Effects of liveweight and dietary rumen degradable nitrogen on the growth and feed intake responses of concentrate fed lambs to escape protein supplements.

Abstract

97 Effects of liveweight and dietary rumen degradable nitrogen on the growth and feed intake responses of concentrate_fed lambs to escape protein supplements R.S. Hegar ty1,3, S.A. Neutze2 and P.J. Nicholls 1 2 3 1 NSW Agriculture, Elizabeth Macar thur Agricultural Institute, PMB 8, Camden NSW 2570 PO Box 548, Wentwor thville NSW 2145 Present address: NSW Agriculture Beef Industry Centre, University of New England, Armidale NSW 2351 roger.hegar ty@agric.nsw.gov.au Summary A study was conducted to evaluate the effects of initial liveweight (W) and rumen degradable nitrogen (RDN) supply on the feed intake and live weight gain (LWG) responses of lambs to escape protein supplements. One hundred and twenty_eight flock_reared crossbred wether lambs were selected at weaning to provide a uniform distribution of initial W from 16_35 kg, while excluding exceptionally fast or slow growing animals. For seven weeks the lambs were fed one of two basal diets containing 12 MJ ME/kg DM with adequate (+U; 1.4% w/w urea), or inadequate (_U; no urea) RDN. Superimposed across both basal diets were escape protein supplements providing 25 g/d of undegradable dietary protein (UDP) as formaldehyde treated vegetable protein (EP, 133 g/d) or formaldehyde treated casein (FC, 40 g/d). Control lambs received no escape protein supplement. LWG of lambs changed in a quadratic manner over the study period and was significantly greater for +U than _U lambs over the final 2 weeks. LWG of EP supplemented lambs was significantly faster than that of unsupplemented or FC lambs. There was a significant interaction between dietary urea content and protein supplementation for both basal feed intake (BFI, kg/d) and W, such that there were positive effects of supplements on W and BFI for _U but not +U lambs. This indicated that the responses in BFI and W were due to the effects of additional RDN supply to the rumen. The slope of the regression relation between W and initial W was greater for _U than for +U lambs. A similar result was observed for the relation between BFI and initial W and, in addition, initial W interacted with urea to moderate the effects of supplements on BFI. Comparison of estimated supplies of apparently digestible protein leaving the stomach with estimated lamb protein requirements showed that as lambs gained LW and increased intake of the _U basal ration, the importance of RDN contributed by the protein supplement increased while the need for protein supplements to provide UDP decreased. These findings confirm the need to consider the weight of the lamb, as well as the RDN and UDP contents of both the basal diet and supplement, in predicting growth responses to escape protein supplements. Keywords: escape protein, lambs, growth, undegraded dietary protein Introduction Feeds containing substantial amounts of protein with low degradability in the rumen (escape protein) such as oilseed meals are widely advocated as being advantageous for production by both grazing and intensively fed ruminants (Wilkerson et al. 1993; Poppi and McLennan 1995; Klopfenstein 1996; Kandylis et al., 1999). With some products, particularly wool (Reis 1969), consistent production responses are observed and are largely attributable to the additional amino acid supplies to the animal from rumen undegraded dietary protein (UDP) digested in the small intestine. Liveweight gain (LWG) responses are less consistent (e.g. Fraser et al. 1991; Hegarty et al. 1999), indicating amino acid supply is not the only factor involved. There is a growing awareness that the nutritional value of the total diet can be constrained by the supply of nitrogen (RDN) available to the rumen microbiota from rumen degradable protein and other nitrogenous materials, and the supply of energy from fermentable