Effects of supplementation on intake and growth of nursing calves grazing native range in southeastern North Dakota

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1 Effects of supplementation on intake and growth of nursing calves grazing native range in southeastern North Dakota T. W. Loy 1, G. P. Lardy 2, M. L. Bauer, W. D. Slanger, and J. S. Caton Department of Animal and Range Sciences, North Dakota State University, Fargo ABSTRACT: A 2-yr study was conducted to determine the first limiting nutrient for gain in nursing calves grazing native range in southeastern North Dakota. Thirty-two calves (20 steers, 12 heifers) in Trial 1 (169 ± 5 kg initial BW) and 31 (16 steers, 15 heifers) in Trial 2 (214 ± 5 kg initial BW) grazed common pastures. Calves were blocked by sex and stratified by weight. Calves were stratified by age of dam in Trial 1 and by pretrial milk intake (MI) in Trial 2. Treatments were nonsupplemented control (CON); energy supplement (ENERGY; 100% soyhulls); degradable intake protein supplement (DIP; 68% soyhulls, 32% SBM); and degradable with undegradable intake protein supplement (DIP+UIP; 80% sulfite-liquor treated SBM, 16% feather meal, 4% blood meal). In Trial 2, 5% molasses was added to all supplements with the ratios of other ingredients held constant. Supplements were formulated to be similar in NE. The DIP and DIP+UIP supplements supplied equal amounts of degradable protein. Supplemented calves were fed individually, with similar supplement DMI. Weight and MI were measured in July, August, and September. Forage intake (FI) was mea- sured in July, August, and September of Trial 1 and July and August of Trial 2. Gain data were analyzed as a randomized complete block and MI and FI as a split-plot in time. Orthogonal contrasts were used to separate means and included CON vs supplemented, ENERGY vs protein, and DIP vs DIP+UIP. No trial effect or trial treatment interactions (minimum P- value = 0.30) were detected for ADG. Supplemented calves gained faster than CON (P = 0.06). No other contrast differences were observed (minimum P-value = 0.50). Treatment did not affect FI (P 0.55). Forage intake was lower (P < 0.001) in Trial 1 than in Trial 2. A linear increase (P = ) in FI (kg OM/d and percentage BW) occurred over time. Calves in Trial 2 consumed more (P = 0.004) fluid milk than calves in Trial 1, though no difference (P = 0.28) was observed relative to BW. No treatment or period differences were detected for fluid MI (minimum P-value = 0.23). Relative to BW, MI declined linearly (P = ) with successive periods. Energy may be limiting weight gain of nursing calves grazing native range in southeastern North Dakota. Key Words: Calves, Growth, Intake, Nursing, Protein, Supplements 2002 American Society of Animal Science. All rights reserved. J. Anim. Sci : Introduction Relative to most classes of livestock, the nutritional requirements of nursing calves have not been well defined. Research that has been conducted has focused on milk consumption and its correlation to weaning weight (Neville, 1962; Clutter and Nielsen, 1987) and its interaction with forage intake (FI) (Boggs et al., 1980; Ansotegui et al., 1991); diet selectivity and FI of suckling calves (Boggs et al., 1980; Grings et al., 1995); and ruminal parameters (Ansotegui, 1986; 1 Current address: University of Nebraska-Lincoln, Department of Animal Science, C220 Animal Science, Lincoln, NE Correspondence: 177 Hultz Hall (phone: ; fax: ; glardy@ndsuext.nodak.edu). Received August 21, Accepted May 10, Cremin et al., 1991). The first limiting nutrient of nursing calves has not been established. Energy and protein supplementations are used to maintain a targeted production level or to minimize weight losses of grazing livestock (Clanton, 1982; Caton and Dhuyvetter, 1997). Gross income of many cowcalf production systems is highly dependent on calf weaning weights (Martin et al., 1981). Providing creep feed to nursing calves has been used to increase preweaning weight gain (Scarth et al., 1968; Stricker et al., 1979; Prichard et al., 1989). To improve supplement efficiency, limiting creep intake (Lusby and Wettemann 1986; Cremin et al., 1991; Faulkner et al., 1994) and increasing protein concentration (Cremin et al., 1991; Hollingsworth-Jenkins, 1994; Lardy et al., 2001) have been investigated. An understanding of what is first limiting weight gain of nursing calves may help in the formulation of more efficient supplementation programs. Faulkner et 2717

