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J. Anim. Sci. 2005. 83:187-195
© 2005 American Society of Animal Science

ANIMAL NUTRITION

Efficacy of using a combination of rendered protein products as an undegradable intake protein supplement for lactating, winter-calving, beef cows fed bromegrass hay1

A. M. Encinias2, G. P. Lardy3, J. L. Leupp, H. B. Encinias4, L. P. Reynolds and J. S. Caton

Department of Animal and Range Sciences, North Dakota State University, Fargo 58105


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 
Seventy-two (36 in each of two consecutive years) lactating, British-crossbred cows (609 ± 19 kg) were used to evaluate effects of feeding a feather meal-blood meal combination on performance by beef cows fed grass hay. Bromegrass hay (9.6% CP, DM basis) was offered ad libitum and intake was measured daily in individual Calan electronic headgates. Acclimation to Calan gates began approximately 20 d after parturition, and treatments were initiated 21 d later. Cows were assigned randomly to one of four treatments (DM basis) for 60 d: 1) nonsupplemented control (CON), 2) energy control (ENG; 790 g/d; 100% beet pulp), 3) degradable intake protein (DIP; 870 g/d; 22% beet pulp and 78% sunflower meal), or 4) undegradable intake protein (UIP; 800 g/d; 62.5% sunflower meal, 30% hydrolyzed feather meal, and 7.5% blood meal). Net energy concentrations of supplements were formulated to provide similar NEm intakes (1.36 Mcal/d). The DIP and UIP supplements were calculated to supply similar amounts of DIP (168 g/d) and to supply 64 and 224 g/d of UIP, respectively. Forage DMI (kg/d) decreased in supplemented vs. nonsupplemented (P = 0.03) and DIP vs. UIP (P = 0.001); however, when expressed as a percentage of BW, forage DMI was not different (P = 0.23). Supplemented cows tended (P = 0.17) to lose less BW than CON. Body condition change was not affected (P = 0.60) by postpartum supplementation. No differences were noted in milk production (P = 0.29) or in calf gain during the supplementation period (P = 0.74). Circulating insulin concentrations were not affected by treatment (P = 0.42). In addition, supplementation did not affect circulating concentrations of NEFA (P = 0.18) or plasma urea nitrogen (P = 0.38). Results of the current study indicate that supplementation had little effect on BW, BCS, milk production, or calf BW when a moderate-quality forage (9.6% CP) was fed to postpartum, winter-calving cows in optimal body condition (BCS > 5). Supplemental UIP did not enhance cow performance during lactation. Forage UIP and microbial protein supply were adequate to meet the metabolizable protein requirements of lactating beef cows under the conditions of this study.

Key Words: Cows • Degradable Protein • Forage • Lactation • Supplementation • Undegradable Protein


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 
The metabolizable protein (MP) system adopted by the NRC (1996)Go provides a more accurate prediction of protein requirements than the previous CP system (NRC, 1984Go). The MP system attempts to more closely define fractions of MP derived from the rumen microflora, endogenous protein, and undegraded fractions from feedstuffs.

Requirements for MP in beef cattle are influenced by age and production state. Metabolizable protein requirements of the beef cow increase by as much as 25% after calving (NRC, 1996Go). Postpartum supplementation with undegradable intake protein (UIP) may be an effective means of increasing MP supply. Many rendered animal by-products have higher UIP values than commonly used plant protein sources for ruminants (Loerch et al., 1983Go).

Feather meal and blood meal have escape protein values of 73 and 90%, respectively (Goedeken et al., 1990Go). However, protein efficiency is maximized in ruminants with combinations of feather meal and blood meal (Blasi et al., 1991aGo) due to an improved AA profile (Goedeken et al., 1990Go) of limiting AA. A majority of the research with feather and blood meal combinations has been conducted using growing steers and feeder cattle (Goedeken et al., 1990Go; Blasi et al., 1991aGo). Data are limited in evaluation of a feather meal and blood meal combination as a postpartum supplement for multiparous, lactating beef cows. The research reported herein was conducted before the proposed blood meal feeding ban (FDA, 2004Go).

