| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Department of Dairy and Animal Science, The Pennsylvania State University, University Park 16802
2 Corresponding author: ajh{at}psu.edu
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
|---|
Key Words: nitrogen utilization • dairy calf • dairy heifer
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
|---|
Two factors that must be considered when addressing protein requirements of growing animals are that growth occurs on a continuum and that individual tissues can grow at a different rate than the body as a whole. Thus, the growth rate of the whole body and the size, growth rate, and nutrient requirements of an individual tissue may differentially affect nutrient requirements at different stages of growth. These relationships pose a difficulty when attempting to meet nutritional requirements during growth, because an experiment conducted at one phase on the growth curve has absolute validity only at that point. General relationships may emerge, however, from conducting or analyzing numerous experiments at several points along the growth curve. An added difficulty posed by the growing ruminant is the discontinuous mode of digestion and source of nutrients derived from advancing development of the rumen. Proteins from milk are generally of high biological value (Roy, 1970) and can contribute to very high levels of growth. Microbial protein is also a high-quality protein source (Storm et al., 1983) and can meet a significant quantity of the AA requirements in the functional ruminant.
Given the nutritional importance of protein in the growing dairy heifer and the limited information related to N partitioning, this investigation statistically examined responses to varying amounts of dietary N. Changes that may be related to BW and ME also were assessed. Outcome variables of interest in these analyses were measures of N partitioning as assessed in nutrient digestibility or balance trials, specifically, apparently digestible N (ADN), fecal N (FN), urinary N (UN), N retained (NR), and N efficiency. The objective was to statistically evaluate N retention and excretion in growing dairy cattle in response to increasing levels of dietary N intake (NI) in otherwise balanced rations fed across various growth stages. A secondary objective was to evaluate N utilization for basal purposes by extrapolations to zero NI and to compare these estimates between cattle fed liquid, milk protein-based diets and cattle that have been weaned.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
|---|
To enter the analysis, the experimental objective must have been to study the effects of differing levels of dietary NI in an otherwise balanced ration. At a minimum, trials had to report NI, FN, BW, and DMI to be included; data were not excluded for not reporting UN or NR. It was also considered acceptable if these responses could be calculated reasonably from other information given in the paper. Data must have been derived from total collection of feces and urine, with adequate procedures in place to minimize N loss by volatilization. Adequacy of collection procedures was evaluated statistically for FN and UN excretion by a regression of the mean within-trial response against the mean within-trial NI. A trial was removed as an outlying trial if the Studentized residual associated with this analysis was >3 standard deviations. Results from slaughter balance-growth trials were not included in the analysis unless results from a total collection trial were also presented.
Given the significance of Holstein-Friesian genetics in the North American dairy industry, the original intention was to analyze the N balance of only Holstein-Friesian heifers. However, several early studies on the protein nutrition of calves were conducted with other breeds, and implementing this restriction severely limited the number of experiments and animals eligible for analysis. Although there are substantial differences in mature size between large-framed breeds of dairy cattle, before cattle reach 100 kg there are relatively small differences between breeds in weight and height (Heinrichs and Hargrove, 1994; Heinrichs and Losinger, 1998). Furthermore, Brisson et al. (1957) indicated in an experiment that included Holstein and Ayrshire calves that there were no statistical differences between these 2 breeds in N utilization. Therefore, large-breed dairy calves were considered eligible for analysis if studied before reaching 100 kg if all other criteria were met (only experiments with Ayrshire and Holstein-Friesian calves met all criteria and were analyzed). After 100 kg, only dairy cattle of Holstein or Friesian genetics were eligible for analysis. Steers and bulls were considered eligible for analysis provided they were studied before reaching 200 kg of BW, because at greater BW effects of sex may become more significant in N retention (Fortin et al., 1980).
Applying these restrictions allowed data from 16 published papers to be used. Of these, 6 studied calves were fed milk or milk protein-based milk replacer (milk-fed group) with 37 different dietary treatments, with animals ranging from 30 to 81 kg of BW (Blaxter and Wood, 1951b; Brisson et al., 1957; Roy et al., 1970; Raven, 1972; Donnelly and Hutton, 1976; Blome et al., 2003). For animals in the milk-fed group, all nutrition was derived from the milk or milk replacer and no additional feed was offered (either starter grain or hay). Ten published papers studied cattle receiving diets based on forage, concentrates, or a combination of forage and concentrates (weaned group) with 57 different dietary treatments, with animals ranging from 56 to 472 kg of BW (Whitelaw et al., 1961; Broster et al., 1963; Bowers et al., 1965; Bines and Balch, 1973; Stobo and Roy, 1973; Veira et al., 1980; Bagg et al., 1985; Hoffman et al., 2001; Gabler and Heinrichs, 2003; Marini and Van Amburgh, 2003).
