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ANIMAL NUTRITION |
The Agricultural Research Institute of Northern Ireland, Hillsborough, BT26 6DR Northern Ireland
Abstract
One hundred thirty-six perennial ryegrass silages with wide variations in quality were evaluated for N and DM degradability in three beef steers offered grass silage of medium quality ad libitum. The silages were incubated in the rumen of each animal in triplicate for 0, 6, 12, 24, 48, and 72 h, respectively. The disappearance rates of N or DM were used to calculate the readily soluble fraction ("a" value), potentially degradable fraction ("b" value), and the fractional degradation rate of "b" ("c" value). The effective degradability (P) of N or DM was then estimated assuming a ruminal outflow rate of 0.02, 0.05, or 0.08/h (P0.02, P0.05, or P0.08). The objective was to use these data to develop prediction equations for N and DM degradability in grass silages. There were considerable variations in "a," "b," and "c" values and the P0.02, P0.05, or P0.08 of N and DM (e.g., the P0.02 of N ranged from 75.0 to 93.4% and the P0.02 of DM from 51.5 to 82.5%). The P0.02, P0.05, or P0.08 of N and DM were negatively related (P < 0.001) to ADF, NDF, and lignin concentrations but positively related (P < 0.001) to protein fractions (CP, soluble CP, and true protein concentrations) and digestibility of DM, OM, GE, CP, and NDF and digestible OM in the total DM (measured with sheep). The N and DM degradability data were also positively related to silage lactic acid concentration, but the relationships between DM degradability data and pH, ammonia N/total N, and VFA concentration in silages were negative (P < 0.05). Several sets of prediction equations (linear and multiple) were thus developed for N and DM degradability using CP or NDF concentration, "a" value or digestibility data as primary predictors, together with or without other nutrient concentration and silage fermentation variables. All these relationships were highly significant (P < 0.001), and each predictor had a significant effect on the relationship (P < 0.05). The R2 values in multiple regression for N and DM degradability were generally over 0.70 and higher than in linear regression equations. Four equations were also developed to convert N and DM degradability at a given ruminal outflow rate, predicted using the above-mentioned equations, to their counterparts at any ruminal outflow rate (0.02 to 0.10/h), respectively.
Key Words: Cattle • Dry Matter Degradability • Grass Silage • Nitrogen Degradability • Prediction Equation
Introduction
In the metabolizable protein system, the dietary CP that animals consume is divided into two fractions: ruminally degradable CP and ruminally undegradable CP. Microbial activity in the rumen depends on the availability of both ruminally degradable CP and ruminally fermentable OM. The degradabilities of CP and DM of feeds are therefore key variables for the metabolizable protein feeding systems. A number of approaches are used for the determination of dietary CP and DM degradability, with the widely adopted method being the Dacron bag technique as developed by Ørskov and McDonald (1979)
. This method requires ruminant animals be fitted with a ruminal cannula, which may not be available under commercial farm conditions. However, there is in the literature some evidence of significant relationships between CP or DM degradability and nutrient concentration in forages. For example, Hoffman et al. (1993) reported a positive relationship of N or DM degradability with dietary CP concentration and a negative relationship with dietary fibrous fraction. Similarly, von Keyserlingk et al. (1996) found that N degradability was negatively related to NDF concentration and positively to CP concentration in grass silage or grass hay. A number of equations for the prediction of N degradability have been developed by Tamminga et al. (1991) using 35 grass silages or 12 grass hays, and by Waters and Givens (1992) using 19 fresh grass samples, although these relationships were significant at only 0.05 or 0.01 levels. In light of these results, it is possible that more accurate prediction equations for CP and DM degradability in forages can be developed using a larger and more comprehensive data set. The objective of the present experiment was to use a comprehensive data set of grass silages (n = 136) to develop prediction equations for CP and DM degradability using nutrient concentrations and digestibility variables and fermentation characteristics.
Materials and Methods
Silages and Digestibility Trials with Sheep
Full details of the preparation of grass silages (n = 136) and digestibility trials with sheep are reported in the companion paper (Yan and Agnew, 2004). Silages were selected from commercial farms across Northern Ireland to achieve a large variation in quality and a uniform distribution over the range using preset criteria of pH, ammonia N/total N, DM, and predicted ME concentrations. The silages were made from perennial ryegrass dominant swards and encompassed primary growth and first and second regrowth materials. The grass was either unwilted or wilted prior to ensiling and ensiled with or without silage additives. Each silage was offered to four wether sheep for 3 wk at maintenance feeding level to assess digestibility of DM, OM, CP, GE, and NDF and digestible OM in the total DM (DOMD).
