. The relationship between total N intake and faecal N excretion, urinary N excretion and milk N excretion was tested for linear and curvilinear effects accounting for the covariation within animal and using h and t(h) as a random effect as described above. Information on using the mixed model methodology in analysis of data from multiple studies is given by St-Pierre (2001). Relationships between predicted and observed parameters were estimated with linear regression. The effect of breeding line, in herds 2 and 3, was not accounted for in the statistical analysis, which consequently increased the error.
3. Results Animal and concentrate characteristics are presented in Tables 1 and 2, respectively. The concentrate supplementation varied from 0 to 10 kg, but the overall average was only
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H. Steinshamn et al. / Animal Feed Science and Technology 131 (2006) 25–41
1.8 kg DM per cow and day with a standard deviation of 2.05. The herbage quality, Table 3, varied, but for most periods it can be characterised as fair with medium digestibility of organic matter (OM) and FEm (0.77–0.93 FEm/kg DM) and medium to low contents of aNDF (371–559 g/kg DM). The concentrates were all grain based and had a negative PBV (Table 3). The estimated herbage DM intake, milk yield and milk protein content are presented in Table 4. The mean intake and excretion of N are presented in Table 5. There was a large variation in herbage N intake and excretion both between herds and within herds, and within grazing periods. However, overall mean and median values were very similar, indicating that the data were normally distributed (figures not shown). For total N intake, the mean value was higher than the median, indicating that the distribution was biased towards low intake. This was due to the fact that the concentrate supplementation was biased towards low levels. 3.1. Effect of concentrate on DM intake and animal performance Corrected for days in milk, each additional kg of concentrate increased the total DM intake by 0.31 kg DM/day, milk production by 1.1 kg/day and the milk protein concentration by 0.28 g kg−1 (Table 6). The reduction in herbage DM intake was 0.69 kg herbage DM per kg concentrate DM intake. First parity cows had lower dry matter intake and milk production than older cows. The difference became less in late lactation than in early lactation as indicated by the significant positive interaction (P<0.05) between parity and days in lactation (Table 6). The effect of concentrate was, however, not influenced by parity or days in milk. 3.2. Nitrogen intake and excretion Significant positive linear relationships were observed between N intake and milk, faecal and urine N excretion (Fig. 1). Milk, faecal and urinary N output increased by 13%, 17% and 71%, respectively, per unit increase in N intake. 3.3. Effect of concentrate on herbage nitrogen intake and excretion Concentrate supplementation was significant (P<0.05) for all parameters regarding N intake and excretion (Table 6). Each additional kg of concentrate reduced herbage N intake by 17.0 g/day, but total N intake increased by 6.2 g/day. The amount of N secreted to milk increased by 6.5 g per day per kg concentrate. The excretion of N in urine was reduced by 4.0 g/day whilst the faecal N losses increased by 2.8 g/day for each extra kg concentrate. Thus, the total loss of N in excreta (urine + faeces) was unaffected (b1 = −1.1 g N/kg DM, P=0.57) by concentrate supplementation (data not shown). As for DM intake, first parity cows had lower N intake and less N excretion in milk, faeces and urine than older cows. Except for urinary N excretion, this effect of parity diminished with increasing number of lactating days (Table 6). 3.4. Effect of concentrate on nitrogen utilisation The utilisation of feed N to milk N increased by 11.7 g N/kg N intake for each additional kg of concentrate (Table 6). Supplementation reduced the proportion of the N intake that