Data were analyzed statistically by using mixed modeling methodology to account for variation among trials in mean response as well as differential responses to varying levels of NI (St-Pierre, 2001). In an attempt to isolate effects of different levels of NI apart from other nutritional factors, the following procedures were enacted: if an experiment included multiple levels of DMI, each level was considered a distinct trial. Likewise, several papers reported outcomes of multiple experiments; in these cases, each experiment was considered a distinct trial. This practice allowed subpopulations within a paper to be modeled separately, but reduced the level of independence between trials in that these subpopulations were more closely related than the remaining population that entered the analysis from other papers. Analyses were originally conducted by grouping all outcomes from an individual paper under a single trial effect and by separating outcomes on the basis of other reported information; responses to additional NI were relatively unaffected, but variability of the fitted response was enhanced. From responses observed in this preliminary analysis, it was therefore assumed that loss of independence because of this practice was small relative to the variability that would be introduced by grouping nutritionally distinct groups together under one trial effect.
Several experiments reported data collected longitudinally at 2 or more time points from animals consuming the same diets. Because these results were each derived from the same animals, considering these repeated measures as distinct observations would be a more egregious violation of the assumption of independence. Therefore, results from these experiments were averaged by treatment over time within trial. These practices resulted in what were considered 27 distinct trials with 94 distinct observations made on 217 animals.
Because the objective of the current analysis was to assess the effects of different levels of NI at various stages of growth, it was important that the independent and dependent variables were comparable across large ranges of ages and BW. To enable this comparison, data were scaled to DMI (g/kg of DMI) for analyses of digestion or to BW0.75 for analyses of UN and NR. This practice allowed considerable overlap and constancy of the independent variables, both within and between milk-fed and weaned cattle over a large range in BW. However, this practice, as well as the lack of reported standard errors in several trials, made weighting data impossible; therefore, data presented in this analysis were not weighted by standard errors. Although scaling data enabled considerable overlap between independent variables, a substantial portion of variation remained and appeared to be due to differences in weaning status. Therefore, analysis of covariance was used to account for these different subpopulations and to statistically compare responses of milk-fed and weaned cattle to different levels of NI (Gujarati, 1970) by using the indicator variable approach (Suits, 1957). The implication of this analytical approach is that coefficients of the equations are calculated for one nutritional group (weaned cattle, in this case) and the deviations from these coefficients are calculated for the other nutritional group. The statistical test for determining differences between the coefficients of the nutritional groups is then a test of whether each of these deviations is different from zero individually or collectively. All data were analyzed and compared statistically according to the equations presented; however, for ease of exposition and interpretation, individual regression equations for both milk-fed and weaned cattle are presented. This was accomplished by adding the coefficient of the covariate to the main response coefficient.
Linear responses in ADN intake (ADNI) and FN were evaluated according to equation [1] (Table 1
), accounting for differences in dietary covariate and random trial effects by using the MIXED procedure of SAS (SAS Institute, 2006). Gross N efficiency for gain (NR/NI; GNE), which responded parabolically, was fit to a quadratic extension of equation [1], as shown in equation [2]. It has been suggested that a physiologically meaningful representation of ADN% data may be given by the nonlinear, hyperbolic function: Y = Bo – 100B1/X, where Y is ADN, %; Bo is true N digestibility, %; B1 is nondietary FN, g/kg of DMI; and X is NI, g/kg of DMI (Moran and Vercoe, 1972). This expression is essentially the linear model rearranged to relate NI to ADN% instead of ADNI. Because ADN% is of interest in dairy cattle nutrition and is the outcome most readily measurable in digestibility trials, this analysis was included for completeness, although parameter estimates should be similar to those fit by using a linear regression with normally distributed data and a close-to-linear parameterization of the nonlinear model. This equation was extended to account for random trial effects and differences attributable to dietary covariate according to equation [3] (Table 1) by using the NLMIXED procedure of SAS (SAS Institute, 2006).
|
) is proposed as a means to meet each of these theoretical and statistical criteria after extension with covariate and random effect variables. This equation approaches linearity, with a slope of B1, as NI approaches infinity and dUN/dNI approaches zero as NI approaches zero. The excretion of UN accelerates maximally in response to NI at the inflection point of dUN/dNI, when NI equals k. Basal UN is represented by Bo when NI approaches zero, although, because this equation is undefined when NI equals exactly zero and NI cannot be negative, a lower bound of NI > 0 exists.