carbohydrate (Fulkerson and Travaskis 1997). Because commercial protein meals often contain more rumen degradable protein than UDP, the adequacy of RDN Recent Advances in Animal Nutrition in Australia, Volume 13 (2001) 98 Hegar ty et al. provided by the basal diet is one factor which could moderate LWG response to escape protein supplements. In many cases these supplements have promoted intake of low digestibility feedstuffs, and this may have been through stimulation of rumen fermentation (Kellaway and Leibholz 1981; Lee et al. 1985; Robinson et al. 1998). The amino acid requirement of the animal is another factor often overlooked in attempting to understand LWG responses to escape protein meals. Daily protein deposition rate and therefore tissue amino acid requirement decline as lambs enter the fattening phase (e.g. Searle and Graham 1987) and, with an increase in voluntary feed intake with increasing liveweight (W) there will be an increase in microbial and dietary protein supply relative to the requirement. It was therefore hypothesised that W and the adequacy of the RDN supply would both affect the responsiveness of ruminants to UDP supplements when provided with a basal diet available ad libitum. The objectives of the present study were therefore to: � compare the feed intake and growth responses to escape protein supplements by lambs offered a basal diet with or without adequate RDN ddentify whether observed responses were also dependent upon lamb liveweight at the time the supplement was given clarify the extent to which LWG responses to escape protein meals were mediated via an increase in the intake of the basal feed (BFI) evaluate the relative efficacy of two UDP sources in increasing LWG. � � � Materials and methods One hundred and twenty_eight normally grown lambs with W evenly distributed over the range of 15_30 kg were selected for the experiment. First_cross wether lambs (Border Leicester Improver x Dorset) reared on ewes at pasture were obtained at weaning from a commercial property at Cowra, New South Wales. To ensure lambs of each initial weight were truly representative of lambs of that weight and age, preweaning growth rate of 200 male lambs (initially 9_23 kg W, 4_10 weeks old) were measured for six weeks prior to selection. The initial draft of 200 male lambs was chosen to contain approximately 15 lambs in each of 13 liveweight classes each spanning approximately 1 kg. At the end of the on_farm preliminary period, all lambs were reweighed and ranked within weight classes on the basis of LWG over the preceding 6 weeks. The fastest and slowest growing lambs within each class were excluded so that only lambs with growth rates typical of that W were taken into the trial. This provided 128 lambs with a uniform distribution of W at entry to the trial (16_35 kg). They were weaned directly into an enclosed animal house where they were individually penned. The lambs were ranked according to W and preweaning growth rate and assigned by stratified randomisation to eight treatment groups (16 lambs/ treatment) so that those in each group had a similar average preweaning growth rate and initial W distribution. All lambs received a booster vaccination (1 mL Ultravac 5 in 1, CSL Ltd, Parkeville, Victoria) and were treated for internal parasites (8 mL Ivomec, MSD AgVet, South Granville, NSW, and 15 mL Mansonil M, Bayer Australia Ltd, Botany, NSW). There was a 7 day adjustment period prior to the start of the 45 day experimental period. Lambs were offered one of two pelleted basal diets (12.2 MJ ME/kg DM) appropriate for rapid growth, but differing in RDN content by the absence (_U) or presence (+U) of urea (Table 1). In addition, lambs were offered (Table 2) either no supplement (Control), or 25 g/d of UDP provided as escape protein (EP), or casein treated with formaldehyde (0.7% w/w; FC). EP was a formaldehyde_ treated vegetable protein meal from Bypass Stockfeeds, Barradine, NSW. Four