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The Effects of Nitrogen and Forage Source on Feed Efficiency and Structural Growth of Prepubertal Holstein Heifers
The Professional Animal Scientist 22 (2006):84–88
TheSource Effects of Nitrogen and Forage on Feed Efficiency and Structural Growth of Prepubertal Holstein Heifers P. J. KONONOFF,1 A. J. HEINRICHS,2 PAS, and M. T. GABLER3 Department of Dairy and Animal Science, The Pennsylvania State University, University Park 16802
Abstract Eighty Holstein heifers averaging 189.6 ± 6.8 kg of BW were used to evaluate the effects of forage level and rumen degradable nitrogen source on feed efficiency and structural growth. A randomized complete block design was used with heifers blocked according to BW (≤136.0 kg and >136.0 kg) and assigned to 1 of 4 treatment diets in a 2 × 2 factorial design. Treatments were constructed with 2 levels of forage (65 or 75%) and 2 nitrogen sources. Forage sources were a mixture of corn silage and chopped timothy hay. Nitrogen sources were either soybean meal (SBM) or a slow-release, polymer-coated urea product (Optigen 1200, CPG Nutrients, Syracuse, NY), which was fed at 1.8% of diet DM on low-forage diets and at 1.3% of diet DM on high-forage diets. Average daily gain and feed efficiency did not differ between rations of different forage level or nitrogen source, averaging 0.87 ± 0.05 kg and 7.4 ± 0.5, respectively, across treatments.
1
Current address: Department of Animal Science, The University of Nebraska, Lincoln 68588. 2 To whom correspondence should be addressed: [email protected] 3 Current address: ADM Alliance Nutrition, Box 44307, Madison WI 53744.
Similarly, no differences were observed in change of withers height, hip height, hip width, or heart girth. No differences were observed in plasma urea nitrogen, which averaged 12.3 ± 0.4 mg/dL across treatments. Results of this experiment suggest that feeding moderately different levels of forage along with either SBM or a polymer-coated urea product does not result in any significant differences in feed efficiency or structural growth. Polymercoated urea can be used in heifer diets to effectively replace SBM as a nitrogen source in either low- or high-forage rations. Key words: heifer growth, feed efficiency, forages, nitrogen source, polymer-coated urea
Introduction Many fiber-digesting rumen bacteria require ammonia for protein synthesis (NRC, 2001). Urea is commonly added to ruminant diets as a source of nonprotein nitrogen that is rapidly hydrolyzed to ammonia in the rumen. Much of the available ammonia is poorly utilized at peak levels, and often the peak level occurs sooner than the peak of maximum fiber fermentation (Van Soest, 1994). Several methods have been evaluated that attempt to improve rumen am-
monia by reducing the rate at which urea is released. Such products include biuret (Lest et al., 2001), starea (Bartley and Deyoe, 1975), and urea formaldehyde (Prokop and Klopfenstein, 1977). Although these methods may be useful in avoiding ammonia toxicity, they rarely improve utilization of dietary nitrogen or improve animal performance compared with standard feed sources (Owens and Zinn, 1988). Additionally, achieving the slow-release effect of some products, such as starea, requires heating to bind nitrogen in an insoluble form and may reduce the availability of some nutrients to rumen microbes (Owens and Zinn, 1988). Optigen 1200 (CPG Nutrients, Syracuse, NY) is a polymer-coated, controlled-release urea product (PCU) that, compared with feed-grade urea, is believed to posses a reduced rate of ammonia release to rumen microbes (Galo et al., 2003). Because it is commonly understood that the rate of rumen ammonia production should be paired with that of carbohydrate digestion (Hoover and Stokes, 1991), PCU may be useful in high-forage, low-concentrate diets that have a slow rate of fermentation. This use may well fit standard heifer diets, which typically include high levels of forage and limited feeding of protein concentrate. The cur-
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rent study was designed to evaluate the effect of feeding PCU in growing heifer diets containing high amounts of slowly digestible fiber. The objective of this study was to determine the effects of diet forage level (FL) and nitrogen source (NS) on feed efficiency, structural growth, and body condition score (BCS) in heifers. It was hypothesized that feeding a high-forage diet containing PCU would result in equivalent growth and feed efficiency when compared with diets containing lesser forage or more protein with a slower rate of digestion.