Retained N was characterized by using a nonlinear asymptotic model (Robbins et al., 1979); however, a quadratic function also fit the data well in the range of NI analyzed. Because it is generally assumed that a growth response cannot decline at increased input of the limiting nutrient beyond the maximal response, the asymptotic exponential function, after adjusting for trial and dietary covariate effects, is reported and is shown in equation [5] (Table 1).
The adequacy of the variance-covariance matrix of the random effects was assessed for its contribution to model fit relative to the number of additional parameters by the Schwarz-Bayesian information criterion. Differences were considered statistically significant at P < 0.05. When the diet covariate tested to be not different from zero at P > 0.10, a model was fit by using a common regression coefficient for both the milk-fed and weaned dietary groups. The assumption of normality was assessed by using the normal probability plot correlation coefficient test (Filliben, 1975). Homoskedasticity of residuals and the presence of outliers were evaluated by assessing the Studentized residual vs. predicted response plot with outliers defined and removed when Studentized residuals were >3 standard deviations (<2% of observations across all analyses). The assumption that variances of each group (i.e., milk-fed or weaned) were statistically equivalent was assessed by using an exact F-test for each dependent variable. Because a large proportion of the variation associated with mixed modeling can be associated with the random effects, it was considered to be valuable to assess the variation that could be accounted for by the whole mixed model (RMixed2 ) as well as the variation that could be attributed to the fixed effects (RFixed2 ). These coefficients were calculated by including the squared residual produced by the mixed procedure for RMixed2 or as the squared difference between the observed value and that predicted from the equations presented below for R2Fixed. Because the parameters estimated and the relationships observed from the mixed modeling methodology are more valuable to the entire population from which the trials were sampled than those in fixed effect analyses, the figures present the dependent variable of interest after adjusting for random effects of trial (St-Pierre, 2001), except where indicated.
|
RESULTS AND DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
|---|
Using total NI, as opposed to an alternative expression related to protein quality (e.g., metabolizable protein), may lead to enhanced variability in responses to additional NI. Most experiments did not report an estimate of protein quality and many gave insufficient data to allow an estimate to be calculated confidently. Furthermore, N can enter and leave the digestive tract at virtually all sites along its length. Although the nutritional requirement is for AA and the site of absorption is the small intestine, the great majority of data in growing dairy heifers is for total tract N digestibility. For this reason, responses to additional NI and extrapolated basal N losses were evaluated in response to total NI.
It is well established that many metabolic transformations and degrees of physiological regulation are associated with N utilization and partitioning in both digestive and postdigestive processes in the ruminant animal. In the current analysis, however, we have attempted to describe animal responses to alterations in NI by using simplified mathematical representations on a whole-animal input-output basis, with the understanding that these observations are the sum of many physiological processes. However, these input-output mathematical representations can be of both practical and scientific value if an adequate description of the data can be achieved with physiologically identifiable parameters.
Finally, the range of time encompassed by papers in the current analysis is considerable (>50 yr). This time also corresponds to a tremendously rapid rate of genetic improvement for lactation performance and mature weight. The possibility therefore exists that the responses analyzed also depend on the year of study; however, limiting the years of study to only the most recent research would have severely diminished the number of experiments that would have been eligible for analysis. It is not altogether clear, however, whether the weight-specific inputs [NI and ME intake (MEI) per BW0.75] produce differences in the weight-specific responses to a level detectable in this type of analysis. For these reasons, data from all years were analyzed and reported.
Scaling
As indicated previously, intake and response data were scaled to enable comparison between animals differing greatly in BW. Typically, FN excretion and ADN% are considered related to DMI, and by implication, to the N concentration of the diet (Mitchell, 1924). However, it is likely that part of nondietary N excretion into feces is simply an alternative mode of N excretion derived from tissue metabolism (Orskov and Mac Leod, 1982). This creates the possibility that FN would be better related to BW, as noted from data compiled for calves by Lofgreen and Kleiber (1953). To test for this possibility, an initial analysis was conducted in which variation from diet source, DMI, and BW could be determined and the most appropriate variable for scaling assessed. Within this data set, FN and ADN% were more closely related to DMI (P < 0.001) than BW (P > 0.652) when both variables were in the model, supporting a previous report (Schneider, 1934). Thus, FN, ADNI, and NI are expressed relative to DMI in the current analysis when N digestion is discussed.
