Complete Rations for Dairy Cattle. IV. Comparison of Supplemental Nitrogen Sources by Metabolizable Protein Concept1

C o m p l e t e Rations f o r D a i r y Cattle. I V . C o m p a r i s o n of S u p p l e m e n t a l N i t r o g e n Sources by M e t a b o l i z a b l e Protein C o n c e p t 1 P. F. RANDEL 2, H. H. VAN HORN, C. J. WI LCOX, H. ROMAN-PONCE, S. P. MARSHALL, and K. C. BACHMAN Dairy Science Department University of Florida Gainesville 32611

ABSTRACT

cattle rations (16). However, this cannot be said of practical rations for lactating dairy cows. Limits to rumen protein synthesis from nonprotein nitrogen (NPN) in combination with higher total protein requirements constitute a drawback to urea utilization with milking cows (6, 17). New concepts of metabolizable protein (MP) and urea fermentation potential (UFP) might be helpful to define amounts of urea that can be fed usefully to lactating cows (3). In this study two complete rations were formulated with high UFP values. Urea was included either as raw urea (U) or as urea in an extruded corn-urea product (E). Each of these basal rations was compared with two others containing 50% as much or no added NPN at the same total protein and with three additional rations with the same NPN contents but supplemented with soybean meal (SBM) to give higher total protein.

Four types of 3 x 3 Latin squares differing in nonprotein nitrogen source and protein percent were used with 4-wk periods and 36 Holstein cows grouped in trios. Basal rations (9.0 to 10.6% crude protein) high in citrus pulp and pelleted cane bagasse, were equalized in estimated metabolizable protein, ignoring urea fermentation potential, by adding raw urea, extruded corn-urea, or soybean meal at three percents. Corresponding rations of higher protein (12.1 to 13.4%)contained a soybean meal increment partially replacing corn. Mean daily feed intake was lower for basal than soy-increment rations (18.0 versus 19.3 kg) as were milk and solids-corrected milk yields (16.0 to 18.3 kg and 12.8 to 14.8 kg). All-soy controls were superior in milk production by 1.35 and .92 kg. Liveweight changes and milk composition were little affected by treatments. Milk fat and protein percents averaged 2.71 and 2.92 overall. Molar porportions of rumen acetate in Period 3 were relatively low, but not acetate/propionate ratios. Increased rumen ammonia concentrations resulted from higher protein, higher nonprotein nitrogen, and urea exceeded extruded corn-urea exceeded soybean meal.

EXPERIMENTAL PROCEDURE

INTRODUCTION Urea may be used to provide all or most of supplemental nitrogen required in many beef

Received September 11, 1974. ~Florida Agricultural Experiment Station Journal Series No. 5556. This research was supported in part by U. S. Sugar Corporation, Clewiston, FL. Formerly visiting professor; present address: Animal Industries Department, University of Puerto Rico, Mayaquez Campus, Lajas 00667.

Thirty-six Holsteins were divided into 12 groups to make the three cows of each as similar as possible. Three groups were assigned at random to each of four 3 x 3 Latin Square sequences (basal rations U, US, S; or E, ES, S; and soy-increment rations U, US, S; or E, ES, S; see Table 1). The 10 rations constitute a total of 12 treatments when ration S in U squares represents a different treatment than ration S in E squares, since any possible carry-over effects w o u l d differ in the two cases. Cows were housed in individual outdoor pens (5.5 x 46.3 m). All received the same ration (soy-increment U) during a.A-wk standardization period. In three 4-wk comparison periods the three rations in the particular set assigned to each cow were tested. Data for statistical analyses were from the last 2 wk of each comparison period. Feed allowances were reviewed daily to minimize feed refusals while equalizing intakes

1109

1110

RANDEL

ET AL.

TABLE 1. As-fed formulas a and composition of complete rations. Soy-increment rations

Basal rations U

US

E

ES

S

U

US

E

ES

S

Citrus pulp b Bagasse pelletsC Ground corn Soybean meal Urea (45% N) Golden-pro d Cane molasses Biofos e Salt