Degradability Study with Cattle
Degradability data for the 136 silages were determined using three Aberdeen Angus x Friesian steers each fitted with a permanent ruminal cannula. The animals were individually tethered in tie stalls, exercised regularly, and given free access to water throughout the experiment. A medium-quality nonexperimental grass silage was offered ad libitum from 21 d before commencement to the completion of the experiment.
Silages were evaluated over a total of 17 periods (eight silages per period) each of 2-wk duration. Each silage was incubated in triplicate for 6, 12, 24, 48, and 72 h. A total of 24 bags (three bags x eight silages) were therefore examined with each animal at each incubation time (i.e., 6, 12, 24, 48, and 72 h, respectively). The bags were made from polyester filter cloth (Locker Wire Weavers Ltd., Warrington, Cheshire, U.K.) with a pore size of 1,600 µm2. The bags were double-sewn with round corners to ensure a smooth interior, free of pockets and crevices. Each bag contained approximately 5 g DM of fresh silage (chopped to 1-cm lengths) and was tied with nylon cord.
Bags were placed into the rumen of the steers in the morning before feeding. At the end of the designated incubation time as stated previously, all 24 bags were removed and a further set of 24 bags placed into the rumen of each animal on the following morning before feeding, for a subsequent incubation time. Bags were immersed in cold water immediately on removal from the rumen, to prevent further microbial action, and washed in a washing machine set to a cold cycle. The wash is a standard procedure as recommended by AFRC (1992) to remove the microbial contamination in in situ residues in Dacron bags and soluble and small particulates added to the bags from the rumen. This procedure is considered to be adequate in removal of all the added material (e.g., soluble and small particulates) from the bags (Varvikko and Lindberg, 1985). However, no attempt was made to correct for microbial contamination in the present experiment, and the potential for microbial contamination existed.
One set of bags for each silage was also prepared and washed immediately to determine the readily soluble or extractable fraction (time 0 h). Following washing, all bags were dried at 60°C for 48 h. The proportion of DM degraded after each time interval, for each silage, was calculated for each bag. The results from each set of three bags for each silage from each animal were averaged to give a mean value for DM degraded per silage. This gave a total of three observations per silage per time interval. The residuals from three bags of each silage from each steer, after each incubation time, were bulked and analyzed for N concentration. The proportion of N degraded at each time interval for each silage was then calculated for each of the three steers.
Data Analysis
Details on analyses of fermentation characteristics and chemical composition in grass silages were as reported in the companion paper (Yan and Agnew, 2004). Silage DM concentration was determined on an alcohol-corrected toluene basis and subsequently used as a basis for expressing all nutrient concentrations in silages.
The disappearance rates of DM and N in the silages during incubation were used to calculate the readily soluble proportion of DM or N ("a" value), potentially degradable DM or N ("b" value), and the fractional degradation rate of b ("c" value). The equation used (Eq. [I]) was as suggested by Ørskov and McDonald (1979), where dg is the degradability of DM or N at time t.
 | [I] |
The effective degradability (P) of DM and N was estimated using the equation as described by Ørskov and McDonald (1979; Eq. [II] herein), where k is the fractional outflow rate from the rumen (per hour). The k-values were taken as 0.02, 0.05, and 0.08/h, as recommended in Agricultural and Food Research Council (AFRC, 1993), to represent about 1x, slightly less than 2x, and more than 2x maintenance feeding level, respectively. The values were referred to as P0.02, P0.05, and P0.08, respectively.
 | [II] |
Correlation coefficients (r) were determined using linear regression analysis (Eq. [III]) between DM or N degradability data and a range of nutrient concentrations, fermentation variables, or digestibility parameters. Linear (Eq. [III]) and quadratic (Eq. [IV]) regression equations were used to develop prediction equations for N and DM degradability (y) using fiber and CP fractions, digestibility data and "a" value (x), respectively.
 | [III] |
 | [IV] |
Stepwise multiple regression analysis (Eq. [V]) was used to develop prediction equations for N and DM degradability (y) using nutrient concentration and fermentation variables, or digestibility data or "a" value with fiber and N fractions in silages. The technique automatically selects the best predictors in the prediction equations.
 | [V] |
All these analyses were performed using Genstat (6th ed.; Lawes Agricultural Trust, Rothamsted, England, U.K.).