Table 3 Chemical composition of the pastures and the concentrate supplements fed in the sample collection periods of the three herds Herd 1
Herd 2
1999
2000
Herd 3
1999
2000
1999
2000
Augusta
Junea
Augusta
Junea
Augusta
Junea
Augusta
Junea
Augusta
Junea
Augusta
Pasture n DM (g/kg) Nb (g/kg DM) OMc (g/kg DM) aNDFd (g/kg DM) OM digestibility FEme (kg−1 DM) PBVf (g/kg DM) AATg (g/kg DM)
8 179 22.1 922 453 0.767 0.89 −14 88
8 194 28.5 900 371 0.767 0.91 23 89
6 188 25.9 912 446 0.783 0.92 5 90
7 156 29.7 901 452 0.783 0.93 30 90
8 181 23.1 922 482 0.761 0.89 −8 88
8 250 20.0 914 475 0.751 0.86 −21 85
8 198 32.6 913 493 0.750 0.91 44 90
7 199 23.4 914 438 0.742 0.86 −2 86
3 170 28.2 894 477 0.799 0.93 17 91
3 257 20.3 930 559 0.700 0.81 −16 83
4 188 30.1 914 549 0.675 0.83 40 85
6 170 22.4 919 536 0.653 0.77 3 79
Concentrate (n = 1) DM (g/kg) Nb (g/kg DM) OMc (g/kg DM) aNDFd (g/kg DM) OM digestibility FEme (kg−1 DM) PBVf (g/kg DM) AATg (g/kg DM)
868 19.6 972 183 0.821 1.09 −37 100
868 19.6 972 183 0.821 1.09 −37 100
847 18.7 972 263 0.832 1.11 −43 99
847 19.7 971 311 0.818 1.08 −36 99
863 21.0 933 162 0.769 1.11 −15 98
863 21.0 933 162 0.798 1.09 −15 98
867 22.1 917 311 0.808 1.03 −15 94
886 21.7 925 351 0.769 1.00 −14 89
872 24.0 – – – 1.11 −10 105
872 24.0 – – – 1.11 −10 105
865 23.2 – – – 1.16 −15 105
865 23.2 – – – 1.16 −15 105
33
See text for explanation of herd. a Period. b Nitrogen. c Organic matter. d Neutral detergent fiber including residual ash (Van Soest et al., 1991). e Net energy of lactation, 1 FEm = 6.9 MJ. f Protein balance in the rumen. g Amino acids absorbed from the intestine.
H. Steinshamn et al. / Animal Feed Science and Technology 131 (2006) 25–41
Junea
34
H. Steinshamn et al. / Animal Feed Science and Technology 131 (2006) 25–41
Table 4 Mean herbage DM intake (kg/day), milk yield (kg/day) and milk protein content (g/kg) in the sample collection periods of the three herds Herda
Grazing period
Herbage DM intake (kg/day)
Milk yield (kg/day)
Milk protein content (g/kg)
1999
2000
1999
2000
1999
2000
1
June August
13.0 (2.7) 14.5 (2.1)
12.7 (2.5) 13.5 (2.8)
19.1 (3.8) 17.6 (3.6)
21.8 (4.2) 18.9 (4.6)
31.5 (1.9) 31.0 (1.9)
29.6 (2.1) 31.4 (1.7)
2
June August
10.9 (2.6) 15.6 (3.1)
12.3 (2.7) 15.4 (3.2)
14.8 (5.6) 14.4 (5.5)
17.5 (5.0) 13.2 (4.6)
33.1 (2.3) 34.7 (2.4)
33.1 (2.7) 36.0 (3.5)
3
June August
16.2 (3.2) 12.6 (2.6)
8.6 (2.6) 15.8 (4.8)
21.4 (3.3) 19.2 (3.7)
20.8 (4.2) 22.3 (4.0)
31.6 (2.2) 31.5 (3.1)
30.8 (3.2) 32.6 (2.1)
Standard deviation is in brackets. a See text for explanation of herd.
Fig. 1. Urinary, faecal and milk N excretion plotted over the N intake range observed for the cows in the study (milk N (g/day) = 42.5 (S.E. ± 7.33) + 0.129 (S.E. ± 0.0156)×N intake (g/day); R2 = 0.43; P<0.0001, Faecal N (g/day) = 41.2 (S.E. ± 5.07) + 0.174 (±S.E.0.0167)×N intake (g/day); R2 = 0.90; P<0.0001, urine N (g/day) = −97.3 (S.E. ± 9.41) + 0.710 (S.E. ± 0.0215)×N intake (g/day); R2 = 0.92; P<0.0001).
was excreted as N in excreta (urine + faeces) by 12.2 g N/kg N intake and changed the partitioning of the N excretion between urine and faeces; the urine N proportion of N intake decreased by 13.8 g, whilst the faecal N proportion increased by 2.6 g N/kg N intake. Thus, the proportion of urine N to total excreta N excretion decreased by 9.2 g N/kg N intake for each kg DM supplementary increase.