Materials and Methods Eighty Holstein heifers, trained to use Calan feeding doors (American Calan, Inc., Northwood, NH), were randomly assigned to 1 of 4 treatment rations in a randomized complete block design and blocked according to BW (≤136.0 kg and ≥136.0 kg). Heifers were housed in a naturally ventilated barn with bedding pack space maintained at 3.7 m2 per heifer. During the 14-d pretrial adaptation period, heifers received a ration meeting NRC (2001) nutrient recommendations for a Holstein heifer gaining 0.80 kg/d. Treatment diets were constructed using 2 FL (65 or 75%) and 2 NS (Table 1). Forage sources were a mixture of corn silage and chopped timothy hay. Primary NS were either soybean meal (SBM) or a PCU (Optigen 1200) both fed at 1.8% of diet DM on low-forage diets and at 1.3% of diet DM on high-forage diets. The quantity of diet DM offered was adjusted weekly in an attempt to achieve an ADG of 0.80 kg/ d. Weekly BW was recorded on 2 consecutive days (3 h postfeeding) to determine the quantity of treatment ration offered for the next 7 d. Refusal of treatment ration was weighed and recorded daily. Samples of treatment rations and forages were collected 3×/wk. Portions of the weekly samples were dried immediately for determination of DM, which was used to adjust treatment
rations to achieve accurate daily DMI as a percentage of BW. The remaining portions of the samples were frozen (−20°C), and composites were made every 21 d for determination of DM, CP (AOAC, 1990), soluble CP (Krishnamoorthy et al., 1982), RDP (Krishnamoorthy et al., 1983), total nonstructural carbohydrates (Smith, 1981; modified to use ferricyanide as a colorimetric indicator), starch (Holm et al., 1986), and NDF and ADF (ANKOM200 Fiber Analyzer, ANKOM Technology Corp., Fairport, NY). Withers height, hip height, hip width, heart girth, and BCS (based on a 5-point scale; 1 = underconditioned and 5 = overconditioned) were recorded 3 h postfeeding at the beginning of the trial and every 28 d until the end of the treatment period. Blood samples also were taken at these 28-d intervals. Blood was acquired from the left jugular vein via venipuncture into evacuated glass tubes containing citrate as an anticoagulant. Plasma samples were aspirated after centrifugation (4,000 × g) and frozen (−20°C) for later analysis of urea N (procedure number 0580, Stanbio Laboratory Inc., San Antonio, TX). Statistical Analysis. Nutrient content of treatment rations was analyzed by the PROC GLM procedure of SAS (SAS Inst., Inc., Cary, NC). Least squares means and standard errors were determined using the LSMEANS and STDERR statement in PROC GLM. The first-order autoregressive covariance structure [AR(1)] and the MIXED procedure were used to analyze all growth and blood data. Repeated measurements of plasma urea nitrogen (PUN) were analyzed by including a REPEATED model statement, as well as a term for time and interaction for treatment by time. Sum of squares for all treatments were then partitioned into single df contrasts planned a priori for FL, NS, and FL by NS interaction. Significance for all models was declared at P ≤ 0.05. All means presented are least squares means. Least squares means
and standard errors for DMI and feed efficiency from d 1 to 140 were determined using LSMEANS in PROC MIXED. Orthogonal contrasts for linear, quadratic, and cubic effects of the treatment rations were determined using the ESTIMATE statement.
Results and Discussion Ingredient and nutrient composition of the treatment rations are listed in Table 1. Body weights, DMI, ADG, and feed efficiencies are presented in Table 2. Heifers began consuming the treatment rations at 189.6 ± 2.1 kg of BW. Average daily gain and feed efficiency were not observed to be different, averaging 0.87 ± 0.05 kg and 7.4 ± 0.5, respectively, across treatments. No differences in initial BW, final BW, or DMI were observed. In addition, no differences were observed in change of withers height, hip height, hip width, heart girth, or BCS (Table 3). The animal performance data of this study are similar to those of Tedeschi et al. (2002) who did not observe differences in growth performance of growing and finishing beef cattle when substituting PCU for urea. The current experiment hypothesized that feeding high-forage diets containing PCU would result in equivalent growth and feed efficiency when compared with diets containing lesser forage or more protein that possesses a slower rate of digestion. The working hypothesis was developed because it is commonly understood that the hydrolysis of ammonia in nonprotein nitrogen sources such as urea yields ammonia that influences fiber digestion; however, the availability of this ammonia is short lived in the rumen, while carbohydrate fermentation proceeds for a longer period (Van Soest, 1994). The results of this study demonstrated that PCU can be used in heifer diets to effectively replace SBM as a slow-release NS, and as a result, allow diet formulations to contain greater amounts of forage.
