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* Department of Animal and Range Sciences, North Dakota State University, Fargo 58105;
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University of Nebraska, West Central Research and Extension Center, North Platte 69101;
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University of Nebraska, Department of Animal Science, Lincoln 68583; and
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South Dakota State University, West River Agricultural Center, Rapid City 57702
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Key Words: Cattle • Degradable • Forage • Grazing • Metabolizable • Protein
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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The NRC (1996) Beef Cattle Requirements Model (1996NRC) represents a significant change in the expression of protein requirements. The model acknowledges that ruminal microorganisms have protein requirements that are different from host animal requirements. Degradable intake protein (DIP) is the fraction of the total protein that is ruminally degraded and is the primary source of nitrogen for the microorganisms. Metabolizable protein (MP) is the sum of the digestible bacterial protein and undegraded intake protein (UIP) present in a feedstuff. The model uses dietary TDN and microbial efficiency to determine DIP requirements.
To accurately estimate DIP and MP supply, the 1996NRC requires reliable estimates of TDN, DIP, UIP, DMI, and microbial efficiency. Forage quality changes with advancing season (Powell et al., 1982; McCollum et al., 1985; Adams et al., 1987). However, information on seasonal changes in DIP and UIP content of native range in the northern Great Plains is limited.
Objectives of this paper are to 1) describe a framework in which 1996NRC applicable data can be generated, 2) describe seasonal changes in nutrient quality including DIP and UIP on native range, 3) use the 1996NRC to predict nutrient balance for cattle grazing these forages, and 4) make recommendations for using the 1996NRC in building nutritional programs for forage-fed cattle.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Rangeland at the Gudmundsen Sandhills Laboratory is generally composed of choppy sands. Dominant grass species on the native upland range sites were as follows: little bluestem (Schizachyrium scoparium [Michx.] Nash), prairie sandreed (Calamovilfa longifolia [Hook.] Scribn.), sand bluestem (Andropogon gerardii var. paucipilus [Nash] Fern.), switchgrass (Panicum virgatum L.), sand lovegrass (Eragrostis trichodes [Nutt.] Wood), indiangrass (Sorghastrum nutans [L.] Nash), and blue grama (Bouteloua gracilis [Willd. ex H.B.K.] Lag. ex Griffiths). Common forbs and shrubs include western ragweed (Ambrosia psilostachya DC.) and leadplant (Amorpha canescens [Nutt.] Pursh).
Subirrigated meadow soils were classified as Gannett-Loup fine sandy loam (course-loamy mixed mesic Typic Haplaquoll). Dominant vegetation on the subirrigated meadows consisted of smooth bromegrass (Bromus inermis Leyss), redtop (Agrostis gigantea Roth.), timothy (Phleum pratense L.), slender wheatgrass (Elymus trachycaulus [Link] Gould ex Shinners), quackgrass (Elytrigia repens [L.] Nevski.), Kentucky bluegrass (Poa pratensis L.), prairie cordgrass (Spartina pectinata Link), and several species of sedges (Carex spp.) and rushes (Juncus spp.). Less-abundant grass species were big bluestem (Andropogon gerardii var. gerardii Vitman), indiangrass (Sorghastrum nutans [L.] Nash), and switchgrass (Panicum virgatum L.). Abundant legumes included red clover (Trifolium pratense L.).
Extrusa samples were collected with four to seven cows or steers on each sampling date. Cows had been fitted with esophageal fistulas 2 to 4 yr before, as described by Adams et al. (1991) with modifications for adult cattle. Surgical preparations and postsurgical procedures were reviewed and approved by the University of Nebraska Institutional Animal Care and Use Committee. Steers had been fitted with ruminal fistula 1 yr before. Following an overnight fast, esophageally cannulated cows were equipped with screen-bottom bags and allowed to graze for 30 to 45 min. Extrusa samples were collected and frozen until laboratory analysis. Ruminally cannulated steers were also used to collect diet samples. Ruminal contents were evacuated and the ruminal wall of each steer washed with a sponge to remove remaining digesta and ruminal fluid. Steers were allowed to graze for approximately 45 min and diet samples were obtained via the ruminal cannula. All extrusa samples were freeze-dried (DM following freeze-drying ranged from 91 to 95%). Extrusa samples used for determining DIP were ground in a Wiley Mill to pass a 2-mm screen (Wilkerson et al., 1995). Samples used for remaining analysis were ground to pass through a 1-mm screen. Dry matter, organic matter, and CP of extrusa were determined by standard methods (AOAC, 1990); NDF was determined according to Van Soest et al. (1991); and ADF was determined by the method of Van Soest (1963). The in vitro organic matter disappearance (IVOMD) of extrusa samples was determined by the modified procedures of Tilley and Terry (1963) with the addition of 1 g of urea per liter of inoculum:buffer mixture (Weiss, 1994). The equations of Rittenhouse et al. (1971) were used to convert IVOMD to DE (1.07 x IVOMD – 8.13).