lambs were excluded from statistical analyses due to sustained low intakes. Rumen degradability of EP was determined in situ (�rskov and McDonald 1979) using nylon bags in cannulated heifers fed a forage_concentrate mixture. UDP was determined as the proportion of protein remaining after 20 h incubation using a single exponential function without a lag phase (McDonald 1981). Because of the tendency of FC to aggregate in nylon bags, its degradability was determined in vitro (Neutze et al. 1993). The proportion of UDP in the protein of each supplement was estimated to be 0.533 (EP) and 0.790 (FC). In a preliminary trial it was found that even after 14 days the lambs were consuming on average only 34_47% of the three supplements offered when provided in manufactured form (i.e. as small particles or powder). Mixing the supplement with 30 g of molasses and 15 g of ground lucerne produced only a mar ginal improvement in acceptance. Hence, in the main experiment, supplements were pelleted with a small quantity of the basal diet to promote complete consumption of the supplement. The feeding protocol involved withdrawing any basal pellet refusals at 0800 h and then offering to each lamb 350 g of supplement pellets which contained 133 g EP or 40 g FC. At 1200 h, any supplement pellets refused were removed and basal pellets were offered again at rates equal to the mean daily intake over the previous three days plus 20%; these rates were computed using a program integrated with the feed scales. Calcium (Ca), phosphorus (P) and sulphur (S) inclusions in the basal diets (Table 1) were based on SCA requirements (1990), with Ca:P ratio = 3.5:1 and RDN:S = 10:1 for _U and 14:1 for + U diets respectively. Since all supplement pellets comprised both basal feed and the protein sources, recorded intakes of the basal diet (BFI) were adjusted to include the basal diet provided in supplement pellets. Responses in lamb growth to protein supplements 99 Table 1 Constituents and chemical composition of basal diets. Feed ingredients (%) Wheat grain Straw Oat hulls Starch Tallow Urea Limestone Sodium chloride Dicalcium phosphate Sodium sulphate Beef Feed Aid _ Urea 51 26.7 8.4 8.4 2.5 0 1 1 1 0.6 0.4 + Urea 50 26.3 8.4 8.4 2.5 1.4 1 1 1 0.6 0.4 Pellet analysis: Dry matter (DM) (%) Organic matter (OM) (%) Crude protein� (%) Crude fat (%) Acid_detergent fibre (%) IVOMD� (%) Estimated ME# (MJ/kg DM) 92.6 92.6 10.50 3.39 17.42 77.8 12.2 92.0 91.6 13.44 3.06 18.30 77.1 12.1 Vitamin/mineral premix (Inter national Animal Health Products, Smithfield, NSW) Values are on a dr y matter basis � Crude protein = N*6.25 � In vitro OM digestibility (Ayres 1991) # ME = 0.157*IVOMD (Barber et al. 1984) muscle area (EMA) was calculated as width x depth x 0.8 (Hopkins et al. 1992). Rumen ammonia concentrations were measured in four lambs per treatment, selected to evenly cover the range in W. Rumen fluid from selected lambs was collected by a tube passed down the oesophagus on days 24 (pre_feeding) and 25 (post_feeding) and again on days 43 (pre_feeding) and 44 (post_feeding). The pre_ feeding samples were taken just prior to feeding at 0800 h and the post_feeding samples at 1430 h. Rumen fluid (approximately 20 mL) was collected into 4 drops of 18M H2SO4 and frozen. Ammonia_N was determined in the supernatant fraction using a Kjeltec 1030 automated N analyser (Tecator AB, Sweden) after thawing and centrifuging samples at 800 g for 20 min. The requirement of lambs for apparently digestible protein leaving the small intestine (ADPLS g/d) was calculated from their LW and rate of LWG using equations in SCA (1990) as summarised in Table 3, assuming standard reference weights and fleece weights of 66 kg and 3 kg respectively. ADPLS supply was calculated from the supply of microbial crude protein (MCP; 8.4 g MCP/ MJ ME intake) and the quantity of digestible undegraded dietary protein (DUDP) from the basal ration and from protein supplements. Digestibility