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TABLE 1. Ingredient and nutrient composition of treatment rations on a DM basis.a Item Ingredient, % Corn silageb Timothy hayc Soybean meal Cracked dry corn Wheat midds Dynamate Aragonite Dicalcium phosphate Optigen 1200 Urea Vitamins A, D, and Ed Vitamin Ee MgO, 54% Mg Trace mineral premixf Selenium, 0.06% Se Salt Nutrient DM, % CP, % Soluble CP, % of CP RDP, % of CP NSC,g % Starch, % NDF, % ADF, % TDN,h % ME,i Mcal/kg of DM Calcium, % Phosphorous, % Magnesium, % Potassium, %
HF + PCU
HF + SBM
LF + PCU
LF + SBM
SE
50.0 24.9 10.2 11.6 — 0.15 0.75 0.47 1.28 — 0.01 0.08 0.18 0.03 0.03 0.25
45.1 29.9 15.6 7.3 — — 0.80 0.38 — — 0.01 0.08 0.19 0.03 0.03 0.25
40.1 24.9 5.1 18.7 7.5 0.19 0.92 0.26 1.78 — 0.01 0.08 0.14 0.03 0.03 0.25
23.4 41.7 15.2 17.7 — — 0.80 0.38 — 0.34 0.01 0.08 0.19 0.03 0.03 0.25
— — — — — — — — — — — — — — — —
52.3 16.0 26.2 29.8 35.4 18.3 42.7 24.3 67.7 2.45 0.52 0.37 0.25 1.11
54.5 16.2 29.9 34.2 32.3 18.8 43.4 25.2 67.7 2.45 0.63 0.33 0.26 1.05
56.9 16.2 28.0 26.0 37.1 19.6 40.8 22.3 68.0 2.46 0.74 0.38 0.25 1.00
66.9 16.1 26.0 36.7 35.0 17.7 40.9 24.2 67.2 2.47 0.57 0.38 0.24 1.18
0.80 0.16 0.66 0.54 — 0.81 0.49 0.24 0.01 0.01 0.01 0.01 0.01 0.01
a Treatments: HF + PCU = 75% forage (high forage) and Optigen 1200 (CPG Nutrients, Syracuse, NY); HF + SBM = 75% forage and soybean meal, LF + PCU = 65% forage (low forage) and Optigen 1200; LF + SBM = 65% forage and soybean meal. b Corn silage contained 37.6% DM; 39.2% NDF; 23.5% ADF; 7.2% CP (DM basis). c Timothy hay contained 93.1% DM; 68.8% NDF; 40.2% ADF; and 6.8% CP (DM basis). d Contained 6,600,000 IU of vitamin A/kg; 2,200,000 IU of vitamin D/kg; 2,200 IU of vitamin E/kg. e Contained 125,000 IU of vitamin E/kg. f Contained 5.80% S; 300 mg of Co/kg; 11,000 mg of Cu/kg; 13,500 mg of Fe/kg; 100 mg of I/kg; 20,000 mg of Mn/kg; 58,000 mg of Zn/kg. g Nonstructural carbohydrates. h Calculated from ingredients. i Estimated metabolizable energy: ME = TDN × 0.04409 × 0.82.
The PUN levels with respect to diet are presented in Table 2. This measurement was included in the experiment because PUN levels correspond positively to an increase in ruminal ammonia concentration (Bunting et al., 1987). A product of rumen fermentation, liberated ruminal ammo-
nia can be absorbed and transported to the liver where it is metabolized to urea. Because urea is a small, watersoluble molecule that permeates all cells and tissues in the body, it can either be reabsorbed by tissues and utilized or excreted in urine or feces (Butler, 1998). Major factors known
to influence PUN levels are dietary CP intake, solubility and degradability of consumed protein, and the amount of fermentable carbohydrate in the diet (Gabler and Heinrichs, 2003). Depending upon the circumstances, urea recycling may play a major role in the delivery of nitrogen to
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TABLE 2. Least squares means for age, BW, intake, and performance of small Holstein heifers fed 2 levels of forage and nitrogen sources.a Contrastb Item Initial BW, kg Final BW, kg ADG, kg/d DMI, kg/d DMI, % of BW FEc PUN,d mg/dL
HF + PCU
HF + SBM
LF + PCU
LF + SBM
SEM
FL
NS
FL × NS
191.1 299.7 0.86 5.34 2.20 7.17 11.7
186.3 295.4 0.86 5.30 2.20 7.49 12.3
191.6 298.7 0.85 5.36 2.20 8.00 12.2
189.3 305.2 0.92 5.42 2.20 7.04 12.8
6.81 11.1 0.05 0.19 <0.01 0.47 0.44
0.76 0.69 0.69 0.72 0.24 0.69 0.30
0.54 0.92 0.48 0.98 0.68 0.50 0.17
0.83 0.63 0.53 0.79 0.43 0.18 0.97
a Treatments: HF + PCU = 75% forage (high forage) and Optigen 1200 (CPG Nutrients, Syracuse, NY); HF + SBM = 75% forage and soybean meal; LF + PCU = 65% forage (low forage) and Optigen 1200; LF + SBM = 65% forage and soybean meal. b FL = forage level; NS = nitrogen source; FL × NS = interaction of forage level and nitrogen source. c Feed efficiency; expressed as the ratio of kilograms of feed to kilograms of gain. d Plasma urea nitrogen.