2 2718 Loy et al. al. (1994) compared nonsupplemented nursing calves with those provided either corn or soyhulls at restricted or ad libitum levels. Their data suggest that energy limited gain of suckling calves on a fescuebased diet. Hollingsworth-Jenkins (1994) compared energy- or undegradable intake protein (UIP)-supplemented suckling calves with nonsupplemented controls. The authors concluded that metabolizable protein (MP) was deficient in spring-born calves grazing native range and that UIP was the first limiting nutrient. The objective of this trial was to determine if energy, degradable intake protein (DIP), or UIP was limiting gain of nursing calves grazing native range in southeastern North Dakota. Secondary objectives were to examine seasonal changes in forage and milk intake (MI) of nursing calves and to determine the impact of supplementation on FI. Materials and Methods Study Site. The study was conducted at the Albert Ekre Grassland Preserve southwest of Kindred, North Dakota. The site utilized was the NE ¹ ₄ of Section 19, Range 51 W, Township 135 N. The area is characterized as sandhills tallgrass prairie, comprised of approximately 60% cool-season and 40% warm-season species. Primary species include Poa pratensis (Kentucky bluegrass), Agrostis stolonifera (redtop bent), Bouteloua gracilis (blue grama), and various species of sedges (Carex spp.). Major soil types include Maddock- Hecla-Hamar loamy fine sands, Hamar fine sandy loam, and Serden loamy fine sand (USDA SCS, 1975). Climatic Conditions. Climate at the study area is characterized by marked seasonal variations in both temperature and precipitation. With approximately 120 frost-free days, monthly temperature means range from 13.8 in January to 22.0 C in July. Mean annual precipitation of 48.8 cm is seasonal with over 69% occurring between May and September (NDAWN, 2001). Temperature and precipitation data for the growing season were collected at an automated weather data collection station located approximately 25 km southwest of the study location. Trial 1. In 1998, 32 suckling calves (Angus cross; 20 steers, 12 heifers; ± 4.5 kg initial BW) were included in the study. Calves were blocked by sex, stratified by weight and age of dam, and assigned to one of four treatments. Age of dam was used as a stratification criterion to account for differences in milk production attributable to differing stages of dam maturity (i.e., the presence of first-calf heifers). Treatments (Table 1) included a CON (nonsupplemented) and three supplemented groups: ENERGY (385 g/d); DIP (DIP; 375 g/d); DIP and UIP (DIP+UIP; 375 g/d). All supplements were pelleted and were formulated to be similar in NE. The DIP and DIP+UIP supplements were formulated to provide equal amounts of rumen degradable protein (60 g/d). The DIP+UIP supplement was formulated to provide 143 g of undegradable protein per day, vs 30 g and 14 g for DIP and ENERGY, respectively. A combination of xylose-treated soybean meal, feather meal, and blood meal was used as the source of UIP. Supplement composition and analyzed nutrient content of supplements are shown in Table 2. Data collection periods (approximately 1 wk in length) occurred once each in July, August, and September, corresponding to average calf ages of 152, 196, and 219 d (Table 3), respectively. Calves in the three supplemented treatments were fed individually. Refusals were collected and weighed following each feeding. All calves were gathered and sorted from the cows. Those in supplemented treatments were sorted into individual pens, and supplements were offered in rubber pans. Supplementation occurred daily the week preceding and the week of fecal collections to ensure constant throughput. Supplements were offered three times weekly at all other times. In