of all dietary proteins in the intestines was assumed to be 0.7 while the apparent digestible protein derived from rumen microbes was calculated as MCP*0.8*0.7 to allow for non_protein nitrogen content and digestibility of microbial nitrogen respectively (SCA 1990). Statistical methods Linear mixed models were used to analyse the repeated W and BFI measures for the experimental period. The fixed effects consisted of the effects of urea and escape protein treatments and of initial W at day 0 of the experimental period, together with their 2_factor and 3_factor interactions. In the models, comparisons among the effects of supplements were made using two single_ degree_of_freedom contrasts: (i) unsupplemented v. EP + FC; (ii) EP v. FC. Corresponding components of their interactions with the other factors were also used. The method of smoothing splines (Verbyla et al. 1999) was used to assess curvature of time trends and interactions of curvature with the fixed effects, with splines included in the models as random effects. Random linear coefficients (intercepts and slopes) over time for individual lambs were also included in each model. A final model for each dependent variable was obtained by successive elimination of non_significant interactions involving time trends or initial W. To facilitate presentation of the effects of initial W on BFI and change in W, mean values for lambs of three initial W (16 kg, 24 kg, 32 kg) were predicted from the final model. Interpretations of effects on LWG were obtained from the final model for W by inferring as significant for LWG each effect or interaction for W which had a significant interaction with a time trend (linear or curvilinear). Estimates of mean LWG between Table 2 Composition of for maldehyde treated casein (FC) and a commercial for maldehyde_treated vegetable protein supplement (escape protein, EP) fed to crossbred lambs. BP DM (%) Organic matter (%) Nitrogen (N ; %) N_degradability (%) ADIN ADF (%) Fat (%) 92.4 85.6 5.97 0.47 0.22 22.9 3.14 FC 89.2 nd 14.58 0.21 nd nd 0.22 nd, not determined Acid detergent insoluble nitrogen Acid detergent fibre On days 10 and 45, all lambs were examined with real_ time ultrasound (Aloka Echo Camera SSD_500, Aloka Co. Ltd, Japan) to determine tissue depth at the GR site (12th rib, 110 mm from the dorsal midline), and fat depth at the C site (12_13th rib junction, 40 mm from the dorsal midline). Width and depth of the eye muscle was determined between the 12th and 13th rib, and eye 100 Hegar ty et al. measurements of W and their standard errors were obtained from differences between successive mean W predicted from the final model. Analyses of EMA and GR assessed the same fixed effects and interactions of the design variables as for W, but the time effect was the difference between two sampling days, so that time and its interactions with the design factors were all fixed. The random effects in the model were those of individual lambs and their sampling day differences. A logarithmic transformation was applied to the rumen ammonia data to overcome variance heterogeneity. No error correlations between samplings were detected for the transformed data so they were analysed as a factorial design with fixed effects of urea, supplements and sampling times. All statistical analyses were performed using ASReml (Gilmour et al. 1999) and all tests of significance were conducted at P<0.05 unless indicated otherwise. Standard errors of weekly mean RDN and ADPLS estimates were calculated from the standard errors of predicted means for W, LWG, BFI and supplement intake and from error correlations between relevant measures. Results There were effects of urea and protein supplements on W that varied significantly with weeks and so had significant effects on LW G. Both the linear and curvilinear components of lamb growth over weeks varied between _U and +U treatments, resulting in LWG responses for these treatments that