Influence of undegraded intake protein on intake, digestion, serum hormones and metabolites, and nitrogen balance in sheep

Small Ruminant Research 35 (2000) 225±233

In¯uence of undegraded intake protein on intake, digestion, serum hormones and metabolites, and nitrogen balance in sheep K.C. Swansona, J.S. Catonb,*, D.A. Redmerb, V.I. Burkeb, L.P. Reynoldsb a

b

Department of Animal Science, University of Kentucky, Lexington, KY 40546, USA Department of Animal and Range Sciences, North Dakota State University, Fargo, ND 58105, USA Accepted 30 June 1999

Abstract In Exp. 1, 20 ewes of mixed breeding were fed grass hay : straw mixtures, assigned to one of four supplemental treatments and evaluated during six collection periods. Supplemental treatments were control (no supplement), and low, medium, and high levels (5.2%, 22.1%, and 41.3% of DM, respectively) of undegraded intake protein (UIP). Supplements were formulated to be similar in degraded intake protein (DIP; 21%). Digestibilities of DM, OM, and CP were increased (P < 0.10) with protein supplementation and in medium and high compared with low UIP supplemented ewes. Digestibility of CP also was increased (P < 0.10) in ewes on high compared with medium treatments. Serum insulin was not in¯uenced (P > 0.10) by UIP treatment, except in collection period four. In contrast, serum glucose and growth hormone were not in¯uenced (P > 0.10) by UIP treatment. In Exp. 2, four wether lambs fed the same treatments as in Exp. 1, were used in two 4  4 Latin squares trials. In trial 1 lambs received a grass hay diet (6.7% CP) and in trial 2 lambs were fed 40 : 60 blend (6.6% CP) of grass hay and spring wheat straw. In both trials, N intake, urinary N, N digestion, apparent N absorption, and N retention were increased (P < 0.10) with protein supplementation. In addition, in trial 1, urinary N, N digestion, and apparent N absorption were increased (P < 0.10) in medium and high compared with low UIP and also in high compared with medium UIP treatments. In trial 2 of Exp. 2, total feed intake (g/kg BW), digestibility of DM and OM, BW, and fecal N were increased (P < 0.10) with protein supplementation. Organic matter digestion, BW, N intake, and N retention were increased by medium and high compared with low UIP. Fecal N, BW, N intake, and N retention were increased (P < 0.10) in high compared with medium UIP. These data indicate that increasing levels of UIP supplementation increases DM, OM, and CP (N) digestibility, serum urea N concentration, and N retention in sheep fed low quality forage. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Sheep; Protein supplements; Intake; Nitrogen balance; Growth hormone; Insulin

1. Introduction Supplemental CP often increases performance of ruminants fed or grazing low quality (low CP) forages *

Corresponding author. Tel.: ‡1-701-231-7653; fax: ‡1-701231-7590. E-mail address: [email protected] (J.S. Caton).

(Owens et al., 1991). Research has shown that protein supplementation can increase intake (Church and Santos, 1981; Kartchner, 1981; McCollum and Galyean, 1985) and (or) digestibility (Church and Santos, 1981; Kartchner, 1981) of low-quality forage. Egan (1965) showed that duodenally infused casein increased intake of low quality forage indicating that there may be regulators of intake which are indepen-

0921-4488/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 4 8 8 ( 9 9 ) 0 0 0 9 1 - 7

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dent of the rumen. These data led to the hypothesis that undegraded intake protein (UIP) supplementation of ruminants fed or grazing low quality forage may increase performance. Von Keyserlingk and Mathison (1993) reported an increase in DM digestibility in ®sh meal and canola supplemented compared with nonsupplemented lambs fed barley straw. However, they reported no difference in digestibility between ®sh meal and canola meal supplemented treatment groups and no differences in intake among all three treatment groups. Donaldson et al. (1991) reported increases in total and forage intake in steers grazing high quality (CP ˆ 27.4±28.5%) winter annuals and receiving protein supplements with low or high amounts of UIP protein compared with steers supplemented with corn. In addition, work with increasing level of CP supplementation has resulted in higher intake, lower growth hormone and higher insulin concentrations (Cheema et al., 1991a,b). In the previous studies these results could not be attributed to either DIP or UIP levels as both increased with increasing supplemental CP level. Therefore, our objectives were to evaluate the in¯uence of increasing levels of supplemental UIP, while holding degraded intake protein (DIP) relatively constant, on intake, digestion, serum hormones and metabolites, and nitrogen balance in sheep fed low quality forage. 2. Materials and methods 2.1. Experiment 1 20 mature ewes of mixed breeding (63  5.4 kg), housed inside the small animal research center (SARC) in individual pens, were offered low quality