the rumen microbes. The current experiment evaluated the effects of feeding PCU in low- or high-forage diets
on PUN levels. Although it is well known that high PUN levels indicate a loss of rumen ammonia and poor
nitrogen utilization, no differences were observed in PUN in the current experiment, averaging 12.3 ± 0.4 mg/
TABLE 3. Least squares means for structural growth measurements and body condition score of Holstein heifers fed 2 levels of forage and nitrogen sources.a Contrastb Item Withers height (cm) Initial Final Change Hip height (cm) Initial Final Change Hip width (cm) Initial Final Change Heart girth (cm) Initial Final Change BCS Initial Final Change
HF + PCU
HF + SBM
LF + PCU
LF + SBM
SEM
FL
NS
FL × NS
105.7 117.9 12.2
104.5 118.0 13.5
105.4 118.8 13.4
106.2 119.6 13.4
1.08 1.08 0.69
0.40 0.21 0.41
0.83 0.65 0.30
0.24 0.69 0.30
110.7 122.7 12.0
109.5 123.0 13.6
110.7 123.9 13.1
111.3 124.3 13.0
1.21 1.12 0.58
0.38 0.21 0.57
0.71 0.69 0.18
0.38 0.95 0.14
33.3 39.5 6.2
33.2 40.9 7.7
33.4 39.3 5.9
33.5 39.8 6.3
0.39 0.72 0.61
0.66 0.37 0.17
0.97 0.18 0.11
0.78 0.55 0.36
127.3 150.8 23.5
127.7 149.6 21.9
128.8 151.3 22.5
128.9 152.4 23.5
1.68 1.87 1.24
0.38 0.38 0.81
0.87 0.98 0.81
0.92 0.54 0.29
0.04 0.06 0.07
0.81 0.74 0.59
0.92 0.21 0.14
0.97 0.91 0.88
2.66 3.29 0.63
2.66 3.36 0.70
2.67 3.27 0.60
2.66 3.34 0.68
a Treatments: HF + PCU = 75% forage (high forage) and Optigen 1200 (CPG Nutrients, Syracuse, NY); HF + SBM = 75% forage and soybean meal; LF + PCU = 65% forage (low forage) and Optigen 1200; LF + SBM = 65% forage and soybean meal. b FL = forage level; NS = nitrogen source; FL × NS = interaction of forage level and nitrogen source.
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dL across treatments. These results seem to indicate that the rates of nitrogen release in the experimental diets were equivalent; however, this study did not directly measure ammonia concentrations in the rumen. Thus, the lack of differences across treatments might have been a result of normal ruminal nitrogen recycling that maintains the concentration of nitrogen in both the blood and rumen (NRC, 1996). It should also be noted the chemical and physical composition of the diet may affect feeding behavior and, as a result, rumen fermentation (Kononoff et al., 2003). In the current experiment, the effect of treatment on eating behavior was not measured, but all animals were individually fed, minimizing bunk competition and allowing all diets, including those containing high amounts of forage, to be eaten throughout the day. Together these factors likely resulted in an even, steady-state digestion in the rumen, which also might have contributed to the lack of differences observed.
Implications Feeding a slow-release urea product (Optigen 1200) in either of 2 different levels of dietary forage to growing dairy heifers does not result in differences in feed efficiency or structural growth when compared with SBM. These results suggest Optigen 1200 can be used in dairy heifer diets to effectively replace SBM as a ration NS if availability and price are favorable.
Kononoff et al.
Acknowledgments This research was a component of NC-1119, Management Systems to Improve the Economic and Environmental Sustainability of Dairy Enterprises. The authors acknowledge financial support of CPG Nutrients, Inc. The assistance with assays and sample collection of T. Schrieffer and M. Long is much appreciated, as well as the editing of C. Jones.
Literature Cited AOAC. 1990. Official Methods of Analysis. 15th ed. Assoc. Offic. Anal. Chem., Arlington, VA. Bartley, E. E., and C. W. Deyoe. 1975. Starea as a protein replacer for ruminants. Feedstuffs 47:42. Bunting, L. D., J. A. Boling, C. T. MacKown, and R. B. Muntifering. 1987. Effect of dietary protein level on nitrogen metabolism in lambs: Studies using 15 N-nitrogen. J. Anim. Sci. 64:855. Butler, W. R. 1998. Effect of protein nutrition on ovarian and uterine physiology in dairy cattle. J. Dairy Sci. 81:2533. Gabler, M. T., and A. J. Heinrichs. 2003. Dietary protein to metabolizable energy ratios on feed efficiency and structural growth of prepubertal Holstein heifers. J. Dairy Sci. 86:268. Galo, E., S. M. Emanuele, C. J. Sniffen, J. H. White, and J. R. Knapp. 2003. Effects of a polymer-coated urea product on nitrogen metabolism in lactating Holstein dairy cattle. J Dairy Sci. 86:2154. Holm, J., I. Bjorck, A. Drews, and N. G. Asp. 1986. A rapid method for the analysis of starch. Starch Staerke 7:224. Hoover, W. H., and S. R. Stokes. 1991. Balancing carbohydrates and proteins for optimum rumen microbial yield. J. Dairy Sci. 74:3630.