were approximately quadratic (Figure 1). Although not significantly different before day 28, mean LWG for +U treatments was significantly greater ( P <0.01) than that for _U treatments for days 28 to 35 (307 � 16 g/d v. 243 � 16 g/d) and from days 35 to 43 (265 � 22 g/d v. 183 � 21 g/d). When averaged over urea treatments, there was a significant interaction effect on W between supplement treatments and the linear component of the time trend, but not the curvilinear component. Consequently, mean LWG over the trial period for lambs given the EP supplement (269 � 8 g/d) was significantly greater than that for both the FC (243 � 7 g/d) and the unsupplemented (Control) groups (245 � 8 g/d). There were other effects on LW that were statistically significant but that did not vary significantly with weeks, so they did not affect LWG. There was an interaction between the urea and supplement treatments, mainly due to a lower mean W for the no_supplement (Control) _U treatment than for the other treatments. There was also a strong relation with initial W that dif fered significantly ( P <0.001) between urea treatments. The regression coefficients between W and initial W were 0.96 � 0.04 kg/kg and 0.75 � 0.05 kg/kg for _U and +U treatments respectively. The greater slope for the _U treatment lambs indicated that lambs of low initial W continued to have relatively low W, whereas the +U lambs of low initial W were able to recover and achieve a heavier W later in the trial. Mean BFI increased as the trial progressed (P<0.001) but the rate of change slowed during the later 350 300 250 LW G (g/d) 200 150 100 50 0 5 10 15 20 25 30 35 40 45 Days Figure 1 Weekly change in mean LWG (g/d) of lambs when consuming a diet containing no urea (open symbols) or diet containing 1.4 % urea (solid symbols) alone (,) or with 25 g/d supplementar y UDP provided as formaldehyde treated vegetable meal (�,�) or for maldehyde treated casein (I,G). The day 7 means are sample LWG estimates whereas the later means are derived from differences in predicted LW. The average standard errors about the means for LWG to days 7, 14, 21, 28, 35 and 43 are shown as ver tical lines below the plot. Responses in lamb growth to protein supplements 101 weeks to an extent that was more pronounced for _U than +U treatments (Figure 2). There was a significant urea x supplement interaction on BFI such that the average effect of protein supplements for _U treatments was positive but no corresponding effect was detected for the +U. On average there was also a positive linear effect of initial W on BFI, which increased with time (P<0.01) without significant curvature in the response. Initial W was involved in a significant 3 _ factor interaction with urea and supplements. For _ U treatments the effect of initial W on BFI was greater for supplemented than unsupplemented treatments, but the reverse occurred for the +U treatments (Figure 2). On days 10 and 45, EMA and GR fat depth both had strong linear relations with initial W (P<0.001). At day 10, EMA was significantly greater for +U lambs than _U lambs (6.9 � 0.1cm2 v. 6.6 � 0.1 cm2). By the conclusion of the study a significant urea x supplement interaction was apparent (P<0.01) such that for _U lambs there was a small, non_significant increase in EMA due to supplements (9.7 v. 9.4 cm2); for +U lambs, supplements were associated with a significantly decreased EMA (9.6 v. 10.5 cm2) for reasons that are not clear. The GR fat depth of lambs was unaffected by treatments at either day of sampling. Rumen ammonia _ N concentrations were significantly influenced by time of sampling (pre_ feeding v post_feeding) and by urea treatment (both P<0.001), but not by day of sampling or supplements. There was no significant interaction between time of sampling and urea treatment. The interaction between urea treatment and day of sampling was significant (P = 0.06), although the main effect of day of sampling did not