Urinary N and NR are commonly scaled to BW0.75 because of the relationship between the so-called endogenous N metabolism (Smuts, 1935) and basal energy metabolism and because basal energy metabolism varies according to BW0.75 in mature animals across species (Kleiber, 1932). To determine whether this base was appropriate for intraspecific comparisons of N metabolism in growing cattle in the current analysis, a fixed effects, unscaled alteration of equation [4] was fit to allow the BW exponent associated with the intercept to vary. On the basis of the results of this preliminary analysis, when the exponent was fit objectively, a scaling factor of BW0.573(±0.093) would be recommended. This value approximates the average exponent of BW0.58 found by Brody (1945) for resting metabolic rate in growing Holstein (BW0.60) and Jersey (BW0.56) cattle as well as the value of BW0.58 more recently reported by Freetly et al. (2003) for growing crossbred heifers. The exponent parameter was not significantly different from 0.75 (P > 0.062); the fit of the model also was not diminished substantially when the exponent was fixed at 0.75. For these reasons, and to maintain consistency with other literature results, UN and NR data are expressed relative to BW0.75.
N Digestion
The effect of NI on ADNI has been investigated several times in the past for preruminating and functioning ruminants. Typical analyses include simple linear, multiple linear, and nonlinear regression approaches including various factors thought to be involved with digestion of N. Simple linear regressions in the form of equation [1] relating NI to ADNI (both as g/kg of DMI) are frequently conducted to obtain estimates of true digestibility (TD, linear slope) and nondietary FN (NDFN, intercept). The term NDFN is defined in the current analysis as the amount of FN excreted when none is consumed (by extrapolation) and is not meant to imply the specific metabolic origin of this N, only that, by definition, it does not come directly from the diet. The utility of this definition and the applicability of the estimated NDFN to diets containing N depends on the assumption that the quantity of NDFN entering the feces remains constant and independent of the amount of N consumed. This assumption is briefly addressed below, but it is generally believed that in nonruminants the results of feeding protein-free diets or linear regression approaches represent a minimum value of NDFN that may increase as protein intake increases (Stein et al., 2007).
Results of this analysis are shown in Figure 1 and reveal considerable differences in NDFN between milk-fed and weaned cattle and smaller, although significant, differences in predicted TD. Because less than complete digestion would be indicated by FN increasing as NI increases, milk-fed animals completely digested the N provided, as indicated by a slope of ADNI vs. NI that did not differ from unity (P > 0.691) and constant FN at all levels of NI. These results are consistent with the results of an experiment conducted with multicannulated Holstein calves in which ileal flows of CP were not affected whether calves were fed a protein-free diet or increasing levels of milk protein (Montagne et al., 2000). This view also has been expressed previously (Cunningham and Brisson, 1957; Roy, 1970); however, other experiments have suggested less than complete digestibility of milk proteins based on different techniques (e.g., 93.5%; Lofgreen and Kleiber, 1953).
|
The NDFN was estimated to be 3.05 g/kg of DMI for milk-fed calves in the current analysis, but a fairly large range exists for estimates of NDFN based on a variety of techniques in other studies. For instance, when protein-free diets were fed, calves were found to excrete 3.34 (Cunningham and Brisson, 1957) or 4.27 g/kg of DMI (Blaxter and Wood, 1951a); regression analysis resulted in 2.24 g/kg of DMI (Donnelly and Hutton, 1976), and 32P-labeled CN yielded 2.7 g/kg of DMI (Lofgreen and Kleiber, 1953). Davis and Drackley (1998) adopted a value of 2.2 g/kg of DMI in their calculation of milk-fed calf N requirements based on the results of experiments using both N-free diets and extrapolation procedures; the NRC (2001) used a value of 1.9 g/kg of DMI.
The ADN% was substantially lower than the predicted TD observed for weaned dairy cattle (Figure 2) because of a large contribution of NDFN toward FN excretions. The NDFN predicted for weaned, growing cattle was 6.51 g/kg of DMI. This value corresponds closely to those reported from some experiments (Mugerwa and Conrad, 1971; Stallcup et al., 1975); however, it is higher than most, which averaged 4.7 g/kg of DMI in a review by Swanson (1977). In contrast to the constant level of FN excretion observed in milk-fed calves, more FN was excreted at higher levels of NI in weaned cattle (P < 0.001). From the results of the current analysis, it can be calculated that at the lowest level of NI, NDFN accounts for 95% of FN and at the highest level of NI, NDFN accounts for 82% of FN, assuming NDFN remains constant.