forage ad libitum and subjected to six 7-day collection periods following 14-day dietary adjustments. Mature, cool season, grass hay : straw blends (chopped 15.2 cm) were used as forage with 100 : 0, 60 : 40, and 40 : 60 grass hay:straw mixes for periods 1 and 2, 3 and 4, and 5 and 6, respectively (Table 1). Straw was added to the forage base as periods advanced to ensure a low quality forage and to keep total forage CP as low as possible. Ewes were also assigned randomly to one of four supplement treatments. Supplements were control (no supplement), low, medium, and high UIP (Table 2). Supplements, fed to provide 100%, 120%, and 140% of recommendations (NRC, 1985), were formulated to be similar in DIP (21%) and energy (1.77 Mcal NEm/kg). Supplements were fed daily 08:30 hours at 119, 179, and 193 g/d (DM basis) for periods 1 and 2 (100%, grass hay), 3 and 4 (60 : 40, grass hay : straw), and 5 and 6 (40 : 60, grass hay : straw), respectively. Level of supplement offered increased with each straw addition because forage intake declined. Therefore, in order to maintain similar CP intakes across period, supplementation level needed to be increased. Water was offered ad libitum. During each collection period, BW were measured on the ®rst and last day. Individual intakes were recorded and diet samples were collected daily and composited across days within period. Fecal grab samples were taken at 08:00 hours and 16:00 hours daily during each collection period. Diet and fecal samples were dried and ground to pass a 2 mm screen and fecal samples were composited within animal across sampling day for each period. Diet and fecal samples were analyzed for DM, ash, CP, and ADF by standard procedures (AOAC, 1990). Analysis of NDF was conducted by the method of Robertson and Van Soest (1982). Digestibilities were estimated using acid

Table 1 Composition of forages fed to ewes (% of DM) Item

100% hay

Ash CP DIPa UIPa NDF ADF a

60% hay, 40% straw

40% hay, 60% straw

Period 1

Period 2

Period 3

Period 4

Period 5

Period 6

9.4 6.7 3.9 2.8 75 45

7.8 6.7 3.6 3.1 73.1 44.0

9.9 6.7 3.6 3.1 77.3 49.0

11.6 7.1 4.1 3.0 76.8 48.1

9.8 6.6 3.3 3.3 80.8 53.4

9.1 6.5 3.2 3.3 83.1 53.9

Based on 16 h dacron bag degradabilities conducted in a fistulated cow.

K.C. Swanson et al. / Small Ruminant Research 35 (2000) 225±233 Table 2 Components and chemical composition of supplements fed to ewes (% of DM) Components

UIP treatment Low

Medium

High

Wheat mids Casein Corn starch Beet molasses Fata Dicalcium phosphate Soybean meal Corn gluten meal Blood meal

70.0 13.0 9.0 5.0 1.0 2.0 ± ± ±

40.3 ± ± 5.0 ± 2.0 30.0 7.6 15.1

21.0 ± ± 5.0 ± 3.3 15.0 18.7 37.0

Chemical composition Ash CP DIPb UIPb NDF ADF

7.0 25.6 20.4 5.2 28.3 8.5

6.1 42.9 20.8 22.1 23.9 8.0

5.8 62.4 21.1 41.3 22.4 5.0

a

Alifet USA, Cincinnati, OH. Formulated to contain 21% DIP across supplements and to have 5.3, 22.3, and 41.2% UIP for low, medium, and high UIP treatments, respectively. b

insoluble ash (Van Keulin and Young, 1977) and the marker ratio technique (Merchen, 1988). Bloods samples were collected via jugular venipuncture prior to feeding on the ®rst 3 days of each collection period in serum separator tubes (Becton Dickinson Vacutainer Systems, Rutherford, NJ), allowed to clot for a minimum of 30 min, and centrifuged at 1560  g for 30 min. Serum was then decanted and stored at ÿ208C until analyzed. Blood serum (500 ml) was analyzed for growth hormone (GH) using radioimmunoassay (RIA) as previously described for cattle (Reynolds et al., 1990). The GH assay utilized NIDDK-oGH-I-4 (biopotency ˆ 1.5 IU/mg) as the radioiodination preparation, USDA-bGH-B-1 (biopotency ˆ 1.9 IU/mg) as the reference standard, NIDDK-oGH-2 as the primary antiserum, and sheep anti-rabbit gamma globulin as the secondary antiserum (Swanson, 1996). All samples were run in a single assay, and intra-assay variation was determined by assaying replicates (n ˆ 6) of a pool of lamb plasma in the same assay. The resulting mean  SE concentration of GH in the lamb plasma pool was 1.02  0.09 ng/ml. To further validate the

