The influence of high-nitrogen forages on the voluntary feed intake of sheep 1,2

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The influence of high-nitrogen forages on the voluntary feed intake of sheep 1,2 D. R. Stevens* 3, J. C. Burns* 4, D. S. Fisher, and J. H. Eisemann Departments of *Crop Science and Animal Science, North
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The influence of high-nitrogen forages on the voluntary feed intake of sheep 1,2 D. R. Stevens* 3, J. C. Burns* 4, D. S. Fisher, and J. H. Eisemann Departments of *Crop Science and Animal Science, North Carolina State University, Raleigh 27695; ARS, USDA North Carolina State University; and J. P. Campbell, Sr., Natural Resource Conservation Center, Watkinsville, GA ABSTRACT: The objective of this research was to examine the effect of high concentrations of nonprotein nitrogen (NPN) on the voluntary food intake of sheep fed high-quality grasses. Wether lambs (n = 6 per treatment) were fed dried switchgrass (Panicum virgatum L.; Exp. 1) or dried tall fescue (Festuca arundinacea Schreb.; Exp. 2). In both experiments, urea was added to the dried forage at 0 (control), 12, or 24 g of N/kg of DM to increase the NPN concentration. Acid detergent fiber concentrations were 305 g/kg of DM in both experiments, although DM digestibility was 663 and 618 g/ kg of DM in Exp. 1 and Exp. 2, respectively. Voluntary feed intake of the control forage was 28.2 and 19.1 g/ kg of BW in Exp. 1 and Exp. 2, respectively, and decreased for the high-urea treatments to 25.2 and 16.2 g/kg of BW in Exp. 1 (P = 0.07) and Exp 2 (P = 0.03), respectively. Total feed N concentrations increased from 29.5 g to 45.7 g of N/kg of DM in Exp. 1 (P 0.01) and from 28.4 to 55.9 g of N/kg of DM in Exp. 2 (P 0.01). Nonprotein N concentrations increased from 28.3 to 53.8% of the total N in switchgrass diets (Exp. 1; P 0.01), and from 26.4 to 64.0% in tall fescue diets (Exp. 2; P 0.01). Plasma urea concentrations of the lambs increased from 3.1 to 6.6 mm (Exp. 1; P 0.01) and from 2.9 to 5.8 mm (Exp. 2; P 0.01) as the amount of urea added to the diets increased. These changes resulted in an increase in plasma osmolality from 298 to 307 mosm/kg (Exp. 1; P = 0.04), and from 299 to 307 mosm/kg (Exp. 2; P = 0.06). Increasing feed N and NPN concentrations through the addition of urea caused a significant decrease in the voluntary feed intake of sheep fed tall fescue and switchgrass. These responses showed no significant cause-and-effect relationship between voluntary feed intake, plasma urea concentrations, and plasma osmolality. Key Words: Forage, Intake, Nitrogen, Sheep, Urea 2004 American Society of Animal Science. All rights reserved. J. Anim. Sci : Introduction The effects of high protein concentrations on voluntary feed intake have not been well investigated. It is generally thought that high protein levels in forage are not a problem because they are often only achieved in legume, which are associated with a high level of voluntary feed intake. Some pasture grasses in temper- 1 Funded by New Zealand Pastoral Agric. Res. Inst., North Carolina ARS, and USDA-ARS. The use of trade names does not imply endorsement by the New Zealand Pastoral Agric. Res. Inst., North Carolina ARS, or by USDA-ARS of the products named or criticism of similar ones not mentioned. 2 G. Ryan and C. Conrad are acknowledged for assistance in obtaining the animal data, along with E. Leonard for assistance in laboratory analysis. 3 Current address: Invermay Research Center, Private Bag 50034, Mosgiel, New Zealand. 