|
The previously addressed experiments relate to the animal contribution to small intestinal endogenous N flow, which would be anticipated to contribute the majority of NDFN in preruminant calves. Several experiments have shown that a considerable proportion of NDFN in functioning ruminants is unabsorbed bacterial N originating in the rumen or cecum (Mason, 1969), which likely contributes substantially toward the increased levels of NDFN observed for weaned cattle. However, even in milk replacer-fed calves there appears to be a substantial contribution of bacterial N toward ileal N flow. It was mentioned previously that ileal N was not different for calves fed a protein-free diet or 3 increasing levels of milk protein (Montagne et al., 2000). In this study, the proportion of ileal N predicted to be bacterial N increased from approximately 27% for the diet containing 28% CP to 50% for the protein-free diet, although bacterial N was not identified in jejunal or duodenal samples. Ruminal bacterial N has been reported to have a TD of 81% (Storm et al., 1983). Thus, even ruminal bacterial N, which would be available for small intestinal digestion, is not completely absorbed, and N incorporated into bacterial protein distal to or in the ileum would not be absorbed as AA N in appreciable quantities. From the results of the current analysis, it was concluded that across a large range of dietary and environmental conditions, total tract TD of N in growing dairy cattle is very high and that all N in the feces of milk-fed calves and a great majority of the N in the feces of weaned cattle is not directly of dietary origin.
UN
Urine N excretion is presented in Figure 3 in relation to NI (both as g/kg of BW0.75). Calves fed milk were predicted to excrete 0.278 g/kg of BW0.75 when NI approaches zero as an estimate of basal UN excretion. The intercept for weaned cattle was 0.123 g/kg of BW0.75; these estimates of basal UN excretion were significantly different from each other (P < 0.001). Brody (1945) found that an interspecies relationship existed between basal N excretion and a large range of mature animal BW and could be represented as 0.146 g/kg of BW0.72. Blaxter and Wood (1951a) observed basal N excretion of 0.194 g/kg of BW0.75 in 30-kg Ayrshire milk-fed calves, and Cunningham and Brisson (1957) obtained 0.186 g/kg of BW0.72 in 35- to 40-kg milk-fed Ayrshire and Holstein calves. In ruminating cattle fed virtually N-free diets containing both roughage and concentrates, Kehar et al. (1943) observed UN excretion of 0.083 g/kg of BW0.75. Swanson (1977) reviewed maintenance UN losses for cattle and determined that the best relationship was 0.440 g/kg of BW0.5 for BW ranging from 30 to 600 kg. Each of the previous estimates was based on N-free or very low N diets fed for various durations, which can have a negative impact on the rumen microbial population in ruminating cattle and can cause diarrhea, shivering, and reduced performance in calves (Blaxter and Wood, 1951a; Cunningham and Brisson, 1957). Orskov and MacLeod (1982) maintained growing steers at zero NI by using an intragastric infusion technique designed to separate whole-animal responses from metabolism in the rumen. With this procedure, UN excretions of between 0.295 and 0.426 g/kg of BW0.75 were observed in growing steers when energy was supplied as VFA but when no N was infused. The NRC (2001) used 0.2 g/kg of BW0.75 (Davis and Drackley, 1998) in its protein requirement calculations for milk-fed calves and 0.440 g/kg of BW0.5 (Swanson, 1977) for weaned cattle.
|
Although the range of NI was considerably greater for weaned cattle, the slope coefficients were not different individually or collectively (P > 0.10) from those for the milk-fed calves. The linear coefficient exceeded unity at the extrapolated infinite NI; however, at the highest level of NI represented within this data set, the slope of the UN vs. NI response was 0.967, a level closely approximating TD observed in the analyses previously addressed. Thus, as NI approaches the highest level occurring in the current analysis, UN parallels truly digestible NI. The coefficient k allows curvature in the UN excretion response between the linear response at maximum NI and the intercept, where the slope of the response curve approaches zero. This coefficient equals the NI for which the excretion of UN is maximally accelerating in response to dietary NI. This response coefficient was 0.698 and did not differ between milk-fed calves and weaned cattle. Within the range of NI included in the current analysis, there appears to be little differential effect of diet type in response to additional NI; the difference arises from a consistently lower level of UN excretion for weaned cattle. A consistently lower level of UN excretion for milk-fed calves may correspond to the previously emphasized microbial capture of dietary NI in the lumen of the gastrointestinal tract of the weaned ruminant (Mason, 1969). Another possible explanation for the reduced UN excretion in the weaned cattle is that there is enhanced recycling of urea N with subsequent microbial capture and FN excretion (Marini et al., 2004; Sunny et al., 2007). This would potentially lead to lower urinary urea excretion in weaned ruminants because this pathway would likely be minimally present in milk-fed calves (Hayashi et al., 2006). This result would tend to agree with the higher UN values produced by use of the intragastric infusion technique, which results in limited production of feces and minimal N capture in the lumen of the gastrointestinal tract (Orskov and Mac Leod, 1982). Milk-fed calves from the current analysis approached the minimum UN excretion reported by the use of this technique; however, neither group achieved this high level of basal UN excretion. In fact, 0.633 and 1.087 g/kg of BW0.75 must be consumed by milk-fed and weaned cattle for UN excretion of 0.300 g/kg of BW0.75, which was approximately the lowest level observed by Orskov and Mac-Leod (1982). Because it has been shown in several species that urea can be removed from circulation and contribute to FN at the expense of UN and that this proportion can be changed by dietary means (Orskov et al., 1970; Mosenthin et al., 1992), considering UN excretion in isolation from FN excretions may be erroneous.