approach significance. The mean of pre_feeding 14 13 12 14 a Fit ted BF I (kg/week) 13 12 11 10 9 8 7 6 5 4 3 14 13 Fit ted BF I (kg/week) 11 10 9 8 7 6 5 4 3 14 13 12 b Fit ted BF I (kg/week) 12 11 10 9 8 7 6 5 4 3 Fit ted BF I (kg/week) 11 10 9 8 7 6 5 4 3 14 13 12 14 c Fit ted BF I (kg/week) 14 16 18 20 22 24 26 28 30 32 34 13 12 11 10 9 8 7 6 5 4 3 14 16 18 20 22 24 26 28 30 32 34 Fit ted BF I (kg/week) 11 10 9 8 7 6 5 4 3 Starting W Starting W Figure 2 Basal feed intake (BFI; kg/week) of lambs consuming a nil urea diet (open symbols) or urea containing diet (solid symbols) alone (,) or with 25 g/d supplementar y undegradable dietar y protein provided as for maldehyde treated vegetable meal (�,�) or for maldehyde treated casein (I,G). Shown are predicted mean weekly intakes (�SE) during the second (a), four th (b) and sixth (c, final) week of study for lambs of 16, 24 and 32 kg initial W . 102 Hegar ty et al. samples was greater than that of post_feeding samples (211 v 135 mg N/L) and the +U mean was greater than the _U mean (222 v 128 mg N/L). The weak interaction between urea and sampling date was due to a tendency for ammonia concentration to decrease between days 24/25 and 43/45 for _U lambs (145 to 114 mg N/L) but to increase in the +U group (213 to 232 mg N/L). To assist in understanding the effect of urea, daily RDN intake and ADPLS supplies were calculated for supplemented and unsupplemented lambs throughout the trial. The calculations of RDN and UDP supplied by the basal diet and protein supplements are demonstrated for lambs of 24 kg initial LW in Table 4. RDN supply for rumen requirements was assessed as being in excess or deficient relative to metabolizable energy intake (MEI) on the basis that 8.4 g RDN is required per MJ of MEI (SCA 1990). On this basis _U basal diet had inadequate RDN at all times relative to available energy (Figure 3). Whilst FC did not improve the ratio, EP ensured the RDN available was initially in excess relative to energy intake. There was an excess of RDN relative to dietary energy intake at all times for lambs on the +U diet. Conversely, the supply of ADPLS arising from both the _U and +U basal diets was below requirements for lambs of 24 kg W at the commencement of the study but increased over time. Components of the calculated protein requirement are presented in Table 3 for lambs on the _U and +U basal diets and when supplemented with EP. A slight rise in calculated ADPLS requirement over time occurred due to greater endogenous faecal loss as feed intake increased, even though the ADPLS requirement for tissue accretion decreased. Protein supplements brought the ADPLS into surplus relative to lamb requirements within 3 weeks while the _U basal diet alone provided inadequate ADPLS throughout (Figure 4). Importantly, the supply of ADPLS increased for lambs on all diets over time due to the increase in BFI. Discussion This study sought to demonstrate the dynamic relationship that exists between protein supply and demand as lambs grow, and to emphasise the importance of W and basal diet RDN content in regulating the response of lambs to protein supplements. Results are initially discussed in the context of nutrient supply, and then in an integrated manner showing the impact of basal diet, protein supplement and changing W, on the adequacy of RDN and ADPLS relative to lamb requirements. Whilst the quantity of feed consumed increased both with initial W and with time as the lambs grew, intake was also dependent upon the adequacy of the RDN content of the basal diet, where adequacy was determined by the RDN:ME ratio. That the _U diet provided inadequate RDN was evidenced by the higher BFI of +U lambs than _U lambs with respective rumen ammonia concentrations of 222 v 128 mg N/L. The intake response to urea suggests the required rumen ammonia concentration in sheep fed the wheat_based diet provided is greater than the 50mg N/L considered 30 20 10 RDN Surplus 