Results
Data on chemical composition, fermentation characteristics, and digestibility variables determined with sheep offered silages at maintenance feeding level, for the grass silages (n = 136) used in the present experiment, are reported in the companion paper (Yan and Agnew, 2004).
Nitrogen and DM Degradability Data
The mean disappearance rates of N or DM in the silages are presented in Figure 1. The degradability of N was consistently greater than that of DM during the 72-h incubation period. The majority of the degradable N was degraded during the first 24 h, whereas the disappearance rate for DM continued to increase after 24 h, although the rate of degradation was decreased. Therefore, the difference in "a" value between N and DM (0.354) was greater than that (0.062) in the total degradable fractions (i.e., "a" + "b" values).
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. There was a large range in each of N and DM degradability variables. For example, the differences between maximum and minimum "a" values in N and DM were, respectively, 0.281 and 0.373 kg/kg; "b" values, 0.227 and 0.609 kg/kg; and "c" values 0.134 and 0.035/h. The P0.02, P0.05, and P0.08 of N therefore ranged, respectively, from 0.750 to 0.934, 0.689 to 0.911, and 0.658 to 0.895 g/g. The corresponding data for DM ranged from 0.515 to 0.825, 0.392 to 0.695, and 0.335 to 0.634 kg/kg.
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Correlation Coefficients in Linear Relationships with Degradability Data
The correlation coefficients obtained in the linear relationships between the degradability variables of N and DM and nutrient concentration, fermentation variables, and digestibility parameters are presented in Table 2. The "a" value and degradability (P0.02, P0.05, and P0.08) for N and DM were negatively (P < 0.05) related to fibrous fractions (ADF, NDF, and lignin concentration) whereas positively (P < 0.001) related to CP variables (total CP and soluble CP [SCP]). A negative relationship (P < 0.05 or 0.01) was observed between fibrous factions and the "b" value of N (not significant with NDF concentration), whereas the relationship was positive (P < 0.01) with the "b" value of DM. In contrast, there was a positive relationship (P < 0.01) of fibrous fractions with the "c" value of N, whereas a negative relationship (P < 0.001) was obtained with the "c" value of DM.
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The relationships of the "a" value and degradability (P0.02, P0.05, and P0.08) of N and DM with all digestibility variables were positive and significant (P < 0.05), respectively. The digestibility data also had positive and significant (P < 0.05) relationships with the "c" value of DM and the "b" value of N, respectively, except for the relationship between CP digestibility and the "b" value of N that was not significant.
Prediction Equations for N and DM Degradability
The correlation coefficients (Table 2
), derived from the linear relationship of degradability of N with nutrient concentration, fermentation characteristics, and digestibility, were generally decreased as ruminal outflow rates increased from 0.02 to 0.08/h, if the relationship was significant. However, the corresponding correlation coefficients with P0.05 and P0.08 of DM were generally greater than with P0.02 of DM. The P0.02 and P0.05 were thus selected for developing prediction equations for N and DM using either CP and NDF concentration or digestibility data as primary predictors, respectively. Four equations were developed to convert the degradability of N or DM predicted at a given ruminal outflow rate to equivalent values at any ruminal outflow rate from 0.02 to 0.10/h (discussed later). All linear, quadratic, and multiple relationships (using stepwise regression technique), reported in Tables 3 to 7, were significant (P < 0.001). Each predictor in the multiple prediction equations presented in these tables had a significant effect (P < 0.05) on the relationship.