Retained N
Retained N is related to NI in Figure 4, fit according to equation [5], where, in the terminology used here, –A is the extrapolated total N excreted when none is consumed (i.e., total basal N excretion in both feces and urine), A + B is the predicted maximum level of NR, and k is associated with the rate of increase from A to A + B. Coefficient A did not differ between milk-fed and weaned cattle (P > 0.20), so a model was fit with a common value between groups. This practice predicted that basal N excretion from the combination of feces and urine was 0.600 g/kg of BW0.75 for both dietary groups and that maximum NR was 1.199 and 0.741 g/kg of BW0.75 for the milk-fed and weaned group, respectively. The coefficient k was not significantly different between dietary groups, indicating that, because weaned cattle retain less N per kg of BW0.75, weaned cattle are less efficient in utilizing dietary N and are less responsive to alterations in dietary N content than milk-fed cattle.
|
Fitted results of this effort are shown in Figure 5 for the approximate range of NI, BW, and 3 representative MEI occurring in this data set (mean MEI ± 2 x SD). The reduced model shown fit the data well, as indicated by a high R2 of 0.941 with 8 estimated parameters; it accounted for 98% of the variation explainable by the full model containing 20 coefficients and provided a substantially better fit than the minimum model containing 3 coefficients (R2 = 0.438). The ln(MEI) was included as a simple means to account for a diminishing returns response to additional dietary energy, as opposed to a linearly increasing function. This procedure was used previously to relate the effects of energy on NR in sheep (Walker and Norton, 1971) and in attempting to determine the N requirement for N balance in human subjects (Rand et al., 2003).
|
This result contrasts, however, with observations noted by Bartlett et al. (2006), in which a constant efficiency of N utilization (0.743) was determined for whey protein-fed calves until reaching a point (calves fed 26% CP at 1.25% BW) at which ME was considered to limit N deposition. Similar observations also have been shown for milk-fed lambs (Black and Griffiths, 1975), intragastrically infused lambs (Lindberg and Jacobsson, 1990), and growing pigs (Campbell et al., 1984, 1985; Kyriazakis and Emmans, 1992). That is, efficiency of dietary N utilization remained constant until energy limited NR, after which no additional N gains were possible in response to NI. In an effort to examine this form of relationship between NI and NR for animals in this data set, a fixed-effect rectilinear regression analysis was conducted, regressing NR against NI and allowing the estimated parameters to vary with BW0.75, ln(MEI), and diet type as previously described for the diminishing returns, exponential response. The primary result of this analysis demonstrated that the slope of the linear portion of the relationship was affected by diet type, MEI, and BW0.75. Because marginal efficiency of NR was affected in the linear range by ME intake, it cannot be concluded that the protein- and energy-dependent phases of NR describe these data most adequately. As such, the exponential model was accepted and is used as a basis to discuss responses to changing BW and efficiency of N utilization.
The declining response of NR to increasing BW was not unexpected based on the diminishing weight-specific growth that occurs as an animal approaches maturity (Brody, 1945). The rate of decline in maximum predicted NR for cattle in the current analysis depended on the level of ME consumed and was not differentially affected by diet type, with the maximum decline in maximum NR occurring for the higher levels of MEI and lower reductions predicted for lower levels of MEI. When MEI was 1.06 MJ/kg of BW0.75, NR declined by 0.011 g/kg of BW0.75 for each unit increase in BW0.75, whereas when MEI was 0.55 MJ/kg of BW0.75, NR declined by 0.004 g/kg of BW0.75. In the analysis of milk-fed lambs by Black and Griffiths (1975), maximum NR declined by 0.146 g/kg of BW0.75 for each unit increase in BW0.75 at the highest level of MEI investigated. Carr et al. (1977), in an analysis of NR in the pig, determined that maximum NR responded to increasing BW in 2 phases. For BW less than 45 kg, maximum NR declined quadratically with BW, which, if expressed as a linear function of BW0.75, is a 0.129 g/kg of BW0.75 decline for each unit increase in BW0.75. After 45 kg of BW, maximum NR declined by 0.026 g/kg of BW0.75 per unit increase in BW0.75 (Carr et al., 1977). More recent work in pigs found that the upper limit to NR (g/d) in the strain investigated was constant between 25 and 85 kg of BW; relative NR, however, declined at a rate of 0.08 g/kg of BW0.75 for each unit increase in BW0.75 (Moughan et al., 2006).