227

GH assay, a pool of lamb plasma was assayed at sample volumes of 200, 300, 400, 500, 600, and 700 ml, which yielded an inhibition curve that was parallel to that of the reference standard. Blood serum was analyzed for insulin concentrations using a commercially available RIA procedure (Coat-A-Count, Diagnostic products corporation, Los Angeles, CA) and procedures similar to those by Reynolds et al. (1990). A glucose oxidase kit (Procedure no. 510, Sigma diagnostics, St. Louis, MO) was used to determine serum glucose levels. Serum urea nitrogen was measured using a urea nitrogen kit (Procedure no. 640, Sigma diagnostics, St. Louis, MO). We have reported similar procedures for measuring glucose and urea nitrogen in serum of cattle (Reynolds et al., 1985, 1990). Data were analyzed with the GLM procedures of SAS (1988) and were subjected to repeated measures analysis (Gill and Hafs, 1971). The model for analysis of BW, intake, digestibility, and serum hormones and metabolites included effects of treatment, sampling period, animal within treatment, and sampling period by treatment interactions with animal within treatment being used as the error term for treatment effects. A sampling period  treatment interaction (P < 0.10) was present for serum insulin concentrations; therefore, these data were analyzed for treatment effects within each sampling period. Serum urea nitrogen and BW also showed a signi®cant sampling period  treatment interaction (P < 0.10); however, the interaction appeared to be due to differences in magnitude between treatment groups; therefore, treatment effects on urea nitrogen and BW data are reported across periods. When signi®cant F-tests were observed (P < 0.10), means were compared using contrast statements. Treatment contrasts were control vs. protein (low, medium, and high UIP), low UIP vs. medium and high UIP, and medium UIP vs. high UIP. 2.2. Experiment 2 Two trials were conducted to determine the in¯uence of increasing supplemental UIP on digestibility and nitrogen balance. Four wether lambs (62.2  2.4 kg) were housed in metabolism crates at SARC and were fed the same supplemental treatments as described above in a 4  4 Latin square design. Lambs

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K.C. Swanson et al. / Small Ruminant Research 35 (2000) 225±233

were offered 100 : 0 and 40 : 60 grass hay : straw blends in trials 1 and 2, respectively. Wethers were allowed a 10-day period to adapt to the feed and metabolism crates prior to each collection period. Forage and supplement were offered at 07:00 hours daily, with additional forage offered at 16:00 hours. Forage and water consumption were measured daily and feces and urine were collected on days 10±17 of each period. Diet, feces, and urine samples were analyzed for DM, ash, total N, and ADF by standard procedures (AOAC, 1990). Analysis of NDF was conducted by the method of Robertson and Van Soest (1982). Data were analyzed as 4  4 Latin square using the GLM procedure of SAS (1988). The model included effects of animal, period, and treatment. When signi®cant F-tests (P < 0.10) were observed means were compared using contrast statements. The contrast statements used were the same as those used in Exp. 1. 3. Results and discussion 3.1. Experiment 1 Body weight of ewes was not affected (P > 0.10) by treatment (data not shown). Forage and total intake also were not in¯uenced (P > 0.10) by treatment

(Table 3). Rittenhouse et al. (1970) and Branine et al. (1985) reported no change in forage intake due to protein supplementation. Other investigators have shown an increase in low quality forage intake as a result of protein supplementation (Church and Santos, 1981; McCollum and Galyean, 1985; Krysl et al., 1987; Cheema et al., 1991a). Forage fed in this study averaged 6.7% CP which may not have been low enough to result in an intake response to supplemental CP. However, other investigations (Cheema et al., 1991a) with oat hay containing 6.25% CP yielded linear increases in forage intake with increasing level of cotton seed meal supplementation. Alternatively, forage DIP present in our study (average 3.7%; 49 g/head daily for control ewes) may have been adequate for ruminal fermentation, resulting in no forage intake response from additional DIP (low UIP) supplementation. If this were the case, additional levels of UIP supplementation when DIP was adequate failed to alter forage intake. This concept is supported by recent work with cows fed a 5.7% CP cool season hay and supplemented with increasing levels of UIP (Sletmoen-Olson et al., 1999a). In the present study, increasing DIP levels by almost twofold had no effect on forage intake. Digestibility of ADF and NDF were not affected (P > 10) by treatment (Table 3). In contrast, DM, OM, and, CP apparent digestibility increased (P < 10) in protein supplemented ewes compared with controls. In