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. Because the R2 values in the relationships of the "a," "b," or "c" values with digestibility data were not greater than those with nutrient concentration, the prediction equations with the former were not presented. With N, prediction of an "a" value using SCP produced a greater r2 value than using CP (0.38 vs. 0.25, Eq. [2] vs. [1], in Table 3). Adding water-soluble carbohydrates, lactic acid and pH values to Eq. [1] as predictors increased R2 value to 0.38 (Eq. [3]). Further inclusion of soluble N/total N as an additional predictor produced a further increase in the R2 value (0.52, Eq. [4]). However, the accuracy of prediction of the "b" and "c" values of N were not as great as that of "a" value, with the R2 values in the multiple prediction equations being only 0.37 and 0.23, respectively (Eq. [5] and [6]). The prediction accuracy for the "b" and "c" values of DM was also poor (R2 = 0.30 and 0.18; Eq. [11] and [12], respectively). However, the prediction of the "a" value of DM had a relatively high r2 or R2 value. The r2 value in the relationship with ADF or NDF was 0.48 and 0.62 (Eq. [7] or [8]). Inclusion of ether extract and fermentation variables increased the R2 values to 0.67 and 0.71 (Eq. [9] and [10], respectively).
The linear and quadratic prediction equations for N and DM degradability using nutrient concentration and digestibility variables are presented in Table 4. Also graphically presented are relationships between the P0.02 of N and CP concentration (Figure 2) and between the P0.05 of DM and NDF concentration (Figure 3). For prediction of the P0.02 of N, using CP and SCP concentration (Eq. [16a] and [17a]) as a predictor produced greater r2 values than using NDF, ADF, and lignin (Eq. [13] through [15]). Fitting CP and SCP in a quadratic line, rather than in a straight line, increased r2 values from 0.59 to 0.63 (Eq. [16b]) and 0.69 to 0.74 (Eq. [17b]). The relationships between the P0.02 of N and each digestibility variable were linear (Eq. [18] through [23]) and the relationship with CP digestibility was stronger than with any of the other digestibility data (r2 = 0.76 vs. 0.56 to 0.62). For prediction of the P0.05 of DM, NDF concentration (Eq. [24]) fitted the linear line better than ADF, lignin, CP, and SCP concentrations (Eq. [25] through [28]; r2 = 0.66 vs. 0.34 to 0.43). Prediction of the P0.05 of DM using OM digestibility (Eq. [30]) produced a greater r2 (0.70) than using any other digestibility variable (Eq. [29], [31] through [34]; r2 = 0.46 to 0.67).
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. The prediction of both the P0.02 of N and the P0.05 of DM was based on CP and NDF concentration in silages. These multiple regression equations (Eq. [35] and [39]) greatly increased the R2 values compared with the linear equation using either CP or NDF as a predictor. Inclusion of lactate/VFA as a predictor marginally increased the R2 value (Eq. [35] vs. [37] and [39] vs. [40]). Similarly, further addition of soluble N/total N for prediction of the P0.05 of DM led to a slightly further improvement of the relationship (Eq. [40] vs. [41]). However, adding soluble N/total N for prediction of the P0.02 of N substantially enhanced the R2 value from 0.69 to 0.81 (Eq. [35] vs. [36]) and from 0.71 to 0.82 (Eq. [37] vs. [38]).
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. The prediction of the P0.02 of N using CP digestibility and NDF produced a R2 value of 0.79 (Eq. [42]) and the addition of soluble N/total N to this equation increased the R2 value to 0.82 (Eq. [43]). Using either digestibility of DM, OM, or DOMD with NDF to predict the P0.05 of DM gave a similar R2 value (0.77 to 0.78, Eq. [44] through [46]). Including any other nutrient variable or fermentation characteristics as a predictor had no significant effect on these five equations.
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). With increasing ruminal outflow rates, the "a" value of N or DM accounted for a greater proportion of degraded N or DM. This led to greater accuracy for predicting the P0.08 than the P0.02 of N and DM using the "a" value, with the r2 value increasing from 0.50 to 0.77 with N (Eq. [47] through [49]) or 0.28 to 0.78 with DM (Eq. [54] through [56]). Adding either NDF or CP or both together as predictors to Eq. [49] and [56] for prediction of the P0.08 of N (Eq. [50] through [52]) and DM (Eq. [57] through [59]) increased the R2 value up to 0.86 and 0.85, respectively. A further inclusion of soluble N/total N for prediction of the P0.08 of N (Eq. [53]) increased the R2 value to 0.88. Discussion
Relationships of N and DM Degradability with Nutritive Values
As reported in the companion article (Yan and Agnew, 2004), the grass silages (n = 136) used in the present experiment were selected from a wide range of commercial farms and presented a considerable range in quality and an even distribution over the whole range. Accordingly, there were considerable variations in N and DM degradability as reported in Table 1 and Figures 2 and 3. The present data set is comparable to that of grass silages (n = 16 for N or 33 for DM) presented in the U.K. Tables of Nutritive Value and Chemical Composition of Feedingstuffs (MAFF, 1990) although the latter contained much less data.