Kononoff, P. J., A. J. Heinrichs, and H. A. Lehman. 2003. The effect of corn silage particle size on eating behavior, chewing activities, and rumen fermentation in lactating dairy cows. J. Dairy Sci. 86:3343. Krishnamoorthy, U., T. V. Muscato, C. J. Sniffen, and P. J. Van Soest. 1982. Nitrogen fractions in selected feedstuffs. J. Dairy Sci. 65:217. Krishnamoorthy, U., C. J. Sniffen, M. D. Stern, and P. J. Van Soest. 1983. Evaluation of a mathematical model of rumen digestion and an in vitro simulation of rumen proteolysis to estimate the rumen-undegraded nitrogen content of feedstuffs. Br. J. Nutr. 50:555. Lo ¨ est, C. A., E. C. Titgemeyer, J. S. Drouillard, B. D. Lambert, and A. M. Trater. 2001. Urea and biuret as nonprotein nitrogen sources in cooked molasses blocks for steers fed prairie hay. Anim. Feed Sci. Technol. 94:115. NRC. 1996. Nutrient Requirements of Beef Cattle. 7th rev. ed. Natl. Acad. Press, Washington, DC. NRC. 2001. Nutrient Requirements of Dairy Cattle. 7th rev. ed. Natl. Acad. Press, Washington, DC. Owens, F. N., and R. A. Zinn. 1988. Protein metabolism of ruminant animals. In The Ruminant Animal, Digestive Physiology and Nutrition. D. C. Church, ed. p 227. Prentice Hall, Englewood Cliffs, NJ. Prokop, M. J., and T. J. Klopfenstein. 1977. Slow ammonia release urea. Nebraska Beef Cattle Report. No. EC 77-218, Lincoln, NE. Smith, D. 1981. Removing and analyzing carbohydrates from plant tissues. Wisconsin Agric. Exp. Stn. Rep. No. R2107. Madison, WI. Tedeschi, L. O., M. J. Baker, D. J. Ketchen, and D. G. Fox. 2002. Performance of growing and finishing cattle supplemented with a slow-release urea product and urea. Can. J. Anim. Sci. 82:567. Van Soest, P. J. 1994. Nutritional Ecology of the Ruminant. 2nd ed. Comstock Publ., Ithaca, NY.
TheSource Effects of Nitrogen and Forage on Feed Efficiency and Structural Growth of Prepubertal Holstein Heifers P. J. KONONOFF,1 A. J. HEINRICHS,2 PAS, and M. T. GABLER3 Department of Dairy and Animal Science, The Pennsylvania State University, University Park 16802
Abstract Eighty Holstein heifers averaging 189.6 ± 6.8 kg of BW were used to evaluate the effects of forage level and rumen degradable nitrogen source on feed efficiency and structural growth. A randomized complete block design was used with heifers blocked according to BW (≤136.0 kg and >136.0 kg) and assigned to 1 of 4 treatment diets in a 2 × 2 factorial design. Treatments were constructed with 2 levels of forage (65 or 75%) and 2 nitrogen sources. Forage sources were a mixture of corn silage and chopped timothy hay. Nitrogen sources were either soybean meal (SBM) or a slow-release, polymer-coated urea product (Optigen 1200, CPG Nutrients, Syracuse, NY), which was fed at 1.8% of diet DM on low-forage diets and at 1.3% of diet DM on high-forage diets. Average daily gain and feed efficiency did not differ between rations of different forage level or nitrogen source, averaging 0.87 ± 0.05 kg and 7.4 ± 0.5, respectively, across treatments.
1
Current address: Department of Animal Science, The University of Nebraska, Lincoln 68588. 2 To whom correspondence should be addressed: [email protected] 3 Current address: ADM Alliance Nutrition, Box 44307, Madison WI 53744.
Similarly, no differences were observed in change of withers height, hip height, hip width, or heart girth. No differences were observed in plasma urea nitrogen, which averaged 12.3 ± 0.4 mg/dL across treatments. Results of this experiment suggest that feeding moderately different levels of forage along with either SBM or a polymer-coated urea product does not result in any significant differences in feed efficiency or structural growth. Polymercoated urea can be used in heifer diets to effectively replace SBM as a nitrogen source in either low- or high-forage rations. Key words: heifer growth, feed efficiency, forages, nitrogen source, polymer-coated urea
Introduction Many fiber-digesting rumen bacteria require ammonia for protein synthesis (NRC, 2001). Urea is commonly added to ruminant diets as a source of nonprotein nitrogen that is rapidly hydrolyzed to ammonia in the rumen. Much of the available ammonia is poorly utilized at peak levels, and often the peak level occurs sooner than the peak of maximum fiber fermentation (Van Soest, 1994). Several methods have been evaluated that attempt to improve rumen am-
monia by reducing the rate at which urea is released. Such products include biuret (Lest et al., 2001), starea (Bartley and Deyoe, 1975), and urea formaldehyde (Prokop and Klopfenstein, 1977). Although these methods may be useful in avoiding ammonia toxicity, they rarely improve utilization of dietary nitrogen or improve animal performance compared with standard feed sources (Owens and Zinn, 1988). Additionally, achieving the slow-release effect of some products, such as starea, requires heating to bind nitrogen in an insoluble form and may reduce the availability of some nutrients to rumen microbes (Owens and Zinn, 1988). Optigen 1200 (CPG Nutrients, Syracuse, NY) is a polymer-coated, controlled-release urea product (PCU) that, compared with feed-grade urea, is believed to posses a reduced rate of ammonia release to rumen microbes (Galo et al., 2003). Because it is commonly understood that the rate of rumen ammonia production should be paired with that of carbohydrate digestion (Hoover and Stokes, 1991), PCU may be useful in high-forage, low-concentrate diets that have a slow rate of fermentation. This use may well fit standard heifer diets, which typically include high levels of forage and limited feeding of protein concentrate. The cur-
85
Nitrogen and Forage Source for Heifers
rent study was designed to evaluate the effect of feeding PCU in growing heifer diets containing high amounts of slowly digestible fiber. The objective of this study was to determine the effects of diet forage level (FL) and nitrogen source (NS) on feed efficiency, structural growth, and body condition score (BCS) in heifers. It was hypothesized that feeding a high-forage diet containing PCU would result in equivalent growth and feed efficiency when compared with diets containing lesser forage or more protein with a slower rate of digestion.