Table 3 Influence of increasing levels of supplemental UIP on DM intake and digestibility in ewes fed low quality forage Item

Intake Forage, g Total, g Forage, g/kg BW Total, g/kg BW Digestibility,% DMb,c OMb,c CPb,c,d ADF NDF a

UIP treatment Control

Low

Medium

High

SE

Pa

1319 1319 21.4 21.4

1266 1430 18.4 20.8

1225 1389 18.5 21.0

1239 1403 18.3 20.7

86 86 1.1 1.1

0.87 0.82 0.15 0.97

46.1 50.5 36.7 46.2 54.6

48.7 53.3 54.3 45.4 54.6

50.7 56.2 60.8 47.8 57.0

51.3 56.1 66.9 47.6 57.2

P equals observed significance for main effect of treatment. Control vs. low, medium, and high (P < 0.10). c Low vs. medium and high (P < 0.10). d Medium vs. high (P < 0.10). b

1.45 1.65 1.10 1.65 1.91

0.08 0.08 0.01 0.67 0.63

K.C. Swanson et al. / Small Ruminant Research 35 (2000) 225±233

229

Table 4 Influence of increasing levels of supplemental UIP on serum hormones and metabolites in ewes fed low quality forage Item

Glucose, mg/dl Urea N, mg/dlb,c,d Growth Hormone, ng/ml

UIP treatment Control

Low

Medium

High

SE

Pa

58.4 11.1 3.23

56.4 14.3 2.41

57.3 18.4 2.97

58.8 21.2 2.55

1.19 1.12 .58

0.50 0.01 0.73

a

P equals observed significance for main effect of treatment. Control vs. low, medium, and high (P < 0.10). c Low vs. medium and high (P < 0.10). d Medium vs. high (P < 0.10). b

addition, digestibility of DM, OM, and CP were increased (P < 10) in ewes supplemented with medium and high levels compared with low levels of UIP. An increase (P < 10) in CP digestibility also was observed in ewes supplemented with high compared with medium UIP. In agreement with these observations, Church and Santos (1981), Kartchner (1981), and Caton et al. (1988a) reported an increase in digestibility in response to protein supplementation. In the present study, increases in DM, OM, and CP digestibility due to protein supplementation likely resulted from the high digestibility of the supplement provided. Serum glucose and growth hormone concentrations were not in¯uenced (P > 0.10) by treatment (Table 4). Other investigators have also shown no effect due to protein supplementation on blood glucose in ruminants (Krysl et al., 1987; Cheema et al., 1991b; Shetaewi and Ross, 1991; Caton et al., 1994). Recent work (Sletmoen-Olson et al., 1999b) has suggested that plasma glucose concentrations in gestating beef cows are increased by DIP based supplements (natural sources) when compared with either non-supplemented controls or medium and high levels of UIP supplementation. Our data, in conjunction with others suggests that, in non-pregnant, non-lactating ruminants, protein supplements usually do not effect serum glucose levels. Serum growth hormone has been shown to decrease (Krysl et al., 1987; Caton et al., 1988b; Cheema et al., 1991b) or not change (Caton et al., 1994; Sainz et al., 1994) with protein supplementation in ruminants. Sletmoen-Olson et al. (1999b) has demonstrated that lactating cows have reduced plasma growth hormone concentrations in response to increasing UIP supplementation. Reasons for the lack