The large variations in degradability of N and DM (Table 1), the relatively even distribution of degradability data over the whole range of silages (Figures 2 and 3), and the relatively large number of data in the present data set are three unique aspects when comparing with published data sets. The linear relationships developed between N or DM degradability and CP (CP, true CP, or SCP) or fiber (ADF, NDF, or lignin) concentrations were all highly significant (P < 0.001) with relatively high correlation coefficients. There are few studies in the literature that have examined similar relationships, but the statistical significance levels for these relationships were much lower than those obtained in the present experiment. One reason might be that these published data sets contained a much smaller range of data than the present data set. For example, Waters and Givens (1992) reported that the relationship between the P0.05 of N and CP or true CP concentration was significant at the 0.01 level using eight primary growth grass silages. Hoffman et al. (1993) also found a significant (P < 0.05) relationship between the P0.06 of N or DM and N (CP or SCP) or fiber (ADF, NDF, or lignin) concentration in a data set containing eight forages, each with three maturities (n = 24). Von Keyserling et al. (1996) even noted no significant relationship of the P0.06 of N with CP (n = 20) or ADF (n = 13) concentration, whereas the relationship with NDF (n = 9) reached significance only at the 0.05 level in grass silages. Tamminga et al. (1991) evaluated the relationships of undegradable dietary CP (UDP) with CP and NDF concentrations in grass silages (n = 35) and found that UDP was negatively related to CP but positively related to NDF concentration. The statistical significance for these two relationships was also at the 0.05 level.
It is worth noting that the P0.02 of N fitted CP (Figure 2) or SCP concentration better as a quadratic than as a linear relationship (Table 4). This indicates that increases in the P0.02 of N are gradually decreased with increasing CP or SCP concentrations in grass silages, with the P0.02 of N reaching a peak value at 0.22 kg/kg of DM of CP or 0.12 kg/kg of DM of SCP. This result is in accordance with the relationship between CP digestibility and CP or SCP concentration in grass silages, in which the r2 was greater in a quadratic than in a linear relationship when the same data set was used (Yan and Agnew, 2004). However, fitting of a quadratic relationship did not improve relationships between the P0.02 of N or the P0.05 of DM and fiber fractions or digestibility data, or between the P0.05 of DM and CP fractions.
As presented in Table 2, some fermentation characteristics in grass silages (pH, ammonia N/total N, lactic acid, and total VFA) had significant relationships with the degradability of N and DM. These fermentation characteristics were therefore used as supplementary predictors to predict N and DM degradability in the present experiment. They had a significant effect on the relationship if included in the prediction equation and addition of these data as predictors generally increased the R2 values (Tables 3 and 5). However, there is little comparable information in the literature on the effect of fermentation characteristics of grass silage on the degradability of N and DM.
The present comprehensive data set enabled development of a number of linear and multiple equations for predicting N and DM degradability. Nutrient digestibility and "a" values were used as primary predictors both with and without nutrient concentration and silage fermentation variables as supplementary predictors. Including these supplementary predictors improved the prediction accuracy, with the R2 values being greatly increased. Silage CP and NDF concentrations were also used alone or with other nutrient concentration and silage fermentation variables to develop the prediction equations. The R2 values obtained in the multiple regression relationships using CP and NDF as primary predictors were as great as those using digestibility data or "a" values, but the former can be widely adopted in practice as they are determined using a routine laboratory procedure. There are few prediction equations for N degradability of grass silages published in the literature. Waters and Givens (1992) reported that the P0.05 of N was 1.9 of CP concentration (kilograms per kilogram of DM) plus a constant of 0.421, whereas Webster et al. (1982) used both CP and modified ADF concentrations to predict N degradability. The prediction accuracy obtained using these two equations cannot be compared with the equations developed in the present experiment because there are too few independent data in the literature to validate these equations. However, these two equations had some limitations. The equation of Waters and Givens (1992) was developed from a small data set (n = 8) and the relationship reached significance only at a 0.05 level, whereas the equation of Webster et al. (1982) required modified ADF, which is no longer determined by a routine laboratory procedure.