42.75 24.85 25.55 . • • 1.90

42.40 24.65 20.30 6.80 .95

42.80 24.90 19.50 5.95 1.90

42.80 24.85 14.45 12.00 .95

";.5'5 .90 .45

42.45 24.65 16.10 6.80 . . . " 5".10 3.55 .90 .45

42.75 24.85 15.35 12.10

"3".60 .90 .45

42.80 24.90 17.10 . . . . . . 10.25 3.60 .90 .45

42.85 24.90 11.10 5.95

42.80 24.90 10.25 12.00

42.76 24.85 9.35 18.10

"3..60 .90 .45

'3'.60 .90 .45

"3..60 .90 .45

10.25 3.60 .90 .45

5.10 3.60 .90 .45

. . . 3.60 .90 .45

Estimated Estimated Estimated Estimated Estimated Analyzed Estimated

--.03 5.26 5.26 8.6 10.4 9.7 63.9

--.33 5.21 4.94 7.9 9.9 10.6 63.6

--.04 5.23 5.23 8.6 10.4 9.8 63.6

-.33 5.20 4.92 7.9 9.9 10.5 63.5

-.61 5.25 5.25 7.4 9.6 9.0 64.5

-1.26 6.00 4.91 10.6 12.5 13.3 63.5

-1.55 6.00 5.18 10.0 12.2 12.8 63.8

-1.27 5.97 4.88 10.5 12.5 12.1 63.1

-1.56 5.98 5.16 9.9 12.1 12.5 63.5

-1.85 6.00 6.00 9.4 1t.7 13.4 64.0

(%)

UFP f MP'g MP la DP CP CP TDN

a4,400 IU of synthetic vitamin A per kg added to all rations. bHalf loose form with only fines pelleted and half completely pelleted. CManufactured by U. S. Sugar Corp. and containing approximately 90% cane bagasse and 10% cane molasses. dTrademark of FFF Corp. -- an extruded corn-urea product containing 18.5% urea. eTrademark of International Minerals Corp. fUrea fermentation potential given is that remaining after dietary percent urea is subtracted from urea fermentation potential of nonurea ingredients. gAssuming all urea contibutes to metabolizable protein of ration. hAssuming only urea up to limit of UFP contibutes to metabolizable protein of ration.

of cows in each trio. T h e l e a s t - c o n s u m i n g cow of each trio usually was p e r m i t t e d ad l i b i t u m intake, a n d o t h e r s were restricted to t h e a m o u n t she was offered. Only e x c e p t i o n s were in o n e g r o u p (assigned to basal U squares) w i t h a c h r o n i c low c o n s u m e r , a n d occasionally in other groups when marked, but temporary, f l u c t u a t i o n s occurred. Approximately twothirds of t h e 24 h allowance was fed at 1200 h a n d r e m a i n d e r at 0 7 0 0 t h e f o l l o w i n g day. F e e d refusals were weighed b e f o r e n o o n feeding. I n g r e d i e n t U F P values were c o m p u t e d b y f o r m u l a of B u r r o u g h s et al. (3) w i t h t o t a l digestible n u t r i e n t figures of N a t i o n a l R e s e a r c h C o u n c i l (15) a n d crude p r o t e i n (CP) c o n t e n t s a n a l y z e d in r e c e n t years at this station. F o r MP estimation, percents protein nondegraded and d e g r a d e d - r e s y n t h e s i z e d in r u m e n a n d digestibilities o f e a c h were t a k e n f r o m T a b l e 4 o f B u r r o u g h s et al. (3). S u g a r c a n e bagasse a n d citrus pulp, n o t listed t h e r e i n , were a s s u m e d to Journal of Dairy Science Vol. 58, No. 8

c o n t a i n 1.2 a n d 4.0% MP (dry m a t t e r basis) e s t i m a t e d f r o m values for o t h e r feeds of similar c o m p o s i t i o n . Based on UFP, m a x i m u m u r e a w h i c h w o u l d c o n t r i b u t e t o MP of r a t i o n s c o n t a i n i n g citrus pulp, bagasse, molasses, a n d m i n e r a l s u p p l e m e n t s in p r o p o r t i o n s selected is a p p r o x i m a t e l y 1.9%. T h e r e f o r e , t w o basal rat i o n s were f o r m u l a t e d w i t h this u r e a a n d n o o t h e r p r o t e i n or NPN s u p p l e m e n t a t i o n ( T a b l e 1). One c o n t a i n e d U a n d the o t h e r equal u r e a in E. T w o o t h e r basal r a t i o n s (US a n d ES) c o n t a i n e d o n e - h a l f as m u c h u r e a as r a t i o n s U and E and were m a d e e q u a l t o t h e s e in MP' ( e s t i m a t e d m e t a b o l i z a b l e p r o t e i n disregarding U F P limit) b y a d d i t i o n of SBM. Basal r a t i o n S c o n t a i n e d n o u r e a b u t was e q u a l i z e d in MP' t o the o t h e r basals b y SBM a d d i t i o n . All urea, thus, was replaced in f o r m u l a t i n g a c o n t r o l basal ration. A c o r r e s p o n d i n g series of h i g h e r p r o t e i n ( s o y - i n c r e m e n t ) r a t i o n s was f o r m u l a t e d b y a d d i n g 5.2 t o 6.0% SBM t o each basal in