The primary objective of this project was to determine the effects of using a feather meal and blood meal combination as a UIP source on performance and circulating blood metabolites of lactating winter-calving beef cows fed a moderate quality bromegrass hay. Secondly, we wanted to use Level 1 of the NRC (1996)Go model to evaluate MP status of the beef cows used in the current study.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results and Discussion
 Implications
 Literature Cited
 
All animal procedures were conducted using protocols approved by the Institutional Animal Care and Use Committee of North Dakota State University. Cows were housed at the Beef Research Unit at North Dakota State University (NDSU), Fargo.

Animal Selection and Training
Selection of 72 (36 in each of two consecutive years) multiparous, British-crossbred beef (primarily Angus-cross) cows (initial BW = 609 ± 19 kg) from the NDSU Beef Unit commercial herd was based on calving date, calving health, and cow age in an attempt to decrease the variation due to these factors. At approximately 20-d postpartum, cows and nursing calves (44 bull calves, 28 heifer calves; 44.2 ± 0.9 kg average birth weight) were moved to the individual feeding barn at the NDSU-Fargo Beef Unit. Cows were trained to individual Calan electronic head gates for 21 d. From d 1 through 7, cows were provided ad libitum access to a common corn silage/bromegrass hay diet. During the final 14 d of the training period, cows were adapted to the bromegrass hay (9.6% CP, DM basis) to be fed during the experimental period. Calves were housed with cows, and only received milk supplied from their respective dam. No access to creep feed supplement (forage or grain) was given to calves during the supplementation period, and calves were unable to gain access to feed supplements because only the cows had transponders to open the Calan gate system.

Feeding
The experimental supplementation period (60 d) was initiated after the 21-d training period. Cows were stratified by weight and calf sex within age and assigned to one of four treatments (DM basis): 1) nonsupplemented control (CON), 2) energy control (ENG; 790 g/d; 100% beet pulp), 3) degradable intake protein (DIP; 870 g/d; 22% beet pulp and 78% sunflower meal), or 4) UIP (800 g/d; 62.5% sunflower meal, 30% hydrolyzed feather meal, and 7.5% blood meal). Net energy levels of supplements were formulated to be similar (NEm intake = 1.36 Mcal/d; Table 1Go). The DIP and UIP supplements were formulated to supply similar amounts of DIP (168 g/d) and to supply an additional 64 and 224 g/d of UIP to the basal diet, respectively.


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  		Table 1. Supplement formulations, amount fed daily, formulated nutrient supply, and predicted degradable intake protein (DIP) and undegradable intake protein (UIP) supply for mature cows fed moderate-quality bromegrass hay and offered various supplements
 
Cows had ad libitum access to bromegrass hay, which was fed twice daily (0700 and 1600). Forage intake was measured daily and adjusted as necessary to ensure 5 to 10% refusal. Supplements were offered individually at 0630 before the morning feeding after hay refusals were removed. Supplement was placed in a supplement pan within each Calan gate feed bunk. Supplement refusal was weighed back daily. Ad libitum fresh water and trace mineral salt blocks (96 to 98% NaCl, ≥0.3% Zn, ≥0.2% Mn, ≥0.2% Fe, ≥0.04%, ≥0.002% Co, and ≥0.002% I; Cargill, Inc., Minneapolis, MN) were provided throughout the 60-d trial.

Following the supplementation period, cows, and calves were transported to the Albert Ekre Grassland Preserve (approximately 72 km southwest of Fargo), and commingled with the remainder of the mature cowherd for the duration of the grazing season. Cows were not supplemented with protein or energy, and calves were not offered creep feed during the grazing season. Calves were weaned at approximately 220 d of age.