0 -10 -20 -30 -40 -50 5 10 15 20 25 30 35 40 45 Days Figure 3 Mean supply of dietar y rumen degradable nitrogen (RDN g/week) relative to rumen requirement deter mined according to SCA (1990) for lambs of 24 kg initial LW over a 43 day feeding per iod. The basal diet was low in RDN and was supplemented with no protein meal (), a for maldehyde treated vegetable protein (�), formaldehyde treated casein (I) or for mulated to include 1.4 % urea (). Average standard errors about the means are shown as ver tical lines below the plot. Responses in lamb growth to protein supplements 103 necessary to maximise microbial protein synthesis (Satter and Slyter 1974), and may be closer to the 200 mg N/L that Mehrez et al. (1977) suggest is needed to maximise fermentation. In contrast, the average rumen ammonia concentration for the +U diet was above 200 mg N/L. Inadequate RDN in _U lambs, but not in +U lambs, may explain why protein supplementation increased intake of the _U diet. The response in BFI to urea was also differentially affected by W, such that there was a much greater response to urea by lambs of low initial than high initial W. This corresponded with the observation that _U lambs of low initial W achieved rather small LWG over time, while lambs fed the +U diet or _U lambs of initially high W had greater LWG. This interaction indicates the necessity of correct diet formulation for lambs weaned at low W to optimise growth. Liveweight and LWG are net reflections of nutrient supply relative to requirement. The pattern of tissue development in lambs has been intensively studied (P�lsson and Verg�s 1952; Butterfield 1988) but the ramifications of growth pattern on the changing nutrient requirements of the growing lambs have been less well defined. Searle and Graham (1987) demonstrated that the daily rate of protein accretion in lambs declines with maturation and this decline is apparent in calculated empty_body protein gains in the present experiment (Table 3). As a result, while milk fed (pre_ruminant) lambs have responded to increasing dietary protein content up to at least 23% CP (Walker and Cook 1967) and young lambs responded to UDP supplements when grazing clover pastures (Poppi et al. 1988), growth of lambs over 35 kg LW is apparently limited by the availability of energy rather than protein (Andrews and �rskov 1970; Hegarty et al. 1999). Associated with the declining daily tissue amino acid requirement as lamb W increases, is a progressive increase in voluntary feed intake, bringing with it increased microbial protein production (SCA 1990). As a consequence, while the ability of the _U basal diet to meet RDN requirements declined over time, the capacity of lambs to satisfy their ADPLS requirement on the _U diet increased over time (Table 4). This indicates the role of protein supplementation of lambs on RDN deficient diets changes from, initially, provider of UDP to being an important supplier of RDN as lamb W increases. These changes are depicted in Figures 3 and 4 and are discussed using, as an exemplar, lambs of 24 kg initial W which, in this study when consuming the _U basal diet, were deficient in RDN for the entire experiment, indicating an inadequate RDN supply. This calculated deficiency was also evidenced by the rumen ammonia concentration being below that required to maximise DM fermentation (Mehrez et al. 1977). While urea increased the RDN supply above requirements and increased the rumen ammonia concentration to an average of 223 mg N/l, the RDN available to _U lambs declined over time pari passu with rumen ammonia concentration and was reflected in the slower LWG of _U lambs over the final 2 weeks. The urea x supplement interactions for W and BFI imply that protein meals 60 40 ADPLS Surplus 20 0 -20 -40 -60 0 10 20 30 40 Days Figure 4 Estimated mean sur plus of apparently digestible protein leaving the small intestine (ADPLS g/d) relative to calculated requirements for lambs