4 Correspondence: Box 7620 (phone: ; fax ; Received December 17, Accepted February 4, ate regions, however, can have high protein concentrations when fertilized with N (Minson, 1990). Animals exhibit preferences that reduce food intake in an attempt to balance the nutrients supplied by the diet. This has been shown for protein, for example, as Kyriazakis and Oldham (1993) measured peak voluntary feed intake at between 141 and 172 g/kg of DM CP when feeding lambs with paired choices of diets ranging from 78 to 235 g/kg of DM CP. Regulation of food intake is a complex process and is under central nervous system control (Provenza, 1995). Blood osmolality may be one metabolic signal of importance linking dietary effects to food intake (Grovum, 1995). Feedback through decreased rumen motility (Wever et al., 1991; Grovum and Wever, 1992) and reduced saliva production (Grovum, 1992) has been observed as plasma osmolality increased. The objective of these experiments was to examine the role of dietary CP concentrations on food intake of sheep fed two forages: a C4 grass, switchgrass, and a C3 grass, tall fescue. The experiments also tested for a correlation between feed intake and plasma osmolality. 1536 High-nitrogen forages and intake by sheep 1537 Materials and Methods Two experiments were conducted during June 1995 and February The experimental design was a randomized complete block with three treatments in two blocks with three replications nested in each block. The treatments were untreated hay (control), hay plus 12 g of N/kg of DM as urea (Low Urea), and hay plus 24 g of N/kg of DM as urea (High Urea). The hay used in Exp. 1 was switchgrass (Panicum virgatum L. var. Kanlow), and in Exp. 2, tall fescue was used (Festuca arundinacea Schreb. var. AU Triumph). In each experiment, 18 4-mo-old castrated lambs (Ovis aries) were assigned in three replicates to each of two controlled environment rooms (blocks) in individual metabolism crates. In Exp. 1, Katahdin, Katahdin Barbado, and Dorset Barbado lambs were used, and in Exp. 2, Katahdin and Dorset Barbado lambs were used. In both experiments, lamb breeds were balanced across all treatments but not across both rooms in Exp. 1. Forages Switchgrass, used in Exp. 1, was obtained from a 20- yr-old stand located at the North Carolina State University Reedy Creek Road Field Laboratory (Raleigh). Forage residue from the previous fall was removed by burning in March The stand received 80 kg of N/ha as ammonium nitrate on April 5, On May 5, 1995, the primary growth of approximately 4,000 kg of DM/ ha was harvested before heading by direct flailing to a 15-cm stubble, leaving a residual of approximately 1,000 kg of DM/ha. The forage was then placed in a drying barn (Burns et al., 1997) to a depth of 1 m, forced-air dried at 60 C (at inlet) for 36 h, and then returned to an ambient temperature (mean approx. 20 C) for the following 3 d. The resulting hay was baled directly from the drier into conventional square bales of approximately 15 kg and stored indoors until the beginning of the trial on June 5, Tall fescue used in Exp. 2 was obtained from a 4-yrold stand (sown as endophyte free) located at the North Carolina State University Reedy Creek Road Field Laboratory. Forage from the area, pastured the previous 2 yr, was removed (April 17, 1995) when approximately 30% of the culms were headed, leaving a residue of approximately 1,000 kg of DM/ha. The subsequent vegetative regrowth served as the experimental forage and was direct-flail harvested on June 3, 1995, when regrowth accumulated approximately 2,800 kg of DM/ha. The forage was dried as noted for Exp. 1, and bales were stored indoors until the beginning of the trial on January 29, In both Exp. 1 and 2, the hay was spread uniformly to a depth of approximately 10 cm on a concrete floor in preparation for treatment. The N treatments were applied to batches of the forage sufficient for 2 to 3 d of feeding at any one time. The urea was applied at a concentration of 173 and 348 g of urea/l for the Lowand High-Urea treatments, respectively, in 150 ml of water/kg of feed, and forage was allowed to air-dry before feeding. The control was sprayed with 150 ml of water/kg of dried grass and air-dried before feeding. Animals and Feeding Lambs averaged 28.3 (±2.6) kg in Exp. 1 and 26.1 (±1.5) kg in Exp. 2. Lambs grazing on an all-grass pasture were brought indoors and initially fed the control hay along with a ration of 0.45-kg alfalfa pellets for a 6-d pretrial period. On d 1 of the experiments, lambs were