NRC Model Evaluation
Before the initiation of the supplementation period, formulated treatments were evaluated with Level 1 of the NRC (1996)Go model using tabular feedstuff values (NRC feed library) and an 11% microbial efficiency to evaluate DIP and MP balance. Level 1 of the beef NRC (1996)Go computer model assumes the DIP required in the diet to be estimable if the efficiency of bacterial protein synthesis and dietary TDN intake are known. The default on Level 1 suggests a 13% efficiency for most diets. In the present study however, an 11% microbial efficiency was used based on recommendations by Lardy et al. (2004)Go. Cow BW (588 kg) and BCS (6.5) were estimated values that were assumed to represent the cows to be used in this study. Milk production, milk components, and expected calf birth weight were default estimates provided by the NRC model for a two-way cross (dam = Angus, sire = Simmental) at 60-d postpartum. Environmental measurements for temperature (1.6°C) and night cooling (none) were estimated for normal seasonal conditions associated with southeastern North Dakota. Forage intake was estimated at 2.4% of cow BW, and supplements were assumed to be fed at the previously described levels. Predicted DIP balance (g) was –114, –130, 5, and 2 for CON, ENG, DIP, and UIP formulated treatments, respectively. Predicted MP balance (g) was –174, –142, –138, and –3 for CON, ENG, DIP, and UIP formulated treatments, respectively. Following the conclusion of the experiment, Level 1 of the NRC model was used to provide an evaluation of DIP and MP balance, using measured cow performance data (BW, BCS, milk production, percentage of milk protein, percentage of milk fat) as well as measured forage and supplement intake and laboratory analyzed nutritional values for the feedstuffs used in this study.

Performance Measurements and Sample Collection
Four cows were removed from the experiment during the supplementation period due to circumstances unrelated to the treatments imposed (the death of two calves: one from CON and one from DIP; mammary infection: one from DIP; and inability to train to individual head-gate: one from CON). Two-day consecutive individual BW were used to measure initial and final cow and calf BW during the supplementation period. Before each BW measurement, the electronic scale (Rice Lake Weighing Systems, Rice Lake, WI) was calibrated. Cow BW was measured before the morning supplementation and feeding. To ensure shrunk weights were measured, calves were sorted from their respective dams overnight (approximately 9 h). Adjusted weaning weight of calves was calculated by taking an average of consecutive 2-d pasture weights and adjusting for age of dam and calf age according to BIF (1990)Go guidelines.

Initial and final BCS were visually estimated during supplementation using a common nine-point scale for beef cattle (1 = emaciated to 9 = extremely obese; Wagner et al., 1988Go) and averages of consecutive 2-d scores from three experienced technicians. Scoring occurred after BW measurement and was assigned following chute-side palpation and visual evaluation in a confined pen.

Milk production was measured three times during the supplementation period, 2 d following the last day of BW measurements. Milk production was quantified using a portable milking machine. Before feeding on the morning of milking, calves were sorted from their respective dams into a pen approximately 10 m from squeeze chute, with access to fresh feed and water. Cows were restrained in squeeze chute with a head catch and administered 2 mL (40 IU) of oxytocin (Vet-Tek, Blue Springs, MO) i.m. at approximately 0800. Four minutes after oxytocin injection, a milking claw was attached to the cows’ teats, and cows were milked until milk letdown ceased (primary milking). Time of claw attachment and duration of milking was recorded. The primary milking removed milk from all cows as a means of starting all cows on an equal basis to estimate milk production (Clutter and Nielsen, 1987Go). Following primary milking, cows remained separated from calves and were given access to basal forage and water. At approximately 1100, in an effort to stimulate natural milk letdown, calves were placed in a pen near cows. After 1 h, oxytocin and milking procedure (secondary milking) was performed in a manner similar to primary milking. Cows were milked in an order similar to primary milking. Milk from secondary milking was weighed and subsampled. Daily production was calculated by dividing secondary milk weight (kg) by the difference in hours between secondary and primary milking (approximately 4 h), and then multiplying by 24. Subsamples were stored on ice and frozen at –20°C for laboratory analyses.

Blood was sampled on the morning prior to the first day of supplementation and every 2 wk thereafter before morning supplementation and feeding. Blood samples were collected via jugular venapuncture using 20-gauge needles (38.1 mm; Becton Dickinson Vacutainer Systems, Rutherford, NJ) and 16 mm x 100 mm EDTA (1.74 mg/mL) collection tubes (Becton Dickinson Vacutainer Systems) to profile metabolic status through analysis of insulin, plasma urea nitrogen (PUN), and NEFA. Following blood sample collection, samples were left at room temperature for 30 min, then placed on ice. Plasma was obtained by centrifigation (1,300 x g, 25 min, 4°C) and stored in scintillation vials at –20°C for laboratory analyses.

Breeding Season
Approximately 2 wk following initiation of supplementation period, cows were observed for signs of estrus for 30 d, when estrus was detected, they were serviced by AI. After the 30-d estrus detection period, cows were exposed to Simmental bulls for the remainder of the supplementation period. Bulls were separated from the cows and fed during morning and afternoon cow feedings. Reproductive performance of the cows is not reported.