Materials and Methods Eighty Holstein heifers, trained to use Calan feeding doors (American Calan, Inc., Northwood, NH), were randomly assigned to 1 of 4 treatment rations in a randomized complete block design and blocked according to BW (≤136.0 kg and ≥136.0 kg). Heifers were housed in a naturally ventilated barn with bedding pack space maintained at 3.7 m2 per heifer. During the 14-d pretrial adaptation period, heifers received a ration meeting NRC (2001) nutrient recommendations for a Holstein heifer gaining 0.80 kg/d. Treatment diets were constructed using 2 FL (65 or 75%) and 2 NS (Table 1). Forage sources were a mixture of corn silage and chopped timothy hay. Primary NS were either soybean meal (SBM) or a PCU (Optigen 1200) both fed at 1.8% of diet DM on low-forage diets and at 1.3% of diet DM on high-forage diets. The quantity of diet DM offered was adjusted weekly in an attempt to achieve an ADG of 0.80 kg/ d. Weekly BW was recorded on 2 consecutive days (3 h postfeeding) to determine the quantity of treatment ration offered for the next 7 d. Refusal of treatment ration was weighed and recorded daily. Samples of treatment rations and forages were collected 3×/wk. Portions of the weekly samples were dried immediately for determination of DM, which was used to adjust treatment
rations to achieve accurate daily DMI as a percentage of BW. The remaining portions of the samples were frozen (−20°C), and composites were made every 21 d for determination of DM, CP (AOAC, 1990), soluble CP (Krishnamoorthy et al., 1982), RDP (Krishnamoorthy et al., 1983), total nonstructural carbohydrates (Smith, 1981; modified to use ferricyanide as a colorimetric indicator), starch (Holm et al., 1986), and NDF and ADF (ANKOM200 Fiber Analyzer, ANKOM Technology Corp., Fairport, NY). Withers height, hip height, hip width, heart girth, and BCS (based on a 5-point scale; 1 = underconditioned and 5 = overconditioned) were recorded 3 h postfeeding at the beginning of the trial and every 28 d until the end of the treatment period. Blood samples also were taken at these 28-d intervals. Blood was acquired from the left jugular vein via venipuncture into evacuated glass tubes containing citrate as an anticoagulant. Plasma samples were aspirated after centrifugation (4,000 × g) and frozen (−20°C) for later analysis of urea N (procedure number 0580, Stanbio Laboratory Inc., San Antonio, TX). Statistical Analysis. Nutrient content of treatment rations was analyzed by the PROC GLM procedure of SAS (SAS Inst., Inc., Cary, NC). Least squares means and standard errors were determined using the LSMEANS and STDERR statement in PROC GLM. The first-order autoregressive covariance structure [AR(1)] and the MIXED procedure were used to analyze all growth and blood data. Repeated measurements of plasma urea nitrogen (PUN) were analyzed by including a REPEATED model statement, as well as a term for time and interaction for treatment by time. Sum of squares for all treatments were then partitioned into single df contrasts planned a priori for FL, NS, and FL by NS interaction. Significance for all models was declared at P ≤ 0.05. All means presented are least squares means. Least squares means
and standard errors for DMI and feed efficiency from d 1 to 140 were determined using LSMEANS in PROC MIXED. Orthogonal contrasts for linear, quadratic, and cubic effects of the treatment rations were determined using the ESTIMATE statement.