of response in serum growth hormone in the present study are unclear, but may be related to basal forage CP levels. In studies where growth hormone has decreased with protein supplementation, it appears that growth hormone may be responding to plane of nutrition, resulting in lower growth hormone concentrations in ruminants on higher planes of nutrition. Alternatively, other unde®ned mechanisms related to source of supplemental protein may be present. As expected, serum urea nitrogen increased (P < 10) in protein supplemented vs. control ewes (Table 4) which agree with previous reports by Caton et al. (1988b, 1994), Cheema et al. (1991b), Sainz et al. (1994) and Sletmoen-Olson et al. (1999b). Serum urea nitrogen also was increased (P < 10) in ewes fed medium and high UIP compared with those fed low UIP and in those fed high UIP compared with medium UIP, which agrees with the observations of Caton et al. (1994) and Sletmoen-Olson et al. (1999b). These data demonstrate that increasing supplemental UIP increases serum urea nitrogen concentrations suggesting that at least a portion of the UIP is not only absorbed but also catabolized. Analysis of serum insulin resulted in a sampling period  treatment interaction (P < 0.10) so data are presented by period (Table 5). Insulin was not affected by treatment, except in period four, in which serum insulin was increased (P < 0.10) in ewes fed high compared with those fed medium UIP. Caton et al. (1988b, 1994), Krysl et al. (1987), and Cheema et al. (1991b) showed increases in serum insulin due to protein supplementation of ruminants and others (Caton et al., 1994; Sletmoen-Olson et al. (1999b)) demonstrated an increase in insulin in steers fed increasing levels of UIP. These data are inconsistent

230

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Table 5 Influence of increasing levels of supplemental UIP on serum insulin concentrations (mu/ml) in ewes fed low quality forage Period

1 2 3 4b 5 6 a b

UIP treatment Control

Low

Medium

High

SE

Pa

3.75 5.41 10.77 7.46 7.95 5.52

5.86 4.91 5.80 9.45 6.69 6.45

8.50 5.92 8.43 5.33 5.08 7.24

6.00 4.86 10.45 14.55 6.83 4.65

1.38 1.35 2.91 2.19 2.59 2.88

0.14 0.91 0.55 0.05 0.89 0.81

P equals observed significance for main effect of treatment. Medium vs. high (P < 0.10).

with our current ®ndings, except for one sampling period. 3.2. Experiment 2 In trial 1, water intake was increased (P < 0.10) in protein supplemented wethers compared with controls (Table 6). However, forage intake, total feed intake, and apparent digestibility of DM, NDF, and ADF were not in¯uenced (P > 0.10) by protein supplementation. In contrast, digestibility of OM increased (P < 0.10) in protein supplemented wethers compared with control wethers (Table 6). In addition, total N intake, urinary N, N digestion, N absorption, and N retention all were increased (P < 0.10) in protein supplemented wethers compared with control wethers (Table 7). In similar studies evaluating CP levels, Cheema et al. (1991a) also observed increased N intake, urinary N, N absorp-

tion, and N retention, and Caton et al. (1988a) showed increased N intake, urinary N, and N retention in response to protein supplementation. In the present studies, urinary N, N digestion, and N absorption also were increased (P < 0.10) in medium and high UIP treatments compared with the low UIP treatment and in high compared with medium treatments indicating that the additional UIP protein was being metabolized. Fecal N and BW were not in¯uenced (P > 0.10) by protein supplementation in wethers fed grass hay (Table 7). In trial 2, water intake was unaltered in protein supplemented wethers compared with controls (Table 8). When expressed as g/d or g/kg BW, forage intake was not in¯uenced (P > 0.10) by treatment; however, total intake was increased with protein supplementation. Digestibility of DM and OM also were increased (P < 0.10) in protein supplemented wethers compared

Table 6 Influence of increasing levels of supplemental UIP on intake and digestibility in wethers fed grass hay (Trial 1) Item

UIP treatment

Total water Intake, g/db Total feed Intake, g/d Forage intake, g/d Total feed Intake, g/kg BW Forage intake, g/kg BW Digestibility DM OMb NDF ADF a b

Control

Low

Medium

High

SE

Pa

1733 1346 1346 20.9 20.9

2146 1494 1375 23.2 21.3

2100 1493 1373 23.3 21.4

2133 1511 1392 23.5 21.6

95.5 63.6 63.6 1.05 1.04

0.06 0.31 0.96 0.34 0.96

48.8 52.6 56.3 50.7

49.3 53.5 55.0 47.9

50.0 54.2 56.0 48.7

52.1 55.9 58.0 51.2

0.9 0.75 1.05 1.45

0.15 0.09 0.32 0.38

P equals observed significance for main effect of treatment. Control vs. low, medium, and high (P < 0.10).