Degradability Predicted at a Given Ruminal Outflow Rate
Nutrient degradability in the rumen can be decreased by decreasing the retention time of the digesta in the rumen when the same diet is offered. The effect of ruminal outflow rate on nutrient degradability is curvilinear (Ørskov and McDonald, 1979 as presented in Eq. [I]. This equation has been widely adopted across the world and was also used in the present experiment to calculate degradability of N and DM at ruminal outflow rates of 0.02, 0.05 or 0.08/h (P0.02, P0.05, and P0.08). These ruminal outflow rates are recommended in AFRC (1993) for ruminant animals offered diets at approximately once, slightly below twice, and over twice the maintenance feeding level, respectively. The ruminal outflow rates recommended in other systems differ. For example, for dairy cows, the ruminal outflow rate is suggested to be 0.06/h in the French feed-rationing system (INRA, 1989), 0.045/h for forages and 0.06/h for concentrates in the Dutch system (Tamminga et al., 1994), and 0.08/h in the Scandinavian system (Madsen, 1985). However, these recommended rates are based only on plane of nutrition, and make no differentiation between liquid and solid phases of digesta. There is also no allowance for the proportion of forage in the diet, although it is known that this affects the ruminal outflow rate (AFRC, 1992). Therefore, the retention time of digesta in the rumen varies greatly with changes in the level of feeding and the type of diet. A number of equations have been published to estimate ruminal outflow rate (Owens and Goetsch, 1986; Sauvant and Archimède, 1989; AFRC, 1992).
It is unrealistic to expect the results of the present experiment to provide prediction equations for N and DM degradability at any ruminal outflow rate. The prediction equations developed were only for a P0.02 or P0.08 with N and a P0.05 or P0.08 with DM. Four equations ([VI] to [IX], displayed following this paragraph) were therefore developed to convert the predicted P0.02 or P0.08 of N, or the P0.05 or P0.08 of DM to equivalent values (Pk) at any ruminal outflow rate (k = 0.02 to 0.10/h). The Pk used in the present experiment to set up these four equations was calculated using Eq. [I] (Ørskov and McDonald, 1979); the mean "a," "b," and "c" values of N or DM; and the ruminal outflow rates of 0.02 to 0.10/h. The technique adopted was to use the ruminal outflow rate (k = 0.02 to 0.10/h) as the x-axis and Pk as a proportion of the P0.02 or P0.08 for N or the P0.05 or P0.08 for DM as the y-axis. These data were fitted to both linear and a number of curvilinear regression relationships and the best-fitting line was with the quadratic regression. The data subscripted in parentheses are SE values in the following four equations. All four relationships were highly significant (P < 0.001) and the r2 values were close to 1 (0.9993 in Eq. [V] and [VI], and 0.9964 in Eq. [VII] and [VIII]). These four curvilinear lines are also presented in Figure 4.
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 | [VI] |
 | [VII] |
 | [VIII] |
 | [IX] |
Implications
The degradability of nitrogen and dry matter are two key variables required for formulating diets for ruminant animals using metabolizable protein-rationing systems as currently adopted across the world. There is, however, limited information available on these two variables in the literature. In the present experiment, sets of linear and multiple prediction equations for these two variables were developed for grass silage, using silage crude protein and neutral detergent fiber concentrations, nutrient digestibility, and the readily soluble fraction as primary predictors, together with other silage nutrient concentration and fermentation characteristics. These equations can be used in practice because they were developed from a large and comprehensive data set.
Footnotes
1 The authors thank their colleagues at the Agric. Res. Inst. of Northern Ireland for access to the data used in the present experiment. 
3 Also a staff member of the Dept. of Agric. and Rural Development of Northern Ireland and the Queen’s University of Belfast, Belfast, Northern Ireland BT9 5PX.
2 Correspondence—phone: 44 (0)28 9268 2484; fax: 44 (0)28 9268 9594; e-mail: tianhai.yan{at}dardni.gov.uk.
Received for publication August 15, 2003. Accepted for publication January 29, 2004.
Literature Cited
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Varvikko, T., and J. E. Lindberg. 1985. Estimation of microbial nitrogen in nylon-bag residues by feed 15N dilution. Br. J. Nutr. 54:473–481.[Medline]
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