Laboratory Analyses
Bromegrass hay and supplements were sampled weekly and composited. Samples were analyzed for CP, ash, Ca, P (AOAC, 1997Go), and IVDMD (Tilley and Terry, 1963Go). Neutral detergent fiber and ADF were analyzed using procedures (Van Soest et al., 1991Go) modified for use in an Ankom 200 fiber analyzer (Ankom Technology, Fairport, NY). Nitrogen degradability of supplements was measured using a 12-h in situ technique in a single ruminally fistulated steer (633 kg) as described by Loy et al. (2002)Go. Determination of escape protein in forage samples were obtained through NDF-N analysis determined using the procedures outlined by Mass et al. (1999)Go. Frozen milk samples from each milking were thawed and analyzed individually for DM, CP, and fat (AOAC, 1997Go).

Plasma insulin concentrations were quantified in a single solid-phase RIA (Coat-A-Count) using a kit from Diagnostic Products Corp. (Los Angeles, CA). Two hundred microliters of each validated standard (Sletmoen-Olson et al., 2000bGo) from Diagnostic Products Corp. was pipetted before and upon conclusion of assay. All nonspecific binding, total counts, quality controls, and serum samples were assayed in duplicate antibody coated tubes. One milliliter of iodinated insulin (125I), supplied by Diagnostic Products Corp., was added to all tubes, which were incubated for 18 h at room temperature. After incubation, tubes were decanted and placed in a Beckman Gamma 5500 (Irvine, CA) for 1 min to determine the amount of radioactivity bound to the tube. The intraassay CV was 9.1%. Plasma NEFA were quantitatively determined by acyl-CoA synthetase-acyl-CoA oxidase micromethods (NEFA C; Wako Chemicals USA, Dallas, TX). Concentration of PUN was measured by urease/Berthelot method (No. 640; Sigma Chemical Co., St. Louis, MO).

Statistical Analyses
Data were analyzed with the Mixed procedure of SAS (SAS Inst., Inc., Cary, NC). The fixed effects in the statistical model for DMI included treatment and treatment x year interaction. For DMI, the random effect was year. The fixed effects for insulin, PUN, and NEFA data included treatment, period, treatment x period interaction, and treatment x year interaction. The random effect for insulin, PUN, and NEFA data was year. For production measures such as cow BW, calf BW, cow BCS, as well as composition data, the fixed effects included treatment and treatment x year interaction. Year served as the random variable for these production measures. For milk production, the fixed effects in the model included treatment, calf sex, calf sex x treatment interaction, and treatment x year interaction. The random variable for milk production analysis was year. Preplanned contrasts (CON vs. supplemented treatments; ENG vs. protein-supplemented treatments; and DIP vs. UIP) were used to compare treatment means. All treatment comparisons were protected by an overall F-test for treatment. When a significant treatment effect was noted, the preplanned contrasts were discussed.


   Results and Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
Implications
 Literature Cited
 
Based on in situ analysis, the ENG, DIP, and UIP supplements supplied 40, 196, and 213 g of DIP, respectively, which was 0, 28, and 45 g more than formulated DIP levels (Table 1Go). The ENG supplement contained 18 g of UIP more than was formulated. The DIP and UIP supplements were lower in UIP (28 and 48 g lower, respectively) than formulated levels. The increased supply of UIP in the ENG supplement was due to the relatively high undegradable CP fraction (54% of CP) of the dehydrated beet pulp used in the present study (Table 2Go) vs. the tabular UIP value (25%) in the feed library of the NRC. The UIP fraction of the bromegrass hay used in the present study was 41% (percentage of CP). This value was higher than the UIP value for the bromegrass hay used to formulate the treatments (NRC, 1996Go).


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  		Table 2. Analyzed nutrients of bromegrass hay and supplements fed to mature, lactating beef cows
 
Accurate estimation of DIP and UIP is important when formulating diets for growing and lactating animals consuming forage-based diets especially when determining supplemental strategies (Klopfenstein et al., 2001Go; Lardy et al., 2004Go). However, Lalman and Lardy (1998)Go noted that other factors, namely microbial efficiency and DMI, have a greater influence on DIP balance.