order to formulate supplements, we estimated the MP requirement of the calves using equations from Wilkerson et al. (1993), where MP requirement (g/d) = 3.8 BW 0.75 kg g ADG kg 1. Estimates of initial BW (158 kg) and weight gain (1.18 kg/d) were used to calculate a requirement of 529 g/d. Metabolizable protein supply was estimated to be 402 g/d and was derived from estimates of MI (8.0 kg/d; 245 g MP/ d), FI (2.1 kg/d), forage UIP (46 g MP from forage UIP/ d), and resulting microbial protein production (111 g MP/d). Microbial protein estimates were based on equations of Burroughs et al. (1974). A constant microbial efficiency (13% of TDN) was used for all treatments in calculating MP supply. The DIP+UIP supplement was formulated to meet the calculated MP deficiency (127 g/d). Thus, CON, ENERGY, and DIP were assumed to be deficient in MP to achieve the targeted weight gain. Sixteen steer calves (four per treatment) were used to estimate FI through total fecal collections. Each calf was fitted with a harness and canvas bag with a plastic liner. Harnesses and bags were placed on the calves several times prior to the first collection so that both calves and cows could grow accustomed to the apparatus. Plastic liners were changed twice daily during collections. Contents were weighed, 10% subsamples were collected, and subsamples frozen for later analysis. Collections were performed in July (6 to 13), August (19 to 26), and September (11 to 18). Samples from a minimum of 5 d were obtained for each calf. Six ruminally fistulated yearling heifers were used to collect diet samples. The heifers and cow-calf pairs grazed common pastures. Ruminal contents were evacuated (Lesperance et al., 1960), and heifers were allowed to graze for approximately 30 min. Diet samples were then collected, frozen, and lyophilized prior to laboratory analysis. Samples were collected in July, August, and September.

3 Supplementation and intake of nursing calves 2719 Table 1. Ingredient composition of supplements Composition, % DM Trial 1 Trial 2 Ingredient ENERGY DIP DIP+UIP ENERGY DIP DIP+UIP Soybean hulls Soybean meal Xylose-treated soybean meal Feather meal Blood meal Molasses Milk intake was measured using a 12-h weighsuckle-weigh technique in July, August, and September according to the methods described by Boggs et al. (1980). Briefly, the evening prior to measurement, calves were separated from their dams for 3 h, allowed to nurse, then sorted for the remainder of the night. The following morning (approximately 12 h later), all calves were weighed and divided into four groups. Calves from each group were then allowed to nurse the appropriate dams and weighed immediately once suckling had ceased. Milk intake was assumed to be the difference between the two weights. Cow-calf pairs grazed a 73-ha pasture for the duration of the study. The pasture was divided into four paddocks, and cattle were rotated every 21 d in a twice-over grazing rotation. Fecal collections began the third or fourth day after rotation to a new paddock in each case to minimize any differences in FI as a result of changes in forage availability. Salt and trace minerals were provided free choice for both cows and calves. Calf weight gain was determined in each period from an average of weights recorded on two consecutive days. Supplement efficiency (gain:feed) was calculated as a ratio of the difference in weight gain of supplemented calves relative to the CON calves and the amount of supplement consumed. Trial 2. In 1999, 31 crossbred calves (214.5 ± 4.5 kg initial BW) were blocked by sex and stratified by Table 2. Formulated and analyzed nutrient content of supplements in both trials a Treatment Item CON ENERGY DIP DIP+UIP Trial 1, Formulated DM (g) NE m (Mcal) NE g (Mcal) DIP (g) UIP (g) Trial 2, Formulated DM (g) NE m (Mcal) NE g (Mcal) DIP (g) UIP (g) Trial 1, Analyzed CP, % DM NDF, % DM ADF, % DM IVOMD b UIP c, % CP Trial 2, Analyzed CP, % DM NDF, % DM ADF, % DM IVOMD b UIP c, % CP a DIP+UIP supplement was formulated to balance a projected MP deficiency. b In vitro organic matter digestibility. c In situ procedure (Britton et al., 1978).