treated for internal parasites with fenbendazole at 10 ml/kg BW (Hoechst Roussel Vet, Warren, NJ) and placed into metabolism crates. An adjustment period of 7 d to the experimental diets was followed by a 7-d intake period. Lambs were then fitted with fecal collection bags and adapted to bags for 5 d before a 6-d digestion period when intake data, feces, and urine were collected. The lambs in both experiments were fed at 120% of the previous day s intake starting at 1.0 kg/d of dried grass. Feeding occurred at 0900 and 1530 daily with ¹ ₃ and ² ₃ of the DM fed at each time, respectively. Orts were removed, weighed, and sampled each morning before feeding. Constant access to water and mineralized salt blocks (consisting of salt and oxides of Zn, Mn, Fe, Cu, carbonates of Fe and Co, calcium periodate, and mineral oil, and containing not less or not more than 970 and 985 g/kg of NaCl and 0.35 and 0.45 g/kg of Ca, and not less than 3.5 g/kg of Zn, 2.8 g/kg of Mn, 1.7 g/ kg of Fe, 0.07 g/kg of I, and 0.07 g/kg of Co) was provided to all lambs. During Exp. 1, one Dorset Barbado lamb was diagnosed as being infected with coccidia on d 9, and consequently, the two other Dorset Barbado lambs (one per treatment) were given the coccidiostat Corid (9.6% amprolium, Merial Ltd., Duluth, GA) per label recommendations over the following 4 d. No other lambs appeared to be affected. Water delivery to one lamb (High Urea) was faulty on 1 d during Exp. 1 and compromised its intake data. The intake data for that lamb on that day and the following day were removed from analysis. Lambs in Exp. 1 were weighed on d 4, 11, 18, and 23, and lambs in Exp. 2 were weighed on d 4, 10, 21, and 25. Lambs were weighed between 0730 and 0830 just before feeding in both experiments. Sampling The voluntary food intake of each lamb was calculated from the dry weight offered and the dry weight of the orts each day. The average daily DMI over the 7-d intake period was used for analysis. Representative samples of the feed offered and orts for each lamb were collected daily during the intake and digestion periods. The daily samples were pooled, mixed thoroughly, and 1538 Stevens et al. then subsamples of both offered feed and orts for each animal from both the intake and digestion periods were oven-dried in paper bags at 50 C for 48 h. All samples were ground in a Wiley mill to pass a 1-mm screen. Feces were collected daily, weighed, and a subsample of approximately 100 g was weighed and dried for 48 h at 55 C to estimate DM concentration. A composite sample, weighted by daily fecal output, was made from the oven-dried samples for each lamb and was used for N and fiber analysis after grinding. Urine was sampled between 1000 and 1100 each morning during the 6-d digestion period. The total weight was recorded and a 5% sample taken each day. Samples were bulked together for each lamb and stored at 18 C. Each day, 50 ml of concentrated HCl (37.2%) was placed in each bucket before the next collection. Samples were thoroughly mixed, and a subsample of approximately 20 ml was taken for total N determination. Blood samples were taken from each lamb on d 14, 17, and 22 of Exp. 1 and on d 15, 20, and 22 of Exp. 2 by venipuncture of the jugular vein in 10-mL tubes treated with sodium heparin. Samples were taken just before the morning feeding and again 2 h after feeding. Some feeding activity was evident, but the disappearance of the offered feed was not measured. Samples were centrifuged in a refrigerated centrifuge at 850 g for 30 min within 30 min of sampling, and the plasma was frozen at 18 C until tested for plasma urea concentration and osmolality. Chemical Analysis Fiber was analyzed with an ANKOM 200 Fiber Analyzer (Ankom Technology Corp., Fairport, NY), sequentially determining the NDF and ADF concentrations according to Van Soest and Robertson (1980). Total N of offered feed, orts, and feces was assayed by the Kjeldahl procedure (AOAC, 1990). The N components of offered feed were fractionated according to Licitra et al. (1996). The true protein