Dry matter intake of cows during the supplemental period is shown in Table 3Go. Forage DMI (percentage of BW) across all treatments (average = 2.38%) was not influenced by supplementation (P = 0.23). However, when intake was expressed in kilograms per day, differences (P = 0.03) in forage DMI (kg/d) were observed between nonsupplemented and supplemented cows. Supplemented cows consumed 0.9 kg/d less forage than CON cows. Cows in the DIP treatment consumed less forage (12.8 kg/d) than cows in the UIP treatment (14.7 kg/d; P = 0.001). Enhanced forage intake, in response to protein supplementation, has been most consistently documented in situations where DIP may have been limiting (i.e., lower quality forages; Caton and Dhuyvetter, 1997Go). Our results support the hypothesis that the quality of bromegrass hay (CP = 9.6%; IVDMD = 50.3%) used in this study may have contributed sufficient protein to ensure maximum utilization of ruminally available energy. Increases in forage intake with protein supplementation have been documented in forages that are less than 5% CP (Mathis et al., 2000Go; Beaty et al., 1994Go) with effects diminishing with forages greater than 5% CP. Total daily DMI (percentage of BW) did not differ (P = 0.53) among treatments; however total DMI (kg/d) was greater for UIP than for DIP (P = 0.001).


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  		Table 3. Dry matter intake by mature, lactating beef cows fed moderate-quality bromegrass hay (9.6% CP, DM basis) and fed different supplements for 60 d
 
Cow BW and BCS measured during the 60-d experiment period are shown in Table 4Go. Non-supplemented cows tended (P = 0.17) to lose more weight than cows in supplemented treatments. Cows fed DIP also tended to lose less BW than cows fed UIP (P = 0.15). No difference in BCS (P = 0.60) was observed between treatments. Lents et al. (2000)Go reported no differences in weight change or BCS change when increasing amounts of UIP were fed to lactating beef cows grazing dormant tallgrass prairie. Sletmoen-Olson (2000a)Go reported that UIP supplementation did not alter cow BW compared with DIP supplementation. Their data also indicated that DIP and DIP + UIP combinations were both effective in improving BW and BCS of gestating and lactating beef cows above non-supplemented controls fed 5.8% CP prairie hay diets. In addition, during late gestation, DIP + UIP supplementation tended (P = 0.15) to improve BCS above cows receiving only DIP supplements. Lents et al. (2000)Go reported two separate studies in which combinations of soybean meal, blood meal, and corn gluten meal supplied up to 400 g of UIP in the first study and 301 g of supplemental UIP in the second study; they reported no differences in BCS change among treatments. Rusche et al. (1993)Go noted no differences in cow BW change or BCS change in cows fed UIP supplements. Blasi et al. (1991b)Go reported no differences in cow weight change when lactating cows were fed increasing levels of UIP when grazing either big bluestem or smooth bromegrass pastures. De Gracia and Ward (1991)Go noted no differences in cow BW change when lactating cows were fed diets based on corn silage and ammoniated wheat straw and supplemented with soybean meal or corn gluten meal-blood meal combinations. Based on these studies, it seems that response to UIP supplementation is variable. Differences in response may be attributed to a combination of several factors, including varying MP requirements, especially attributable to milk production level, variation in MP supply due to forage type, as well as variation in amounts of metabolizable AA supplied with the various UIP sources. Our study indicates a need to focus on a more complete understanding of factors regulating MP requirements and supply in lactating beef cows.


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  		Table 4. Body weight and body condition score of mature, lactating beef cows fed moderate-quality bromegrass hay (9.6% CP, DM basis) and fed different supplements for 60 d
 
Daily milk production is reported in Table 5Go. No differences (P = 0.29) were observed for milk production among treatments whether expressed as kilograms per day or percentage of BW. Lents et al. (2000)Go reported variable results when increasing levels of UIP were fed to lactating beef cows. In one study, increasing level of UIP had no effect, whereas in a second study, increasing level of UIP resulted in a quadratic response, first increasing and then decreasing milk production. Rusche et al. (1993)Go reported no differences in milk production in cows fed low-quality grass hay (6% CP) and UIP supplements.