Undegradable intake protein was calculated by the method of Mass et al. (1999). Five-gram samples (DM basis) were incubated in dacron bags (Ankom, Inc., Fairport, NY). Samples deemed vegetative were incubated for 4, 10, and 16 h; samples from dormant vegetation were incubated for 8, 16, and 24 h. Three separate incubation sequences were performed over 3 d. Bags were washed according to Wilkerson et al. (1995) and residue was analyzed for NDF-N. Amounts of NDF-N remaining after incubation were log-transformed, and a rate of degradation calculated. Undegradable intake protein (UIP) was calculated using the following formula: UIP = B[kp/(kd+kp)], where B is the pool size or potential UIP calculated from the intercept of the log transformation of degradation, kp is the rate of passage, and kd is the rate of degradation of NDF-N (modified from Broderick, 1994). Passage rates were determined in a separate research project at the Gudmundsen Sandhills Laboratory during the 1994 growing season (Lamb, 1996; data not shown). In 1992, 8 samples from native range and 9 from subirrigated meadow were used; in 1994, 12 samples from native range and 10 from subirrigated meadow were used (Lardy, 1997).
Crude protein, UIP, DIP, and IVOMD for both subirrigated meadow and native range were analyzed using the PROC REG procedures of SAS (SAS Inst., Inc., Cary, NC). For meadow samples, day of the year was the independent variable, with March 1 considered d 1. For range samples, day of the year was the independent variable, with April 1 considered d 1.
Forage Evaluation and Use of the 1996 NRC Model to Predict Nutrient Balances
The 1996NRC was used to predict nutrient balances in beef cows consuming these forages at various stages of the reproductive cycle using the following assumptions. Mature cow body weight was 500 kg. Peak milk production was 8.2 kg/d. Calving date was March 1 for spring-calving cows and July 1 for summer-calving cows. Weaning date was October 15 for spring-born calves and January 1 for summer-born calves. When meadow hay was included in rations, it was assumed to be of average quality (56% digestibility, 8% CP, 80% DIP; Villalobos, 1993). No supplemental energy or protein were included in the calculations. The DIP requirement was equal to digestibility x 0.13 for vegetative forages and digestibility x 0.10 for dormant forages, including meadow hay. Estimates of dry matter intake were based on predictions from NRC (1996). All calculations were made using thermoneutral conditions.
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Results and Discussion |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Forage Composition
In order to use the 1996NRC, data were averaged within months over sampling method and year. Table 1
shows the seasonal changes (means ± SD, where applicable) in chemical composition and DIP of subirrigated meadow diet samples collected during 1992 and 1994. Because the subirrigated meadow sites are cool-season species with a small component of legumes, a rapid increase in CP and IVOMD in early spring should be expected. Growth begins in March, with rapid increases in CP occurring in April. Forage quality declines during mid-summer as temperatures increase and cool-season grasses cease growth. Regrowth occurs as temperatures decrease during the fall, and forage quality increases. Estimates of UIP were generally highest during periods of active growth. Olson et al. (1994) reported lower CP over the season compared with results reported here on subirrigated meadow. However, due to the subirrigated nature of the meadows, it is difficult to compare the two data sets because the native range used by Olson et al. (1994) was a mix of warm- and cool-season species.