4 2720 Loy et al. Table 3. Calf age and weight within period (July, August, and September) and trial Age (d) Weight (kg) Period Trial 1 SEm a Trial 2 SEm b Trial 1 SEm a Trial 2 SEm b July c d 4.3 August c d 4.7 September c d 4.8 a n = 32. b n = 31. c,d Within a row, means without a common superscript letter differ (P < 0.05). weight. Milk intake (measured by 12-h weigh-suckleweigh) prior to initiation of the trial was also used as a stratification criterion. Sixteen steers (4 per treatment) and 15 heifers (4 CON, 4 ENERGY, 3 DIP, 4 DIP+UIP) were assigned to treatments as described for Trial 1. Collection periods were July 11 to 18, August 20 to 27, and September 10 to 17 (average calf age = 157, 197, 218 d, respectively; Table 3). To improve palatability and pellet quality, 5% molasses was added to all three supplements, with ratios of the other ingredients the same as in Trial 1 (Table 2). Calves were fed individually as previously described in Trial 1. Similar to Trial 1, the DIP+UIP supplement was formulated to meet a calculated MP deficiency. An average MP requirement (470 g/d) was calculated based on the previous trial s initial BW (169 kg) and ADG of 0.95 kg. Metabolizable protein supply was based on average period 1 FI (0.75% BW) from Trial 1 and MI from the pretrial weigh-suckle-weigh (10.5 kg/d). Primarily because MI was overestimated prior to Trial 1, it was necessary to increase the amount of supplement offered to 500 g DM/d to supply enough MP to meet the predicted deficiency of 186 g/d in Trial 2. Sixteen steer calves were used to estimate FI as described in Trial 1. In contrast to Trial 1, fecal collections occurred only in July and August. The September collection in Trial 2 was not conducted because we felt forage would have become limiting during that period. Milk intake and diet samples were collected in July, August, and September as described for Trial 1. In contrast to Trial 1, three ruminally fistulated nursing, contemporary calves were used to collect extrusa samples. It was necessary to allow the calves to graze for approximately 45 min to obtain an adequate sample. Cow-calf pairs grazed the same pastures and in the same manner as in Trial 1. Analyses. Fecal samples were analyzed for dry and organic matter (AOAC, 1990). Samples were ground to pass a 1-mm screen using a Wiley mill (Thomas Scientific, Swedensboro, NJ). Extrusa samples were lyophilized and ground through a 1-mm screen using a Wiley mill and analyzed for DM, OM, NDF, and ADF (Robertson and Van Soest, 1982), CP (AOAC, 1990), and in vitro organic matter digestibility (IVOMD; Tilley and Terry, 1963) with the addition of 1 g L 1 of urea to the inoculum-buffer mixture. Differences in fecal output due to differences in supplement digestibility were accounted for by multiplying supplement intake by supplement indigestibility and then by subtracting fecal output due to supplement from total fecal output (Lardy et al., 1999). Then FI was calculated by dividing fecal OM output by forage OM indigestibility. Samples of ingredients and mixed supplements were ground through a 1-mm screen using a Wiley mill and were analyzed for DM, OM, and CP as described above. Mixed supplements were also analyzed for NDF, ADF, and IVOMD as described above. Estimates of escape protein content in extrusa samples were obtained through analysis of NDF-N according to the methods outlined by Mass et al. (1999). In situ analysis was used to estimate the amount of UIP in the supplement ingredients (Britton et al., 1978). Briefly, 1-g samples (2 mm grind) were placed in Dacron bags (Ankom Technologies, Fairport, NY) and incubated in a ruminally cannulated steer receiving a grass hay diet. Samples were incubated for 12 h and residue analyzed for CP. The difference in CP between the initial sample and the incubated sample was considered to be UIP as a percentage of total CP. Statistical Analyses. Data were analyzed using the GLM procedures of SAS (SAS Inst. Inc., Cary, NC). Sex was used as a blocking criterion; initial weight and pretrial MI were used as stratification measures. Independent variables included treatment, period, and trial, as well as the associated interactions. Animal was used as the experimental unit in all cases. Weight gain data were analyzed as a randomized complete block design using residual error to test the means. Forage and MI were analyzed as a split-plot in time, using animal within treatment by trial as the main-plot error term. The subplots, or periods, were tested using residual error. Where trial or trial treatment effects were not observed, the data were pooled and only main effects reported. Three preplanned contrasts were included to separate means: Control vs the average of the supplemented treatments (ENERGY, DIP, and DIP+UIP), ENERGY vs the average of protein-supplemented treatments (DIP and DIP+UIP), and DIP vs DIP+UIP. Forage and MI were also tested for linear and quadratic changes over time. The rela-