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  		Table 5. Estimated milk production, milk composition, and calf body weight of mature, lactating beef cows fed moderate-quality bromegrass hay (9.6% CP, DM basis) and fed different supplements for 60 d
 
Treatment did not influence milk protein (P = 0.20) or milk fat (P = 0.50). Similarly, Rusche et al. (1993)Go reported no differences in milk fat or protein in lactating primiparous cows fed low-quality hay.

No differences (P = 0.74) in calf gain were observed during the 60-d supplementation period of the dams or after the 80 d of summer grazing (P = 0.48). Lents et al. (2000)Go reported no differences in calf weight gains during the supplementation period when increasing amounts of supplemental UIP were fed to their dams. In contrast, Rusche et al. (1993)Go reported increased weight gains by calves nursing dams fed supplemental UIP.

Mean circulating concentrations of plasma hormones are shown in Table 6Go. No differences (P = 0.42) were observed between treatments for circulating insulin. Conversely, Sletmoen-Olson et al. (2000b)Go fed beef cows 5.8% CP prairie hay and reported that UIP supplementation resulted in increased plasma insulin concentrations above control and DIP-supplemented cows during the last 3 mo of gestation and first 3 mo of lactation. Rusche et al. (1993)Go reported no differences in insulin concentrations in primiparous cows fed DIP or UIP supplements. Alderton et al. (2000)Go reported greater insulin concentrations in primiparous beef cows fed DIP + UIP compared with a DIP supplement early in lactation (d 30), but noted no differences in insulin thereafter. Reasons why our response in insulin disagrees with the previously mentioned research are not immediately clear. Possible factors include differences in MP supply from supplementation, as well as differences in basal forage offered in our study compared with the previous research.


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  		Table 6. Circulating plasma insulin, non-esterified fatty acids, and urea nitrogen (PUN) of mature, lactating beef cows fed moderate-quality bromegrass hay (9.6% CP, DM basis) and fed different supplements for 60 d
 
No differences were noted in circulating NEFA concentrations (P = 0.18). Rusche et al. (2003) also reported no differences in plasma NEFA concentrations in cows fed DIP or UIP. In contrast, however, others have reported that serum NEFA concentrations decrease in response to protein supplementation (cottonseed meal), which likely reflects improved nutritional status (Krysl et al., 1987Go; Barton et al., 1992Go; Cheema et al., 1992). In work with beef cows in late gestation, Sletmoen-Olson et al. (2000b)Go reported that NEFA concentrations were greatest in nonsupplemented controls, intermediate in those receiving DIP supplements, and least in those fed DIP + UIP supplements. The cows used in our study had slightly greater BCS than the cows used by Sletmoen-Olson et al. (2000b)Go. This may explain in part the discrepancies reported between the two studies with regard to changes in NEFA.

No differences (P = 0.38) were observed for PUN, which is contrary to data reported by Alderton et al. (2000)Go, in which DIP + UIP-supplemented cows had greater serum urea N than DIP-supplemented cows. Rusche et al. (1993)Go reported greater PUN concentrations in DIP-supplemented cows than in cows fed UIP supplements. Our results likely differ from those of Rusche et al. (1993)Go because we fed both DIP and UIP in the UIP supplement, whereas Rusche et al. (1993)Go fed either DIP or UIP.

Degradable Intake and Metabolizable Protein Evaluation
Degradable intake protein balance evaluated by Level 1 of the NRC (1996)Go model for beef cattle is shown in Table 7Go. After considering laboratory analysis of the hay and supplements used in the study and using actual BW, milk production, and milk and diet composition values, it seems the diet was adequate in DIP. At d 40, 70, and 100, the CON diet supplied enough DIP to meet the needs of the mature lactating cows used in this study. For the ENG treatment, the model predicted a slight DIP deficiency (approximately 15 g). This is a smaller deficiency than had been expected, primarily due to the difference between the tabular values for DIP for the hay and our actual laboratory analyses. We had formulated the DIP and the UIP supplements to meet a predicted DIP deficiency; however, when our actual production data and laboratory analyses values were used, the DIP supplement had a predicted DIP balance of approximately 125 g at each of the dates measured. Likewise, the UIP supplement supplied approximately 180 g of DIP at each of the dates measured. Hollingsworth-Jenkins (1996)Go determined that DIP was the first-limiting nutrient for spring-calving cows grazing native range; however, that study was conducted with cows grazing dormant range, which was lower in CP than the hay used in this study. In addition, Johnson et al. (1998)Go reported that DIP supplementation needs likely were between 90 and 250 g/d for cattle grazing mixed-grass prairie in western North Dakota. The differences we encountered between tabular values and laboratory analyses of the hay and supplements used in the study affected our predicted DIP balances greatly. It is also possible that the microbial efficiency that we used (11%) overestimated actual microbial efficiencies, which would have affected our DIP balance predictions, causing the DIP requirement to be overestimated.