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shows seasonal changes in chemical composition and DIP of native range diet samples. Native range sites are primarily warm-season species that grow rapidly during the summer months. Some cool-season species are present, which explains the relatively high forage quality in April, but forage quantity would not be expected to support heavy grazing at this time (Burzlaff, 1962; Nosal, 1983). Forage quality decreases rapidly on native Sandhills range as evidenced by the decline in CP during September. Similar declines in CP on short-grass prairie during the fall months were reported by Adams et al. (1987). In their work, dietary CP was higher during the early part of the growing season (May to June), but lower in CP later in the growing season than CP reported in this paper. Holechek et al. (1981) did not report decreases in CP to the magnitude of those reported herein on native range. Crude protein remained above 10% on Oregon grasslands during September and October in the study of Holechek et al. (1981), whereas native range diets in our study averaged 7.4% CP in September. Major grass species on the study site used by Holechek et al. (1981) included bluebunch wheatgrass (Agropyron sipcatum [Pursh] Scribn. & Sm.) and Sandberg bluegrass (Poa secunda Presl). Holechek et al. (1981) reported that precipitation was adequate to promote regrowth of grasses, which were relatively high in CP. Contrary to our results and in agreement with Holechek et al. (1981), Hirschfeld et al. (1996) reported higher CP during September and October on mixed-grass prairie in North Dakota. In our study, precipitation was below average in 1992 and average in 1994. However, due to the warm-season dominated nature of the native range grasses, decreases in CP would be expected as plants become dormant.
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Acid detergent fiber levels were higher than those reported by Adams et al. (1987) and similar to values reported by McCollum et al. (1985) on blue grama rangelands in New Mexico. Hirschfeld et al. (1996) reported higher NDF and ADF compared with our data from either subirrigated meadow or native range. Holechek et al. (1981) reported much higher ADF levels on both forested and grassland range in Oregon than we found on either subirrigated meadow (Table 1
) or native range (Table 2). Olson et al. (1994) reported higher values for NDF and ADF compared with values reported here. Again in contrast to the data presented here, Fredrickson et al. (1993) reported higher NDF and ADF values for diets of steers grazing blue grama range in northeastern New Mexico from August through November. In their work, NDF and ADF tended to increase with advancing maturity, which agrees with the present data. Fredrickson et al. (1993) reported several factors that hastened the onset of dormancy in their study: 1) precipitation in July was 70% below normal, 2) low temperatures in November, and 3) low precipitation in October and November. This, coupled with differences in latitude and forage species, may explain why we found lower NDF and ADF than those authors.
In vitro organic matter disappearance followed the general pattern reported by McCollum et al. (1985) and Adams et al. (1987). In vitro organic matter disappearance values reported by Hirschfeld et al. (1996) were similar to values reported in this paper. The work of Hirschfeld et al. (1996) was conducted on sites dominated by blue grama, needle-and-thread, sedges, Kentucky bluegrass, and western snowberry. Holechek et al. (1981) compared diet quality from forested pastures to that of grassland pastures in Oregon. On forested pastures, those researchers found that IVDMD decreased with advancing plant maturity and were in general agreement with data we report for native range.
Undegradable intake protein content of samples collected from native range was also greatest during periods of active growth. This is a function both of a larger NDF-N pool size with the potential to escape and faster passage rates (data not shown). Degradable intake protein increased during active growth and declined during dormancy in a manner similar to that of CP. This may be expected because DIP levels, when expressed as a percentage of the CP, were greater than 80%. Olson et al. (1994) found that insoluble N generally declined with advancing season. One could expect measures of insoluble nitrogen to follow the same general trends as NDF-N. Neutral detergent fiber-N was least for dormant samples and greatest for actively growing forages in our study. This finding is in agreement with the work of McCollum et al. (1985). Acid detergent-insoluble nitrogen values are in agreement with those reported by Olson et al. (1994) for steers grazing native range in North Dakota.