5 Supplementation and intake of nursing calves 2721 Table 4. Nutrient analysis of grazed diet samples in July, August, and September (period) and trial Trial 1 Trial 2 Item July August September July August September %OM Mean SEm a Mean SEm b Mean SEm b Mean SEm c Mean SEm c Mean SEm c CP NDF ADF IVOMD UIP d a n = 6. b n = 5. c n = 3. d Based on NDF-nitrogen (Mass et al., 1999). tionship between milk and FI was analyzed using the regression procedure of SAS. Results and Discussion In Trial 1, June to September temperatures averaged 0.4 C higher than long-term (30 yr) averages. Monthly precipitation totals were 14.6, 5.9, 3.1, and 2.6 cm for June, July, August, and September, respectively. The total for the 4 mo was 95% of the longterm (30 yr) average (NDAWN, 2001). In Trial 2, June to September temperatures averaged 0.6 C lower than long-term (30 yr) averages. Monthly precipitation totals were 8.4, 14.4, 5.9, and 13.2 cm for June, July, August, and September, respectively. The total for the 4 mo was 160% of the long-term (30 yr) average (NDAWN, 2001). Average start date of the two trials was June 23, which was about 3 wk after the cows and calves had been placed on pasture. This delay was necessary to get calves to readily consume supplement. Initial BW in Trial 1 was less than in Trial 2 (168.7 and kg, respectively, P < 0.001), although little difference existed in the age of the calves. Fewer calves nursing first-parity cows were included in the study in Trial 2, and MI was higher in Trial 2 (3.78 and 4.77 kg/d; P = 0.004), although differences were not significant relative to body weight (1.83 vs 1.96% of BW; P = 0.28). In Trial 1, diets collected from ruminally cannulated yearling heifers averaged 9.8% CP and 55.7% IVOMD (Table 4). The diets collected from ruminally cannulated nursing steers in Trial 2 averaged 10.5% CP and had an IVOMD of 50.3% (Table 4). Hollingsworth- Jenkins (1994) reported lower CP values but generally higher digestibility of diets taken from cannulated nursing calves grazing native range in the Nebraska Sandhills. Ruminally cannulated calves grazing fall subirrigated meadow regrowth selected diets that averaged 12.5% CP and 54.8% IVOMD (Lardy et al., 2001). Those researchers also found that the nutrient composition of diets selected by suckling calves was similar to mature cows. Grings et al. (1995) found that the diets grazed by esophageally fistulated springborn calves were higher in CP in June and July but not in August, September, or October when compared with diets selected by mature steers. The same study reported that calves selected diets lower in ADF, but differences in NDF were not consistent. Total intake (forage, milk, and supplement) was lower in Trial 1 than in Trial 2 (2.98 vs 3.95 kg OM/ d; P, 0.001). As a percentage of their BW, calves in Trial 1 consumed 1.40% BW while consumption in Trial 2 was 1.58% BW (P = 0.004). Supplemented calves had higher (P = 0.04) total intakes than CON calves and tended (P = 0.08) to consume more as a percentage of BW (data not shown). No two-way interactions were detected in total intake. Calves in Trial 2 had higher (P = 0.004) fluid MI (kg/d) than in Trial 1, although MI as a percentage of BW did not differ (P = 0.28) between trials (data not shown). No trial treatment interaction was detected (minimum P-value = 0.39) for fluid MI. The amount of fluid MI was not different (minimum P-value = 0.23) between treatments or periods. Other researchers have reported large variation in MI of nursing calves at similar times to those in this study using weighsuckle-weigh techniques (Boggs et al., 1980; Ansotegui et al., 1991; Hollingsworth-Jenkins, 1994). When expressed relative to BW, MI declined linearly (P < 0.001) with period. Fluid MI was 2.32, 1.73, and 1.60% of BW in July, August, and September, respectively (Table 5). These results are in agreement with data from Hollingsworth-Jenkins (1994), who also reported a linear decrease in MI relative to BW from June through September, although our values for milk intakes are higher. In Trial 1, calves consumed more forage than in Trial 2 (1.32 vs 1.70 kg OM/d; P < 0.001). This difference was also observed when FI was expressed relative to BW, with calves consuming 1.28% of BW in Trial 1 and 1.47% BW in Trial 2 (P < 0.001). No trial treatment (minimum P-value = 0.18) or treatment period (minimum P-value = 0.87) interactions were detected for FI.