4. Discussion Substitution rate (SR), or the reduction in pasture DM intake per kg concentrate DM intake, is a factor which may explain the variation in milk response (MR) to supplementation. In general, a negative relationship exists between SR and MR (Leaver, 1985), but there are several pasture, animal and supplement factors that will influence this relationship (Bargo et al., 2003). According to Bargo et al. (2003), there is an inconsistent relationship between
Herd 1
Herd 2
1999
N (g/day) n Total intake Herbage intake Faecal excretion Urine excretion Milk excretion Retention
2000
Herd 3
1999
2000
1999
2000
June
August
June
August
June
August
June
August
June
August
June
August
10 304 (58) 287 (60) 110 (22) 101 (29) 94 (16) −1 (7)
17 419 (61) 414 (61) 122 (17) 204 (42) 85 (16) 7 (6)
16 337 (57) 328 (65) 116 (12) 115 (44) 101 (18) 6 (7)
16 409 (79) 401 (83) 120 (14) 189 (62) 92 (20) 7 (7)
36 318 (54) 250 (61) 101 (19) 130 (32) 76 (28) 12 (9)
22 355 (49) 313 (62) 119 (17) 145 (36) 77 (27) 14 (6)
50 444 (69) 400 (87) 110 (18) 233 (57) 90 (24) 11 (7)
39 397 (55) 361 (69) 107 (15) 203 (35) 73 (23) 14 (5)
25 506 (76) 458 (90) 107 (19) 285 (49) 106 (18) 8 (7)
29 313 (45) 257 (53) 105 (14) 101 (32) 95 (22) 12 (9)
21 304 (71) 259 (79) 76 (11) 115 (54) 100 (21) 13 (9)
26 414 (106) 353 (108) 86 (14) 200 (82) 114 (24) 13 (8)
Standard deviation in brackets. See text for explanation of herd.
H. Steinshamn et al. / Animal Feed Science and Technology 131 (2006) 25–41
Table 5 Mean nitrogen (N) intake and excretion (g/day) in each grazing period of each year and herd
35
36
S.E.a
C
S.E.a
P
Lb
S.E.a
P
D
S.E.a
DM intake (kg/day) Total 15.9 Herbage 15.9
0.94 0.94
0.31 −0.69
0.09 0.09
<0.001 <0.001
−3.5 −3.5
0.82 0.82
<0.001 <0.001
−0.005 −0.005
0.003 0.003
Milk production Milk (kg/day) Protein (g/kg)
0.91 0.59
1.09 0.28
0.12 0.081
<0.001 <0.001
−7.4 −0.4
0.99 0.31
<0.001 0.186
−0.046 0.026
Y
N intake (g/day) Total Herbage
a
24.9 27.8
L*Db
S.E.a
0.933 0.898
0.010 0.010
0.004 0.004
0.004 0.002
<0.001 <0.001
0.030 ns
0.005
P
400 402
26 26
6.2 −17.0
2.3 2.3
0.009 −95 <0.001 −95
21 21
<0.001 <0.001
−0.11 −0.11
0.08 0.08
0.726 0.758
0.27 0.27
0.11 0.11
N excretion (g/day) Milk 114 Faeces 111 Urine 185
5 5 18
6.5 2.8 −4.0
0.6 0.6 1.6
<0.001 −40 <0.001 −19 0.012 −32
5 5 6
<0.001 <0.001 <0.001
−0.17 −0.05 ns
0.02 0.02
<0.001 0.599
0.16 0.07 ns
0.03 0.03
N output/input (g/kg) Milk 300 Faeces 279 Urine 419 Faeces + urine 703
16 16 29 18
11.7 2.6 −13.8 −12.2
1.6 1.1 2.3 1.5
<0.001 −46 0.025 25 <0.001 −22 <0.001 31
15 4 9 15
0.002 <0.001 0.014 0.035
−0.40 ns 0.25 0.22
0.06
<0.001
0.08
0.08 0.06
0.003 0.018
0.24 ns ns −0.18
U/(U + F)
30
−9.2
2.3
<0.001 −31
9
<0.001
0.20
0.08
0.015
ns
582
0.08
RMSE
R2
0.028 0.028
1.7 1.7
0.749 0.789
<0.001
1.7 1.2
0.906 0.841
P
0.015 41 0.014 41 <0.001 0.004
0.796 0.833
8 8 39
0.911 0.847 0.725
0.003 32 25 51 0.028 33
0.802 0.812 0.776 0.792
52
0.753
P, probability value of P>|t|: estimate significant different from zero or estimate of the difference 1. Lactation vs. ≥2 lactation significant different from zero. RMSE, root mean square error. a S.E., standard error of estimate or mean standard error of difference among the effects of 1. Lactation cows and cows ≥ 2 lactations. b Estimates for L refers to cows in first lactation, cows ≥ 2 lactations have the value 0.