The rate of decline of NR observed in the current analysis was substantially lower than the rates of decline observed for these other farm animal species; however, given differences in the mature size and rate of maturation between these species, it is not surprising that the pig and the sheep have a greater rate of decline in NR than do cattle, as seen in the current analysis. Other experimental data directly confirm the rates of maximum NR reduction in response to increasing BW observed in this statistical analysis: from maximum levels of NI and MEI and the 2 BW reported by Blaxter et al. (1966), a decline in NR of 0.011 g/kg of BW0.75 per unit increase in BW0.75 could be calculated. In the current analysis, a differential effect of diet type and BW could not be detected on the decline in NR and may indicate that the decline in maximum NR was more closely related to developmental cues related to BW than dietary type. This has been confirmed from a more mechanistic perspective for milk-fed pigs (Wray-Cahen et al., 1998; Suryawan et al., 2006), indicating that at least one potential factor influencing this decline is that feeding-induced protein synthesis diminishes because of a reduced responsiveness to insulin with advancing age.
Because no differences were observed in the NR response, the marginal efficiency function (dNR/dNI from the equations in Figure 5) calculated for milk-fed and weaned cattle are not different between nutritional groups. Some support for the validity of this response comes from the analysis of the heavy (80 to 240 kg) milk-fed calves of Gerrits et al. (1996), in which mean marginal efficiency of retaining NI across weights and intakes was reported to be 26.8%. Using MEI, CP intake, and mean BW reported for those experiments, we calculated a mean marginal efficiency of 31.7% from the equation associated with Figure 5. The marginal efficiency results of Gerrits et al. (1996) were calculated by using the linear slope coefficients across several levels of NI, whereas the results from the current analysis are an instantaneous estimate of marginal efficiency. Differences in computations may lead to deviations in the estimated marginal efficiencies between methods; however, the similarity in estimates is surprising considering that the maximum BW for milk-fed cattle was 81 kg. This observation would seem to lend some support to a hypothesis that marginal efficiency of using additional NI depends more on development, as indicated by advancing BW, than on being milk-fed or weaned per se.
The GNE, adjusted for random trial effects, is shown in Figure 6 in response to NI. The variability across experiments was considerable, but when this variation was modeled by using the random trial effect to isolate the response to additional NI, a significant curvilinear relationship emerged. Gross N efficiency was greater for milk-fed calves than weaned heifers, but the response to NI did not differ between diet types (P > 0.151). From these equations, maximum GNE was predicted to occur when NI was 1.84 g/kg of BW0.75 regardless of diet type; however, because the energy and CP density of the rations were greater for milk-fed calves, the relationship between CP concentration and GNE differed significantly between diet types. Maximum GNE occurred at 18.9% CP for milk-fed calves, whereas weaned cattle obtained maximum GNE at 14.2% CP. Thus, although the NI required for maximum GNE was not statistically different between diet types, differences in nutrient density of the rations offered altered the CP concentration at which the maximum GNE was predicted to occur. Another potentially valuable means of expressing CP concentration is relative to the energy density of the diet (Baker, 1986). Expressed in this fashion, assuming 16.74 kJ of ME/g of CP and 6.25 g of CP/g of N, the maximum GNE occurs when 22.5% of dietary energy is energy derived from CP (i.e., MJ of CP/MJ of ME) for both dietary groups. This value corresponds closely to the protein-to-energy ratio of 20% (Campbell et al., 1984), 22% (Campbell et al., 1985), 23% (Kyriazakis and Emmans, 1992), and 25% (Sandberg et al., 2005) where maximum N efficiency occurred in pigs, and a protein-to-energy of 22% (Blaxter and Wood, 1951b) to avoid utilizing protein to meet energy requirements in maximally gaining milk-fed calves; however, this value is lower than the theoretically derived optimum protein-to-energy ration of 27% (Miller and Payne, 1963).
|
Absolute levels of NR and GNE are, however, substantially lower in older and weaned cattle than in younger and milk-fed cattle. The sum of basal N requirements predicted from linear and exponential extrapolation to zero NI (from Figures 1 and 3) predicted that milk-fed calves would excrete 0.411 g/kg of BW0.75 and weaned cattle were predicted to excrete 0.649 g/kg of BW0.75. Furthermore, it can be calculated based on the results of the current analysis that 32.4 and 81.0% of basal N excretions were lost in feces for milk-fed and weaned animals, respectively. Dry matter intake was a variable significantly related to basal N excretion in the fixed-effect exponential model, and predicts that when NI is zero, N is excreted in both urine and feces. When DMI is zero, however, as may be approximated experimentally with intragastric infusion, 0.300 g/kg of BW0.75 is predicted to be excreted in urine alone.