6 2722 Loy et al. Table 5. Fluid milk and forage OM intake of nursing calves by period Period Item July August September SEm Fluid milk intake a kg/d %BW b Forage OM intake c kg/d b d 0.05 %BW b d 0.05 a n = 32 in yr 1; 31 in yr 2. b Linear effect of period (P < 0.001). c n = 16 in yr 1; 16 in yr 2. d Includes Trial 1 only. No treatment (minimum P-value = 0.55) or contrast (minimum P-value = 0.19) differences were detected in FI, expressed as kg/d or % BW. Some investigators have reported decreased FI as supplement intake increased (Cremin et al., 1991; Tarr et al., 1994). Due to the levels of intake and(or) the types of supplements used in this study, a negative effect on FI was not observed. Forage intake increased linearly (P < 0.001) with period (Table 5). Other authors have reported similar increases in forage intake by nursing calves (Boggs et al., 1980; Ansotegui et al., 1991; Hollingsworth-Jenkins, 1994). When data from the two trials were pooled, forage intake was negatively correlated to MI (r = 0.43; P = 0.01). Data correlating forage and MI have been inconsistent. The large errors associated with MI measurements may prevent detection of any significant correlation. Most research has reported an inverse relationship between forage and MI (Le Du et al., 1976; Lusby et al., 1976; Boggs et al., 1980). Weight gain did not differ (P = 0.31) between trials and no trial treatment interaction was detected (P = 0.30). Supplemented calves gained faster than nonsupplemented calves (Table 6; 0.89 vs 0.83 kg/d, respectively; P = 0.06). These results are in agreement with data from ZoBell and Goonewardene (1989), who found that nursing calves supplemented with canola and soybean meal gained faster than unsupplemented calves. Suckling calves grazing subirrigated meadow regrowth gained more weight when supplemented with UIP (Lardy et al., 2001). No differences were detected among supplemented groups (minimum P- value = 0.70), suggesting that energy was limiting weight gain. This contrasts Hollingsworth-Jenkins (1994), who concluded that UIP was the first limiting nutrient of suckling calves grazing native Sandhills range. However, our findings are in agreement with Faulkner et al. (1994), who reported increased weight gains in nursing steer calves grazing fescue and supplemented with soyhulls or corn. Supplement efficiency (added gain above CON relative to supplement intake) did not differ between treatments (P = 0.71; Table 6). The response to supplementation was less pronounced (P = 0.11) in Trial 2, because supplement intakes were higher and increases in weight gain were smaller. Supplement efficiency in Trial 1 was 0.57 ± 0.10 and in Trial 2 was 0.25 ± 0.10 kg of added gain per kg of supplement consumed. No trial treatment interaction was detected (P = 0.47). Supplement efficiency was 0.52 ± 0.12 for steer calves and 0.31 ± 0.15 for heifer calves (P = 0.27). The absence of treatment differences in forage or MI suggests that performance differences resulted from supplementation and not differences in forage utilization. Supplements used in the study were selected in part because we felt they would not hamper forage digestibility. Lardy et al. (2001) also reported no differences in forage intake between supplemented and nonsupplemented calves when supplement intakes were limited. In contrast, Cremin et al. (1991) found that forage intake was negatively associated with supplement intake, whether that supplement consisted of corn or soy hulls. However, creep feed in that trial was offered ad libitum. In trials such as this, where direct measurements of microbial protein flow are not taken, it is difficult to attribute responses to energy, rather than increased Table 6. Average daily gain and supplement efficiency of treatments a Item CON ENERGY DIP DIP+UIP SEm ADG b, kg Supplement efficiency a Supplement efficiency is measured as added gain above non-supplemented CON relative to supplement intake. b CON vs supplemented; P = 0.06.