H. Steinshamn et al. / Animal Feed Science and Technology 131 (2006) 25–41
Table 6 Estimates for the relationship between concentrate supplementation (C, kg DM/day), parity (L, 1 or ≥2) and days in milk (D) on total and herbage DM intake (kg/day), milk yield (kg/day), milk protein content (g/kg), total and herbage N intake (g/day), N excretion (g/day), N excretion as the proportion of N intake (g/kg) and urine N excretion as the proportion of total urine N and faecal N excretion (U/(U + F) g/kg)
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the amount of supplement and the MR and SR in grazing studies with highyielding dairy cows, and when using data from four experiments they could not find any relationship between MR and SR. The simultaneously high SR (0.69 kg pasture/kg concentrate) and MR (1.1 kg milk/kg concentrate) estimated in the present study contrast findings in older reviews (Leaver, 1985). However, recent studies and reviews report similar SR and MR to supplementation by dairy cows at milk yield levels, days in lactation and level of concentrate feeding as in the present study (Bargo et al., 2003; Delaby et al., 2003). Using the estimated herbage and concentrate net energy content (FEm) in Table 3, it was calculated that the total net energy intake increased by 0.51 FEm (3.51 MJ net energy lactation) per kg concentrate DM. The marginal response was therefore 0.46 FEm (3.17 MJ) per kg milk, which is, when adjusting for milk fat and protein content, close to the standard energy requirement for milk production (Van Es, 1978). The estimated SR of 0.69 kg pasture per kg concentrate indicates a relatively high pasture quality and pasture allowance (Dixon and Stockdale, 1999; Bargo et al., 2003), although the chemical analysis of the herbage samples indicates medium and even for some periods rather low quality (Table 3). However, as the cows are selective, particularly at high pasture allowances, the quality of the pasture actually consumed might have been better than indicated by the analysis, as the herbage sampling method did not account for this fact. The SR is also reported to increase with the amount of concentrate (Leaver, 1985), but the present data do not indicate such a relationship. Although, the range in concentrate feeding was wide in the present study, most of the supplementation was in the lower range (average 1.8 kg DM/day S.D. 2.05). The marginal response of 0.28 g protein per kg milk per extra kg concentrate is similar to the 0.22 g/kg reported by Delaby et al. (2003), but higher than the 0.10 g/kg presented by Bargo et al. (2003). This positive response of milk protein content was likely due to the additional energy input with supplementation that stimulated the production of glucogenic nutrients in the rumen, which in turn stimulated protein synthesis (Coulon and Remond, 1991; Rigout et al., 2003). Ruminal fractional passage rate is high at pasture, and starch dominated concentrate supplementation may increase the ruminal escape of dietary starch. This has shown to increase both milk protein production and decrease urinary N excretion (Reynolds et al., 2001). Thus, the increased secretion of milk N per unit N intake was likely due to both increased milk production and increased protein synthesis. The linear increase of urinary N output by 71% per unit increase in N intake (Fig. 1) is very similar to the 72% found by Mulligan et al. (2004). Others have found an exponential increase in urinary N excretion as N intake increased (Castillo et al., 2000; Kebreab et al., 2001). Albeit discrepancy in the functional form, it is evident that urine appears to be the main route of N excretion in dairy cows. Outputs of faecal and milk N increased by less than 20% per unit increase in N intake, and the rates found in the present study agree with other observations (Castillo et al., 2000; Kebreab et al., 2001; Mulligan et al., 2004). Linear regression between total N intake (Ni , g/day) and the efficiencies of conversion of dietary N to milk (Ncm , g/g) and faecal N (Ncf , g/g) resulted in the following equations: Ncm = −0.00028 × Ni + 0.36, R2 = 0.48 Ncf = −0.00029 × Ni + 0.40,
R2 = 0.88
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The coefficients are almost identical to those reported by Castillo et al. (2000), who used data from N balance experiments published in the literature; −0.0002/g for milk and −0.0003/g for faecal N excretion. The reduction in urinary N excretion by supplementation was probably due to incorporation of NH3 -N, which was produced from the highly degradable herbage protein, into microbial protein in the rumen (Van Vuuren et al., 1993; Reis and Combs, 2000; Bargo et al., 2002, 2003). With increased concentrate supplementation, more fermentable OM was available for microbial growth. Consequently, more dietary N was incorporated into microbial protein, even though the total N supply also increased with supplementation. A reduction of NH3 -N