was precipitated using sodium tungstate and analyzed. Nonprotein N was then calculated as the difference between total N and precipitated true protein N. Acid detergent insoluble N was determined by removing and weighing the ADF residue from the synthetic bags following the sequential NDF and ADF analyses and assayed for the remaining N using the Kjeldahl procedure. Plasma urea concentration was tested by colorimetric assay of ammonia after the urea was hydrolyzed by urease (Procedure 640, Sigma Diagnostics, St. Louis, MO). Plasma osmolality was measured using the freezing point determination method (Osmomette A semiautomatic osmometer, Precision Systems Inc., Matick, MA). Statistical Analysis Data were analyzed using the PROC GLM procedure of SAS (SAS Inst., Inc., Cary, NC). The general form of the model used was: Y ijk = +R i +Br j +T k +e ijk (Model 1) For testing, the independent variables of room (R) and breed (Br) were considered random, whereas treatment (T) was considered fixed. Polynomial contrasts were used to test the effects of urea addition. Voluntary feed intake, feed N, and feed fiber results were averaged over both the intake and digestion periods, but digestibility and N balance information was calculated only from the digestion period. Room and breed effects and interactions with treatment were not significant (P 0.05) and are not given any further consideration. The final analysis was conducted using all six lambs per treatment as replicates. A comparison of the pre- and postprandial plasma osmolality in Exp. 1 was obtained using a repeated-measures analysis within PROC GLM of SAS. Loss of original data for pre- and postprandial plasma osmolality from the tall fescue prevented statistical comparison of these data. Regression analysis was used to determine the effect of plasma osmolality on intake using PROC GLM with the model Y ijk =b 0 +R i +Br j(i) +b 1 O ijk +e ijk (Model 2). Intake (Y) was predicted and the model included the effects of room (R), breed (Br) and osmolality (O). Results Feed Nitrogen and Fiber Fractions The N concentration of the control switchgrass in Exp. 1 was relatively high (Table 1). Nonprotein N accounted for 28.3% and ADIN accounted for 4.3% of the total N. Spraying urea on the control forage resulted in a linear increase in total and nonprotein N concentrations (P 0.01, Table 1). True protein and ADIN were unaffected. The nonprotein N fraction accounted for 41.7 and 53.8% of the total N for the Low- and High- Urea treatments, respectively. Similarly, the N concentration of the control tall fescue in Exp. 2 was relatively high (Table 1). Nonprotein N accounted for 26.4% of the total N and ADIN made up 3.0% of the total N (Table 1). As in Exp. 1, adding urea elicited a linear increase in the total and nonprotein N concentrations (P 0.01, Table 1). The true protein concentrations of the treatments receiving urea were lower than the control. Nitrogen recovery, expressed as the percentage of the N added as urea present on the forage analyzed, averaged only 67% in Exp. 1, but averaged 106% in Exp. 2. All added N that was recovered was in the nonprotein N fraction (Table 1). The average NDF and ADF concentrations of the untreated switchgrass were 674 g/kg of DM and 305 g/kg of DM, respectively (Table 1). The fiber profile was not affected by the addition of urea to switchgrass (Table 1). The NDF and ADF concentrations of the control tall fescue were 635 g/kg of DM and 305 g/kg of DM, respectively, while treatment with urea caused a linear decline in both NDF and ADF concentrations (P 0.01, Table 1). High-nitrogen forages and intake by sheep 1539 Table 1. Nitrogen and fiber fractions in switchgrass and tall fescue forage and after treatment with urea at 12 or 24 g of N/kg of DM Probability for contrasts Item Control Low urea a High urea b Linear Quadratic SE c Exp. 1: Switchgrass N fractions, g/kg of DM Total N 29.5 d True protein N Nonprotein N ADIN Total N recovery, % Fiber fractions, g/kg of DM NDF ADF Exp. 2: Tall fescue N fractions, g/kg of DM Total N 28.4 