7 Supplementation and intake of nursing calves 2723 Figure 1. Calculated metabolizable protein (MP) balance of each treatment. Supply was based on treatment averages for fluid milk intake and forage intake relative to BW, using Burroughs et al. (1974) equations. Requirements were based on treatment average BW and ADG using equations of Wilkerson et al. (1993). Metabolizable protein balance represents the difference between supply and requirement. Supplemented calves had higher predicted MP supply than CON (P < 0.001), protein-supplemented calves had higher predicted MP supply compared with ENERGYsupplemented calves (P < 0.001), and DIP + UIP had higher MP supply than DIP-supplemented calves. Predicted MP requirement was higher for supplemented calves compared with CON (P = 0.01). Metabolizable protein balance was greater for supplemented calves compared with CON and for protein-supplemented calves compared with ENERGYsupplemented calves (P < 0.001). Calves supplemented with DIP + UIP had greater MP balance than calves supplemented with DIP (P = 0.001). microbial protein synthesis or efficiency. By design, MP flow should have been increased with supplementation and MP supply for DIP+UIP > DIP > ENERGY. We calculated average MP requirement, supply, and balance within each treatment (Figure 1). Each parameter was calculated on an individual calf basis, and those were used to generate treatment averages. Metabolizable protein supply was based on individual MI, supplement intakes (where appropriate), and forage intake for heifers was based on the average intake relative to BW of steers in the same treatment. The equations of Burroughs et al. (1974) were used to estimate MP supply. Based on these calculations, MP supply was 300, 356, 401, and 465 g/d for CON, ENERGY, DIP, and DIP+UIP, respectively (Figure 1). This resulted in improved MP balances, which ranged from 181 to 21 g/ d. These results were expected, because no differences were detected in MI, and supplementation did not affect forage intake. Given the low levels of supplementation used in this study, coupled with the lack of differences in forage intake, it seems unlikely that a change in microbial efficiency occurred. However, in spite of an increase in flow of MP elicited by all supplement treatments, calves did not respond with increased weight gain. This supports the conclusion that energy was indeed limiting. Calculated MP balance was negative, even for the DIP+UIP treatment. There are a number of possible explanations. One is that the equations outlined by

8 2724 Loy et al. Burroughs et al. (1974) may underpredict MP flow in young calves. Developing calves may have proportionally smaller rumens than mature animals, resulting in less ruminal fill, more rapid fluid dilution rates, shorter turnover times, and a higher microbial efficiency. However, Varel and Kreikemeier (1999) reported that heifers had a proportionally larger fluid fill that turned over more slowly, resulting in lower microbial efficiencies than mature cows. Alternatively, MP requirements may be overpredicted by the equations of Wilkerson et al. (1993), which would not be surprising given that the model was developed using steer calves averaging 253 kg. Nursing calves may be more efficient in their utilization of MP compared with more mature ruminants. Milk protein may also represent a source of UIP that is more digestible and(or) more closely matches the amino acid requirements of the calf relative to other UIP sources. Additionally, differences in composition of gain, as well as lower protein turnover rates, specifically within the rumen, may account for improved protein efficiency of young calves. In conclusion, supplemented nursing calves had greater weight gains than nonsupplemented. No advantage in gain was observed among supplemented treatments. No treatment effects were observed on forage or MI, suggesting that supplementation did not alter digesta kinetics. Forage intake increased both linearly and quadratically as the season progressed while MI, when expressed relative to BW, declined with successive periods. By September, forage comprised a larger portion of the diet than did milk. However, milk still represented an important source of MP. Based on our results, we concluded that energy was the first limiting nutrient for gain in nursing calves grazing native range. Implications This information should be useful in designing efficient supplementation or creep feeding strategies for suckling calves. These data may also be useful in describing the nutrient requirements and intake patterns of grazing, nursing calves. Additionally, forage intake data from nursing calves should be useful to range managers as they plan stocking rates and grazing systems which provide adequate nutrition for the nursing calf while maintaining or enhancing carrying capacity of native rangeland resources. Literature Cited Ansotegui, R. P Chemical composition and rumen digesta kinetics of diets selected and influence of milk intake on forage intake by suckling calves grazing native range. Ph.D. dissertation, New Mexico State Univ., Las Cruces. Ansotegui, R. P., K. M. Havstad, J. D. Wallace, and D. M. Hallford Effects of milk intake on forage intake and performance of suckling range calves. J. Anim. Sci. 69: AOAC Official Methods of Analysis. 14th ed. 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