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  		Table 7. Degradable intake protein (DIP) and metabolizable protein (MP) balance in lactating beef cows fed various supplements at 40, 70, and 100 d postpartum as evaluated by Level 1 of the NRC (1996)Go beef cattle requirements modela
 
Our original formulations predicted a MP deficiency for the CON, ENG, and DIP treatments. Metabolizable protein balance is also shown in Table 7Go; however, analysis conducted using the NRC model indicated that only the CON treatment was actually deficient in MP. Moreover, this deficiency only occurred on d 40 and 100 and was much smaller than originally predicted. Using actual laboratory analyses, BW, milk production, and milk composition values, it seems the supplemented treatments had positive MP balances on d 40, 70, and 100. Lents et al. (2000)Go determined that the MP needs of lactating beef cows could be met with microbial protein, forage UIP, and an addition of 142 g of supplemental UIP daily. Lents et al. (2000)Go did not test lower levels of supplemental UIP. Lardy et al. (1999)Go concluded that DIP was the first-limiting nutrient for lactating summer calving cows grazing dormant native range.

Due to the design of our study we are unable to determine whether the NRC model overpredicted MP requirements or underestimated actual MP supply such that a deficiency was predicted when, in fact, a deficiency was not present. More research is necessary to determine MP requirements and MP supply in lactating beef cows fed forage-based diets.


   Implications
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
 Implications
Literature Cited
 
Based on these data, it seems that the bromegrass hay used in this study was adequate to meet the needs of mature, lactating, beef cows with moderate milking ability. Although there were tendencies for improved performance with the use of supplements high in ruminal undegrability, few biologically significant responses were noted. These data may be useful to further refine beef cattle nutrient requirement models and to improve understanding of metabolizable protein requirements of beef cows. The need to accurately determine forage digestibility, as well as the degradable and undegradable protein of forages before the initiation of postpartum supplementation, is critical to formulate supplements to meet the metabolizable protein requirements of lactating beef cows.


   Footnotes
 
1 Research partially supported by U. S. Poultry and Egg Association and Regional Research funds NC-189. The authors thank G. T. Wallace for assistance with animal care during this study. Back

2 Current address: New Mexico State University, Clayton Livestock Research Center, 15 NMSU Lane, Clayton, NM 88415. Back

4 Current address: 33 Truman Dr., Clayton, NM 88415. Back

3 Correspondence: 177 Hultz Hall (phone: 701-231-7660; fax: 701-231-7590; e-mail: glardy{at}ndsuext.nodak.edu).

Received for publication May 27, 2004. Accepted for publication October 7, 2004.


   Literature Cited
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 


Alderton, B. W., D. L. Hixon, B. W. Hess, L. F. Woodard, D. M. Hallford, and G. E. Moss. 2000. Effects of supplemental protein type on productivity of primiparous beef cows. J. Anim. Sci. 78:3027–3035.[Abstract/Free Full Text]

AOAC. 1997. Official Methods of Analysis. 16th ed. Assoc. Off. Anal. Chem., Arlington, VA.

Barton, R. K., L. J. Krysl, M. B. Judkins, D. W. Holcombe, J. T. Broesder, S. A. Gunter, and S. W. Beam. 1992. Time of daily supplementation for steers grazing dormant intermediate wheatgrass pasture. J. Anim. Sci. 70:547–558.[Abstract]

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J. J. Reed, M. R. O'Neil, G. P. Lardy, K. A. Vonnahme, L. P. Reynolds, and J. S. Caton
Effect of undegradable intake protein supplementation on intake, digestion, microbial efficiency, in situ disappearance, and plasma hormones and metabolites in steers fed low-quality grass hay
J Anim Sci, April 1, 2007; 85(4): 1092 - 1101.
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