d True protein N Nonprotein N ADIN Total N recovery, % Fiber fractions, g/kg of DM NDF ADF a Urea applied at 12 g N/kg DM. b Urea applied at 24 g N/kg DM. c SE = [mean square error (Model 1)/6]. d Each value is the mean of six observations (feed offered each animal). Intake, Apparent Digestibility, and Nitrogen Balance Lamb BW did not change throughout Exp. 1 or Exp. 2 and was similar for each treatment (Table 2). Adding urea to the control forage did not alter voluntary food intake in Exp. 1 (P = 0.07), but did cause a linear decrease in voluntary food intake in Exp. 2 (P = 0.03). Nitrogen intake (g/kg BW) increased in Exp. 1 with switchgrass (P = 0.04) and in Exp. 2 with tall fescue (P 0.01, Table 2). The apparent digestibility of DM, NDF, and ADF was unaffected by adding urea to the control forage in both experiments, although it was higher in Exp 1 than in Exp. 2 (Table 2). The apparent N digestibility increased linearly with the addition of urea in both Exp. 1 and Exp. 2 (P 0.01, Table 2). The increase in N intake (g/d) was significant in both experiments (P 0.04, Table 2). Increasing N intake resulted in a linear increase in urinary N excretion in both experiments (P 0.01, Table 2). Consequently, N retention was significantly altered in Exp. 1 (P = 0.02) but not in Exp. 2 (P = 0.83). Plasma Urea Nitrogen and Plasma Osmolality The data were averaged over time and day of sampling before analysis. Average plasma urea concentrations in both the switchgrass and tall fescue experiments increased linearly (P 0.01, Table 3) with the addition of urea to the control forage. Average blood plasma osmolality also increased linearly (P 0.04 Exp. 1; P 0.06 Exp. 2; Table 3) in sheep fed switchgrass and tall fescue treated with urea. There was no significant (P = 0.61, Table 3) difference between pre- and postprandial plasma osmolality when measured during the switchgrass experiment. The relationship between plasma osmolality and DMI was not significant (Exp. 1, P = 0.62; Exp. 2, P = 0.21). Discussion Urea additions were successful in changing feed N concentrations from 29.5 g of N/kg of DM in the switchgrass control forage to 45.7 g of N/kg of DM in the High-Urea treatment. The N concentrations of the forage in the tall fescue experiment ranged from 28.4 g of N/kg of DM in the control to 55.9 g of N/kg of DM in the High-Urea treatment. Within this framework, the nonprotein N levels ranged from 26 to 64% of the total N pool, providing a good platform to test the hypothesis that high levels of rapidly available N may cause a decline in voluntary food intake. The use of urea in the switchgrass experiment gave a smaller range of both total N and nonprotein N than in the tall fescue experiment because of the lower recovery of the applied N, although both experiments had a wide range. Lower recovery in the switchgrass experiment may have been due to hot and humid conditions, which slowed drying of the hay after urea treatment. This could have resulted in some volatilization of the applied urea during the drying process. Minson (1990) summarized a large number of data sets from the literature to show that 1540 Stevens et al. Table 2. Daily voluntary feed intake and nitrogen intake, and the apparent digestibility of dry matter, nitrogen, and fiber fractions of switchgrass and tall fescue altered by additions of urea Probability for contrasts Item Control Low urea a High urea b Linear Quadratic SE c Exp. 1: Switchgrass Sheep BW, kg 30.8 d Feed intake, g/kg of BW N intake, g/kg of BW Diet digestibility, g/kg of DM DM NDF ADF N N balance, g/d N intake N in feces N in urine N retention Exp. 2: Tall fescue Sheep BW, kg 26.2 d Feed intake, g/kg of BW N intake, g/kg of BW Diet digestibility, g/kg of DM DM NDF ADF N N balance, g/d N intake 15.8 d N in feces N in urine N retention a Urea applied at 12 g of N/kg of DM. b Urea applied at 24 g of N/kg of DM. c SE = [mean square error (Model 1)/6]. d Each value is the mean of six animals. the average CP concentration was 21 g of
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