Regression equations for prediction of CP, UIP, DIP, and IVOMD for subirrigated meadow and native range are presented in Table 3. Forage growth on subirrigated meadow begins in March; therefore, d 1 in the regression equations for subirrigated meadow is March 1. The best-fit regression equations for these variables ranged from second- to sixth-degree polynomial equations, with R2 values ranging from 0.477 to 0.835. Based on R2, the best estimation equation was for prediction of UIP. Adams and Short (1987) reported slightly higher R2 values for prediction of CP and ME content of diets collected on northern Great Plains rangelands in Montana. The data used by Adams and Short (1987) included 78 samples, whereas the data reported here used 19 samples.
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). For upland range regression equations, April 1 was considered d 1. The estimation equation with the highest R2 value was for prediction of UIP. Adams and Short (1987) reported slightly higher R2 values for predicting dietary CP and ME content from northern Great Plains rangelands, a finding that is most likely due to the number of samples used, as discussed previously.
Forage Evaluation and Use of the 1996 NRC Model to Predict Nutrient Balances
Table 4 shows the consequence of changes in microbial efficiency on estimated DIP and MP supply, requirement, and balance for gestating, spring-calving cows that were grazing native winter range. Microbial efficiency can range from 8 to 13%. Values as low as 8% can be expected with low-quality forages (< 60% digestibility; NRC, 1996). With slow passage rates, more energy is used by the ruminal microbes for maintenance, including cell lysis (NRC, 1996). Data collected on dormant winter range in Nebraska would suggest that this range of efficiencies is reasonable (Villalobos, 1993; Hollingsworth-Jenkins et al., 1996). As microbial efficiency is reduced from 13% to 8%, DIP balance moves from highly negative to slightly positive. At the same time, MP balance goes from positive to highly negative. The consequence of choosing an efficiency that is too high would be to oversupplement DIP and to overestimate the supply of MP available to the animal. The consequence of choosing an efficiency that is too low would be to underestimate the DIP requirement and fail to supplement DIP when supplementation is necessary (NRC, 1996). In addition, one would conclude that MP was deficient at 8% microbial efficiency, when in fact it is not. Hollingsworth-Jenkins et al. (1996) estimated DIP requirements for spring-calving cows grazing native winter range. They measured deficiencies ranging from approximately 60 to 140 g of DIP daily. This is also in general agreement with Köster et al. (1996), who estimated DIP requirements at 11% of digestible OM intake for cows consuming low-quality tallgrass prairie forage. Scott and Hibberd (1990) fed cows 4% CP native grass hay supplemented with increasing DIP level. They found that forage digestion and intake were maximized at DIP levels of 9% of rumen digestible OM. Based on the data presented above, we chose to use 10% and 13% microbial efficiency values for dormant and growing forages, respectively, for our nutrient balance calculations (Tables 5 and 6).
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shows the nutrient balances for spring-calving cows. The 1996NRC predicted energy deficiencies when lactating spring-calving cows were fed meadow hay. Metabolizable protein was deficient when lactating cows were fed meadow hay and when gestating cows grazed winter range. Degradable intake protein deficiencies were predicted when cows grazed dormant winter range. Predicted deficiencies are lower than in vivo deficiencies determined by Hollingsworth-Jenkins et al. (1996) for gestating spring-calving cows grazing dormant winter range in the Nebraska Sandhills. Those authors reported that cows lost body condition during the winter. This is reflected in the application of our data from December and January with the 1996NRC, which predicts small energy deficiencies during those time periods. When IVOMD was used as a proxy for TDN in the 1996NRC model runs (data not shown), energy balance was overpredicted. A companion abstract from this symposium discusses these efforts more extensively (Patterson and Klopfenstein, 2003).
For summer-calving cows (Table 6), predicted deficiencies in energy were common when lactating cows grazed dormant fall-winter forages for either meadow or native range. Predicted MP deficiencies were also common when lactating cows grazed dormant range or subirrigated meadow. Predicted DIP deficiencies were also common on dormant fall and winter range. This is in agreement with cow performance data gathered using summer-calving cows grazing native range during the fall and winter and fed either DIP or DIP plus UIP supplements (Lardy et al., 1999). Those trials indicated that DIP and UIP were co-limiting for lactating summer calving cows grazing dormant native range during the fall and winter. In addition, DIP seems to be limiting for gestating, summer-calving cows grazing dormant winter range (Lardy et al., 1997). This is in agreement with our model calculations using dormant winter diets. Predicted energy deficiencies for summer-calving cows grazing native range or subirrigated meadow in November appear to be unreasonable (–7.9 and –8.4 Mcal/d, respectively). The low balances are a function of low energy concentrations predicted using the equations of Rittenhouse et al. (1971) and the low DMI predictions that result. The combination of low energy and DMI resulted in predicted energy deficits that are biologically unreasonable. The diet data collected in November for both subirrigated meadow and native range are based on only one sampling date. The low number of samples may contribute to the problem. As data sets such as this are developed for other areas of the country, focus should be on compiling several years’ worth of data to minimize year-to-year variation.