by supplementation is either associated with improved utilisation of NH3 -N or with a reduction in total N intake due to low N content in the supplement. Rumen NH3 -N concentration was not measured in the present study, but presumably it was reduced by supplementation and, most likely, the reduction was caused by an increased microbial synthesis as the total N intake increased. As the rates of urinary and faecal N excretion with increasing supplementation were inversely related, but of the same order of magnitude, the total N excretion was not affected. The amount of N excreted in dairy cow faeces is relatively constant and consists mainly of metabolic faecal N and undigested material and is thus closely linked to DM intake (Peyraud et al., 1995). However, the proportion of urine N of total N excreted in faeces and urine was reduced. The proportion averaged 0.60 of the total excreta return, which is close to the typical 0.57 on pasture (Jarvis, 1994). Reduced proportion of urinary N of total N excreted is important, even when total N excretion remains the same, as it presumably reduces the NH3 volatilisation rate (Lockyer and Whitehead, 1990; Petersen et al., 1998). Although statistically significant, the effect of concentrate supplementation on herbage N intake and N excretion and utilisation was numerically small. Supplementation reduced the herbage N intake as anticipated, but the total N intake, from herbage and concentrate, increased. Nitrogen contents in the supplements was probably too high, although the commercial feed used trials 2 and 3 were designed for pasture conditions. Berry et al. (2001) used concentrate with a N content of 8.5 g/kg DM to dairy cows at similar N production and N intake as in the present study. The N content ranged between 19.6 and 24.0 g/kg DM in our study, approximately the same as the herbage N content (Table 3). Berry et al. (2001) found that supplementation with the low protein concentrate improved N utilisation (N in milk relative to N intake) by 13% kg−1 , whilst in the present study the improvement was only 5% kg−1 concentrate. Excretion of urine N and its proportion of excreta (urine + faeces) N decreased in the present study by approximately 2.4% and 1.5% kg−1 supplementation, respectively, whilst Berry et al. (2001) found 11% and 4% reductions. This illustrates that it should be possible to achieve higher transfer efficiencies than found in our study by using concentrate with a lower N content. The commercial mixtures currently used in Norway are too high in protein if the aim is to reduce urinary N and the proportion of urine N of the total N excretion and thereby significantly reducing the potential N losses to the wider environment. On the other hand, as V´erit´e and Delaby (2000) argue, an improvement of N utilisation at the individual cow level does not imply improvement on a higher system level. Supplementation makes it possible to have a higher stocking rate or to extend the grazing season, and therefore, the amount of N excreted per area grazed may remain the same or even increase. Supplementation also transfers N from grain production areas to pasture
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areas via animal excreta, either within the farm itself or at a larger (regional, national) scale. An analysis of dairy farm N balance studies across Europe showed that the amount of N that dissipates from the farm system in order to produce 1 kg of N in milk + meat increased linearly with feed N purchase, especially when taking into account the dissipation related to the production of the purchased feed (Bleken et al., 2005).
5. Conclusions The present study provides data on the potential to improve N utilisation in dairy cows on pasture by supplementation with grain based concentrates. It was demonstrated that increasing concentrate supplementation resulted in a higher proportion of N in milk, less total urinary N excreted, and a lower proportion of urinary N of the total N excreted. This reduced the potential NH3 -N loss from the pasture. However, the improved utilisation of N by increasing concentrate supply was rather small compared to similar studies. This was mainly due to a high N content in the concentrate used.
Acknowledgements We thank the staff of the Animal Production Centre, Norwegian University of Life ˚ regional prison farm for grazing management. The authors Sciences and the staff at the Ana are grateful to Henrik Stryhn, Atlantic Veterinary College, University of Prince Edward Island and Normand St-Pierre, Department of Animal Sciences, Ohio State University for their statistical assistance, to Harald Volden, Department of Animal and Aquacultural Sciences, Norwegian University of Life Sciences, for critical review of the manuscript and to Karl N. Kerner for improving the English of this manuscript. Financial support from the Research Council of Norway is acknowledged.
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