K.C. Swanson et al. / Small Ruminant Research 35 (2000) 225±233

231

Table 7 Influence of increasing levels of supplemental UIP on nitrogen retention in wethers fed grass hay (Trial 1) Item

BW, kg N Intake, g/db Fecal N, g/d Urinary N, g/db,c,d N Digestion, %b,c,d Apparent N Absorption, g/db,c,d N Retention, g/db

UIP treatment Control

Low

Medium

High

SE

Pa

64.4 16.2 8.9 5.2 45.0 7.2 2.0

64.4 21.3 10 7.6 53.2 11.4 3.8

64.2 24.8 10.4 10.0 58.1 14.4 4.4

64.5 28.6 10.6 12.6 62.9 18.0 5.3

0.28 0.75 0.46 0.33 0.95 0.38 0.54

0.89 0.01 0.14 0.01 0.01 0.01 0.02

a

P equals observed significance for main effect of treatment. Control vs. low, medium, and high (P < 0.10). c Low vs. medium and high (P < 0.10). d Medium vs. high (P < 0.10). b

with controls and OM digestibility was increased (P < 0.10) in medium and high UIP treatments compared with the low UIP group. However, NDF and ADF digestibility were not in¯uenced (P > 0.10) by treatment. Body weight, N intake, fecal N, urinary N, N digestion, N absorption, and N retention all were increased (P < 0.10) in protein supplemented wethers compared with control wethers (Table 9). Body weight, N intake, urinary N, N digestion, N absorption, and N retention also were increased (P < 0.10) in medium and high compared with the low UIP group, and were increased (P < 0.10) in the high UIP compared with medium UIP treatment group. If anything, the increasing levels of UIP in¯uenced nitrogen meta-

bolism to a greater extent in trial 2 than in trial 1, perhaps because of the decreased forage quality in trial 2. This suggestion is supported by the negative N balance of control wethers in trial 2 compared with the positive N balance of controls in trial 1. 4. Conclusions In summary, results indicate that protein supplementing sheep fed a 6.5±7.0% CP forage with increasing levels of UIP increases DM, OM, and CP digestibility and serum urea N concentrations, but does not affect forage intake, ADF and NDF digest-

Table 8 Influence of increasing levels of supplemental UIP on intake and digestibility in wethers fed 40 : 60 grass : hay straw blend (Trial 2) Item

Total water Intake, g/d Total feed Intake, g/db Forage intake, g/d Total feed Intake, g/kg BWb Forage intake, g/kg BW Digestibility DMb OMb,c NDF ADF a

UIP treatment Control

Low

Medium

High

SE

Pa

2013 851 851 14.2 14.2

2210 1037 844 17.3 14.0

2566 1000 807 16.5 13.4

2513 1055 862 16.9 13.8

173.6 42.3 42.3 0.65 0.65

0.18 0.05 0.81 0.05 0.83

46.4 50.5 53.6 46

51.4 55.7 54.0 44.0

53.4 57.5 55.2 45.9

52.2 57.4 55.0 43.8

0.67 0.48 1.04 0.90

0.01 0.01 0.66 0.26

P equals observed significance for main effect of treatment. Control vs. low, medium, and high (P < 0.10). c Low vs. medium and high (P < 0.10). b

232

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Table 9 Influence of increasing levels of supplemental UIP on nitrogen retention in wethers fed 40 : 60 grass hay : straw blend (Trial 2) Item

BW, kgb,c,d N Intake, g/db,c,d Fecal N, g/db,d Urinary N, g/db,c,d N Digestion, %b,c Apparent N Absorption, g/db,c,d N Retention, g/db,c,d

UIP treatment Control

Low

Medium

High

SE

Pa

60.0 8.6 5.3 4.8 38.5 3.4 ÿ1.5

60.2 16.5 6.4 8.9 61.0 10.1 1.1

60.6 21.4 6.2 12.8 70.8 15.2 2.4

62.7 28.1 7.1 17.3 74.8 21.0 3.7

0.50 0.45 0.21 0.30 1.48 0.26 0.33

0.03 0.01 0.01 0.01 0.01 0.01 0.01

a

P equals observed significance for main effect of treatment. Control vs. low, medium, and high (P < 0.10). c Low vs. medium and high (P < 0.10). d Medium vs. high (P < 0.10). b

ibilities, or serum growth hormone and glucose concentrations. Nitrogen digestibility, absorption, and retention also are increased with protein supplementation. Increasing levels of UIP also appear to increase N digestibility, absorption, and retention, especially when lower quality forages are fed. In conclusion, it appears that providing additional UIP, while holding DIP constant can alter digestion and nitrogen metabolism. Increases observed in nitrogen retention resulting from providing additional protein can be explained, at least in part, by increasing UIP levels. Growth hormone and insulin concentrations in serum appear unresponsive when sheep consuming 6.5±7.0% CP cool season forages are supplemented with DIP or increasing UIP level.

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