of 24 kg initial LW over a 43 day feeding period. The basal diet was low in RDN and was supplemented with no protein meal (), a for maldehyde protected vegetable protein (�), for maldehyde treated casein (I) or was for mulated to include urea (). Lamb protein requirement was calculated according to SCA (1990) as shown in Table 3. Average standard errors about the means are shown as ver tical lines below the plot. 104 Table 3 Calculation of the ADPLS requirement for growing lambs consuming a diet either with (+U) or without urea (_U) and with (+P) or without (_P) a supplement of formaldehyde treated vegetable protein meal (escape protein, EP). Requirements for protein gain in the empty body, endogenous urinary and faecal losses and wool growth were calculated using equations 1.36A, 2.17, 2.18 and 2.24 respectively of SCA (1990). Standard reference weights and standard fleece weights of 66kg and 3 kg were assumed. Net protein requirement was calculated as the sum of these protein allocations. The requirement for apparently digestible protein leaving the intestine was calculated from net protein requirement by dividing by the efficiency with which digestible protein is used for these purposes, being 0.6 for wool growth and 0.7 for all other purposes (Table 8, SCA 1990). Metabolizable energy intake (MEI) is the fitted average for the preceding week. Protein gain in empty body g/kg EBW g/d 27.7 6.9 5.2 1.5 41.4 45.2 49.6 53.4 55.9 57.0 52.3 3.6 4.7 5.6 6.2 6.5 9.0 12.0 15.6 19.0 21.6 8.4 23.5 2.6 3.5 4.6 5.5 6.3 6.9 55.3 58.9 61.8 63.4 63.7 52.9 55.8 59.4 62.7 64.9 66.3 2.6 3.7 4.8 5.5 5.9 2.7 8.7 12.8 16.3 18.7 20.0 8.2 11.3 14.9 18.1 20.2 21.1 7.1 7.4 7.6 7.9 8.1 7.0 7.3 7.6 7.9 8.2 8.4 7.0 7.2 7.5 7.8 8.1 26.8 25.8 24.7 23.8 23.0 34.4 33.1 31.6 30.2 28.9 27.8 34.2 33.0 31.8 30.4 28.9 27.6 4.2 148.3 143.4 137.8 132.3 127.3 123.2 144.0 138.5 132.3 126.2 120.8 116.1 144.3 139.5 134.1 128.3 122.2 116.4 7.0 10.2 13.1 15.0 16.1 7.5 10.0 12.9 15.5 17.1 17.8 7.3 9.6 12.5 15.2 17.3 18.9 Endogenous urinary protein (g/d) Endogenous faecal protein (g/d) Wool g clean fibre per day Net protein requirement (g/d) 59.5 65.2 71.8 77.4 81.1 82.8 75.4 79.9 85.2 89.6 92.1 92.6 76.1 80.5 86.0 90.9 94.3 96.4 Hegar ty et al. Treatment Day LW (kg) MEI MJ/d LWG (g/d) ADPLS requirement (g/d) _U_P 7 23.8 187 _U_P 14 25.4 224 _U_P 21 27.2 257 _U_P 28 29.0 256 _U_P 35 30.6 236 _U_P 43 32.0 176 _U+P 7 24.9 239 _U+P 14 26.7 257 _U+P 21 28.7 291 _U+P 28 30.7 290 _U+P 35 32.6 269 _U+P 43 34.3 210 +U_P 7 24.8 237 +U_P 14 26.3 223 +U_P 21 28.1 251 +U_P 28 30.0 277 +U_P 35 32.1 299 +U_P 43 34.2 258 Table 4 Estimated daily supply of rumen degradable nitrogen (RDN) and of apparently digestible protein leaving the stomach (ADPLS) of lambs relative to their requirements. Data shown are for lambs receiving a low RDN diet with (_U+P) or without (_U_P) a for maldehyde treated vegetable protein meal supplement, or for the same basal diet containing 1.4% urea (+U). Estimation procedures are from SCA (1990). ADPLS from the basal diet and supplement were estimated from the crude protein contents and measured ruminal degradabilities and an assumed intestinal degradability of 0.7. Microbial ADPLS was calculated from RDN availability with the assumption that 0.8 of microbial nitrogen was protein with an intestinal degradability of 0.7 Source of ADPLS (g/d) Treatment Day Basal diet 16.4 27.3 39.9 50.9 58.5 62.7 32.3 44.8 58.2 68.2 74.7 77.4 35.6 47.2 61.3 74.6 85.0 92.4 Microbes Supplement Total ADPLS required (g/d) 59.5 65.2 71.8 77.4 81.1 82.8 75.4 79.9 85.2 89.6 92.1 92.6 76.1 80.5 86.0 90.9 94.3 96.4 ADPLS surplus (g/d) _38.4 _30.1 _20.5 _12.0 _5.9 _2.3 _21.5 _9.7 2.5 11.3 17.2 20.1 _35.0 _25.9 _15.0 _4.6 4.0 10.5 Source of RDN (gN/week) Basal diet 32.7 54.7 79.8 101.8 117.0 125.3 51.0 70.6 93.4 113.1 126.0 131.6 80.0 106.2 138.0 167.9 191.2 208.0 Supplement Total RDN required (g/week) 42.5 71.0 103.7 132.3 152.0 162.8 66.2 91.7 121.3 146.9