Orskov and MacLeod (1982) found in dairy cattle that total N excretion at zero N infusion with VFA energy infusion was between 0.308 and 0.429 g/kg of BW0.75, and more recently, Orskov et al. (1999) found a value of 0.261 g/kg of BW0.75 when glucose energy was infused but no N was given. Furthermore, Hovell et al. (1987) indicated that a mean value of basal UN excretion for 23 lambs nourished by intragastric infusion of VFA with no NI was 0.356 g/kg of BW0.75. These values agree closely with the 0.411 g/kg of BW0.75 determined for milk-fed calves based on summing fecal and urinary basal N excretion and the value of 0.300 g/kg of BW0.75 determined with the fixed effects model when DMI is zero. The question arises as to whether a value of the obligate basal N excretion may be near 0.300 to 0.400 g/kg of BW0.75 for growing ruminants. This low level may be attainable only when a small quantity of DM passes through the digestive tract, whereas higher values observed for weaned cattle may arise because of substantially greater N captured in the lumen of the digestive tract, as has been suggested previously (Orskov, 1992). Results from the current analysis tend to support this view; thus, a weaned animal requires 1.57 times as much NI simply to replace basal losses before additional N can be utilized for gain.
|
CONCLUSIONS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
|---|
Milk-fed calves averaged greater NR and marginal efficiency than weaned heifers, but there was limited evidence to suggest that the change in marginal efficiency was related to differences between diets as much as to changes associated with BW. Milk-fed calves had a substantially greater GNE than weaned heifers; however, differences in GNE between milk-fed and weaned cattle could be accounted for to a large degree by changes in basal N excretion. The CP concentration that maximized GNE was 18.9% for milk-fed calves and 14.2% for weaned cattle; weight-specific NI (g/kg of BW0.75) and ratio of CP to ME (MJ of CP/MJ of ME) were expressions of NI for which a single level of NI was found to maximize GNE for both dietary populations. The NI that maximized the GNE of both milk-fed and weaned calves was 1.84 g/kg of BW0.75 and 22.5% MJ of CP/MJ of ME. From the results of this analysis, it can be concluded that responses in FN and UN to altered levels of NI are largely similar for milk-fed and weaned dairy cattle; however, the NR or efficiency differences appeared to be less related to differences between diets than to changes associated with BW and basal N excretions.
|
ACKNOWLEDGEMENTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
|---|
|
FOOTNOTES |
|---|
Received for publication August 17, 2007. Accepted for publication December 14, 2007.
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
|---|
This article has been cited by other articles:
|
|
 |
|
  G. I. Zanton and A. J. Heinrichs Digestion and nitrogen utilization in dairy heifers limit-fed a low or high forage ration at four levels of nitrogen intake J Dairy Sci, May 1, 2009; 92(5): 2078 - 2094. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G. I. Zanton and A. J. Heinrichs Rumen Digestion and Nutritional Efficiency of Dairy Heifers Limit-Fed a High Forage Ration to Four Levels of Dry Matter Intake J Dairy Sci, September 1, 2008; 91(9): 3579 - 3588. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
) or weaned (x), after adjustment for random trial effects. Extrapolations are indicated by broken lines. Parenthetical values associated with the equation are SE. Intercept (P < 0.001) and slope (P < 0.008) coefficients differed for both apparently digestible N intake (ADNI) and fecal N (FN) excretion. The milk-fed ADNI slope coefficient was not different from 1, and the FN slope coefficient was not different from zero (P > 0.691). Apparently digestible N intake, milk-fed: ADNI = –3.05 (±0.40) + 1.004 (±0.009) x NI. Apparently digestible N intake, weaned: ADNI = –6.51 (±0.31) + 0.964 (±0.011) x NI. RMixed2 = 0.999; RFixed2 = 0.994. Fecal N, milk-fed: FN = 3.05 (±0.40) – 0.004 (±0.009) x NI. Fecal N, weaned: FN = 6.51 (±0.31) + 0.036 (±0.011) x NI. RMixed2 = 0.975; RFixed2 = 0.859.
) or weaned (
). Milk-fed: ADN% = 100.26 (±1.11) – 100 x 2.90 (±0.42)/NI. Weaned: ADN% = 95.75 (±0.91) – 100 x 6.35 (±0.26)/NI. RMixed2 = 0.995; RFixed2 = 0.954.
. Weaned: 
. RMixed2 = 0.992; RFixed2 = 0.750.