Table 7 gives suggested input guidelines for using the 1996NRC for grazing beef cows. This is intended as a guideline for county and area extension educators, feed industry personnel, and others who might use the computer model to evaluate diets for grazing cattle. More detail on various aspects of using the model can be found in the NRC (1996) text. In some cases, users should be cautious when using some of the model features and interpreting output if these features are used. The most significant finding in this regard is the sensitivity of the model to environmental inputs. Users are urged to focus on long-term average temperatures for a particular time period. In addition, wind speed inputs should also be limited to 8 km/h or less. Values greater than this will unrealistically impact energy balances. In addition, our recommendation is that the "On Pasture" feature in the model not be used because it unrealistically increases energy requirements. In almost all cases, if the feature is not used, model output data more closely resembles biological data gathered at field stations. For instance, for the spring-calving cow grazing native range in December, with the On Pasture feature not in use, the predicted NEm balance is –1.0 Mcal/d (Table 5). However, if the On Pasture feature is used with level terrain, a NEm balance is –5.4 Mcal/d is predicted (data not shown). If the Hilly Terrain feature is used, the NEm balance is –6.7 Mcal/d, a biologically unreasonable value. Therefore, we recommend that the On Pasture feature not be used.
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An accurate estimate of TDN intake is critical for accurate model prediction of grazing cows’ performance and the prediction of DIP requirements and MP supply. Supplementation can affect digestibility of the forage, either positively or negatively (Moore et al., 1999); however, we did not attempt to model this response.
Recommendations to Extension Specialists to Improve Adoption of the MP System
Beef cattle extension specialists should look for additional opportunities to train extension educators to help them become comfortable with the MP system. These may include specialized workshops or in-service training, distance education opportunities, or simply incorporation of the MP system into routine programming efforts. In addition, extension specialists should also focus on development of state-specific feed libraries for use in the 1996NRC model. This is relatively easy to do and would emphasize the importance of the model in developing nutritional programs.
Recommendations to Scientists with Interest in Beef Cattle Nutrition and Management
There are many areas of potential research contained within the framework of the 1996NRC. The most obvious of these appears to be the need for a greater understanding of factors influencing microbial efficiency and the use of appropriate measurement methods. In addition, scientists should focus attention on data reporting. This will enhance the ability of the next NRC committee to properly evaluate and use data to generate requirements. Data needed include development of forage databases, as well as reporting interim weights, condition scores, DMI, environmental conditions, and other data that are used in the 1996NRC.
Recommendations to Future NRC Beef Cattle Nutrient Requirement Committees
The NRC (1996) publication represented a significant move forward in the understanding of protein nutrition in beef cattle. The use of a computer model to generate requirements also represents a significant contribution to the needs of the industry. However, the lack of acceptance of the current edition of the NRC requirements, is partially driven by the fact that the software package is not Microsoft Windows-based. Consequently, future committees should strongly consider the development of a Windows-based computer model for the next edition. In addition, further incorporation of the effect of environment also is needed.
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Implications |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Footnotes |
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2 Application of the 1996 NRC to Cow Nutrition (developed by the NCR-87 Regional Research Committee).
3 Published with the approval of the director of the University of Nebraska-Lincoln Institute of Agric. and Natural Resources Div. as Journal Ser. No. 14225.
4 Correspondence: 177 Hultz Hall (phone: 701-231-7660; fax: 701-231-7590; e-mail: glardy{at}ndsuent.nodak.edu).
Received for publication April 13, 2003. Accepted for publication August 15, 2003.
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Literature Cited |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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