Results and Discussion Ingredient and nutrient composition of the treatment rations are listed in Table 1. Body weights, DMI, ADG, and feed efficiencies are presented in Table 2. Heifers began consuming the treatment rations at 189.6 ± 2.1 kg of BW. Average daily gain and feed efficiency were not observed to be different, averaging 0.87 ± 0.05 kg and 7.4 ± 0.5, respectively, across treatments. No differences in initial BW, final BW, or DMI were observed. In addition, no differences were observed in change of withers height, hip height, hip width, heart girth, or BCS (Table 3). The animal performance data of this study are similar to those of Tedeschi et al. (2002) who did not observe differences in growth performance of growing and finishing beef cattle when substituting PCU for urea. The current experiment hypothesized that feeding high-forage diets containing PCU would result in equivalent growth and feed efficiency when compared with diets containing lesser forage or more protein that possesses a slower rate of digestion. The working hypothesis was developed because it is commonly understood that the hydrolysis of ammonia in nonprotein nitrogen sources such as urea yields ammonia that influences fiber digestion; however, the availability of this ammonia is short lived in the rumen, while carbohydrate fermentation proceeds for a longer period (Van Soest, 1994). The results of this study demonstrated that PCU can be used in heifer diets to effectively replace SBM as a slow-release NS, and as a result, allow diet formulations to contain greater amounts of forage.
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TABLE 1. Ingredient and nutrient composition of treatment rations on a DM basis.a Item Ingredient, % Corn silageb Timothy hayc Soybean meal Cracked dry corn Wheat midds Dynamate Aragonite Dicalcium phosphate Optigen 1200 Urea Vitamins A, D, and Ed Vitamin Ee MgO, 54% Mg Trace mineral premixf Selenium, 0.06% Se Salt Nutrient DM, % CP, % Soluble CP, % of CP RDP, % of CP NSC,g % Starch, % NDF, % ADF, % TDN,h % ME,i Mcal/kg of DM Calcium, % Phosphorous, % Magnesium, % Potassium, %
HF + PCU
HF + SBM
LF + PCU
LF + SBM
SE
50.0 24.9 10.2 11.6 — 0.15 0.75 0.47 1.28 — 0.01 0.08 0.18 0.03 0.03 0.25
45.1 29.9 15.6 7.3 — — 0.80 0.38 — — 0.01 0.08 0.19 0.03 0.03 0.25
40.1 24.9 5.1 18.7 7.5 0.19 0.92 0.26 1.78 — 0.01 0.08 0.14 0.03 0.03 0.25
23.4 41.7 15.2 17.7 — — 0.80 0.38 — 0.34 0.01 0.08 0.19 0.03 0.03 0.25
— — — — — — — — — — — — — — — —
52.3 16.0 26.2 29.8 35.4 18.3 42.7 24.3 67.7 2.45 0.52 0.37 0.25 1.11
54.5 16.2 29.9 34.2 32.3 18.8 43.4 25.2 67.7 2.45 0.63 0.33 0.26 1.05
56.9 16.2 28.0 26.0 37.1 19.6 40.8 22.3 68.0 2.46 0.74 0.38 0.25 1.00
66.9 16.1 26.0 36.7 35.0 17.7 40.9 24.2 67.2 2.47 0.57 0.38 0.24 1.18
0.80 0.16 0.66 0.54 — 0.81 0.49 0.24 0.01 0.01 0.01 0.01 0.01 0.01
a Treatments: HF + PCU = 75% forage (high forage) and Optigen 1200 (CPG Nutrients, Syracuse, NY); HF + SBM = 75% forage and soybean meal, LF + PCU = 65% forage (low forage) and Optigen 1200; LF + SBM = 65% forage and soybean meal. b Corn silage contained 37.6% DM; 39.2% NDF; 23.5% ADF; 7.2% CP (DM basis). c Timothy hay contained 93.1% DM; 68.8% NDF; 40.2% ADF; and 6.8% CP (DM basis). d Contained 6,600,000 IU of vitamin A/kg; 2,200,000 IU of vitamin D/kg; 2,200 IU of vitamin E/kg. e Contained 125,000 IU of vitamin E/kg. f Contained 5.80% S; 300 mg of Co/kg; 11,000 mg of Cu/kg; 13,500 mg of Fe/kg; 100 mg of I/kg; 20,000 mg of Mn/kg; 58,000 mg of Zn/kg. g Nonstructural carbohydrates. h Calculated from ingredients. i Estimated metabolizable energy: ME = TDN × 0.04409 × 0.82.
The PUN levels with respect to diet are presented in Table 2. This measurement was included in the experiment because PUN levels correspond positively to an increase in ruminal ammonia concentration (Bunting et al., 1987). A product of rumen fermentation, liberated ruminal ammo-
nia can be absorbed and transported to the liver where it is metabolized to urea. Because urea is a small, watersoluble molecule that permeates all cells and tissues in the body, it can either be reabsorbed by tissues and utilized or excreted in urine or feces (Butler, 1998). Major factors known
to influence PUN levels are dietary CP intake, solubility and degradability of consumed protein, and the amount of fermentable carbohydrate in the diet (Gabler and Heinrichs, 2003). Depending upon the circumstances, urea recycling may play a major role in the delivery of nitrogen to
87
Nitrogen and Forage Source for Heifers
TABLE 2. Least squares means for age, BW, intake, and performance of small Holstein heifers fed 2 levels of forage and nitrogen sources.a Contrastb Item Initial BW, kg Final BW, kg ADG, kg/d DMI, kg/d DMI, % of BW FEc PUN,d mg/dL
HF + PCU
HF + SBM
LF + PCU
LF + SBM
SEM
FL
NS
FL × NS
191.1 299.7 0.86 5.34 2.20 7.17 11.7
186.3 295.4 0.86 5.30 2.20 7.49 12.3
191.6 298.7 0.85 5.36 2.20 8.00 12.2
189.3 305.2 0.92 5.42 2.20 7.04 12.8
6.81 11.1 0.05 0.19 <0.01 0.47 0.44
0.76 0.69 0.69 0.72 0.24 0.69 0.30
0.54 0.92 0.48 0.98 0.68 0.50 0.17
0.83 0.63 0.53 0.79 0.43 0.18 0.97
a Treatments: HF + PCU = 75% forage (high forage) and Optigen 1200 (CPG Nutrients, Syracuse, NY); HF + SBM = 75% forage and soybean meal; LF + PCU = 65% forage (low forage) and Optigen 1200; LF + SBM = 65% forage and soybean meal. b FL = forage level; NS = nitrogen source; FL × NS = interaction of forage level and nitrogen source. c Feed efficiency; expressed as the ratio of kilograms of feed to kilograms of gain. d Plasma urea nitrogen.
the rumen microbes. The current experiment evaluated the effects of feeding PCU in low- or high-forage diets
on PUN levels. Although it is well known that high PUN levels indicate a loss of rumen ammonia and poor
nitrogen utilization, no differences were observed in PUN in the current experiment, averaging 12.3 ± 0.4 mg/
TABLE 3. Least squares means for structural growth measurements and body condition score of Holstein heifers fed 2 levels of forage and nitrogen sources.a Contrastb Item Withers height (cm) Initial Final Change Hip height (cm) Initial Final Change Hip width (cm) Initial Final Change Heart girth (cm) Initial Final Change BCS Initial Final Change
HF + PCU
HF + SBM
LF + PCU
LF + SBM
SEM
FL
NS
FL × NS
105.7 117.9 12.2
104.5 118.0 13.5
105.4 118.8 13.4
106.2 119.6 13.4
1.08 1.08 0.69
0.40 0.21 0.41
0.83 0.65 0.30
0.24 0.69 0.30
110.7 122.7 12.0
109.5 123.0 13.6
110.7 123.9 13.1
111.3 124.3 13.0
1.21 1.12 0.58
0.38 0.21 0.57
0.71 0.69 0.18
0.38 0.95 0.14
33.3 39.5 6.2
33.2 40.9 7.7
33.4 39.3 5.9
33.5 39.8 6.3
0.39 0.72 0.61
0.66 0.37 0.17
0.97 0.18 0.11
0.78 0.55 0.36
127.3 150.8 23.5
127.7 149.6 21.9
128.8 151.3 22.5
128.9 152.4 23.5
1.68 1.87 1.24
0.38 0.38 0.81
0.87 0.98 0.81
0.92 0.54 0.29
0.04 0.06 0.07
0.81 0.74 0.59
0.92 0.21 0.14
0.97 0.91 0.88
2.66 3.29 0.63
2.66 3.36 0.70
2.67 3.27 0.60
2.66 3.34 0.68
a Treatments: HF + PCU = 75% forage (high forage) and Optigen 1200 (CPG Nutrients, Syracuse, NY); HF + SBM = 75% forage and soybean meal; LF + PCU = 65% forage (low forage) and Optigen 1200; LF + SBM = 65% forage and soybean meal. b FL = forage level; NS = nitrogen source; FL × NS = interaction of forage level and nitrogen source.
88
dL across treatments. These results seem to indicate that the rates of nitrogen release in the experimental diets were equivalent; however, this study did not directly measure ammonia concentrations in the rumen. Thus, the lack of differences across treatments might have been a result of normal ruminal nitrogen recycling that maintains the concentration of nitrogen in both the blood and rumen (NRC, 1996). It should also be noted the chemical and physical composition of the diet may affect feeding behavior and, as a result, rumen fermentation (Kononoff et al., 2003). In the current experiment, the effect of treatment on eating behavior was not measured, but all animals were individually fed, minimizing bunk competition and allowing all diets, including those containing high amounts of forage, to be eaten throughout the day. Together these factors likely resulted in an even, steady-state digestion in the rumen, which also might have contributed to the lack of differences observed.
Implications Feeding a slow-release urea product (Optigen 1200) in either of 2 different levels of dietary forage to growing dairy heifers does not result in differences in feed efficiency or structural growth when compared with SBM. These results suggest Optigen 1200 can be used in dairy heifer diets to effectively replace SBM as a ration NS if availability and price are favorable.
Kononoff et al.
Acknowledgments This research was a component of NC-1119, Management Systems to Improve the Economic and Environmental Sustainability of Dairy Enterprises. The authors acknowledge financial support of CPG Nutrients, Inc. The assistance with assays and sample collection of T. Schrieffer and M. Long is much appreciated, as well as the editing of C. Jones.
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