SUPPLEMENTAL SOURCES OF NITROGEN substitution for a like a m o u n t of shelled corn. Estimated TDN contents of all rations were nearly alike (Table 1). Milk samples were collected during final days of each period and analyzed as described previously (22). A sample of rumen c o n t e n t s of each cow was obtained during the last 2 w k of each comparison period 2 to 3 h after n o o n feeding. V a c u u m pressure was applied to a stomach tube. Samples were stored briefly in jars with tight caps, then filtered through several layers of cheese cloth; the resulting fluid was diluted 1:3 with distilled water, and NH 3 c o n t e n t was d e t e r m i n e d i m m e d i a t e l y with an Orion A m m o n i a Electrode. During Period 3 only, part of the u n d i l u t e d r u m e n fluid was frozen with added HgC12 for subsequent volatile fatty acid ( V F A ) analyses by a m o d i f i c a t i o n of the m e t h o d of Baumgardt (2), in which 10% diethylene glycol adipate + 2% phosphoric acid on 60 to 80 mesh C h r o m o s o r b W was used. All data were subjected to general least squares analysis (11). Main t r e a t m e n t effects were t w o protein percents (basal and soy-increm e n t ) and six protein sources (controls in U and E squares considered different). Conventional Latin square analysis procedures were

1111

used for comparing all treatments within squares. The comparison of all basal vs. all soy-increment diets was by averaging performance across the three periods (three basal or three soy-increment diets) and comparing perf o r m a n c e of 18 basal cows vs. 18 soy-increment cows. Thus, m o s t sensitive tests were within squares, e.g., basal U vs. basal US vs. basal S; tests of significance b e t w e e n squares were less exact. E x c e p t for liveweight, data of standardization periods were n o t used as a covariate because of atypical milk p r o d u c t i o n in cows destined for basal E squares. RESULTS

Cows generally accepted rations well, though t h e y showed signs of craving m o r e fibrous material , ingesting sandy soil and tree bark. Overall c o n s u m p t i o n increased f r o m Period 1 to 2 b u t declined again in latter part of Period 3 during h o t weather. A t t e m p t s to equalize feed intake within groups were least successful in basal U squares where t r e a t m e n t means differed by as m u c h as 1.2 kg per cow daily (Table 2). Half as great a range of mean differences occurred in basal E squares and only a quarter as great in either type of soy-increment squares.

TABLE 2. Mean daily feed intakes, tactational performance and liveweights. Milk composition Ration

Feed intake

Milk yield (kg) -

SCM yield

Fat

-

SNF

Protein

Liveweight (kg)

(%)

Combined basals Combined soy-increment

18.0 a 19.3 b

16.0 a 18.3 b

12.8 a 14.8 b

2.70

2.72

8.35 8.44

2.90 2.94

568 577

Basals (see Table 1) U US S (in U squares)

17.4 17.9 18.6

15.3 a 16.8 c 15.9 b

12.1 13.3 13.2

2.74 2.58 2.81

8.30 8.31 8.39

2.90 2.94 2.89

565 565 563

E

17.7

ES S (in E squares)

18.1 18.3

14.8 a 16.4 b 16.9 b

12.1 13.0 13.1

2.88 2.65 2.54

8.37 8.35 8.38

2.93 2.84 2.92

568 571 575

18.8 18.5 18.8

17.2 17.4 18.3

14.9 d 13.6 e 14.2 e

3.06 a, 2.42 b 2.41 b

8.60 8.50 8.53

2.98 3.00 3.01

582 583 585

19.7 19.9 20.0

18.7 18.8 19.5

15.1 15.4 15.6

2.80 2.89 2.74

8.38 a 8.37 a 8.24 b

2.90 d 2.93 d 2.81 e

566 568 577

Soy-increment (see Table 1) U US S (in U squares) E ES S (in E squares)

a,b,CMeans in the same ration category not followed by the same superscripts differ (P<.O5). d,eMeans in the same ration category not followed by the same superscripts differ (P<.IO). Journal of Dairy Science Vol. 58, No. 8

1112

RANDEL

Basals were consumed in less (P<.05) amounts (1.3 kg) than soy-increment rations though protein sources were not a significant source of variation. Milk yield difference (P<.05) in favor of soy-increment rations over basals was 2.3 kg per cow daily (Table 2). Within basal U squares, US was superior (P<.05) whereas S exceeded U (P<.05); within basal E squares E was inferior (P<.05). In SCM yield relative inferiority of basal rations was 2.0 kg per day (P<.05). Within soy-increment U squares U was superior (P<.10) in SCM production which was attributtable to this ration maintaining a higher milk fat percent (P<.05). Within soy-increment E squares S was lowest in SNF percent (P<.05) and milk protein percent (P<.IO). Treatments did not affect liveweight significantly after adjustment for standardization period by covariance. Since differences between corresponding U and E rations were not large, additional analyses combined the two. Results showed no significant effects of protein source on feed intake, liveweight, milk protein, or SNF contents. Within basal rations, combined U and E were lower (P<.05) than either combined US and ES or combined S in both milk yield (15.05 vs. 16.61 and 16.38 kg) and SCM yield (12.13 vs. 13.17 and 13.12 kg) while within soy-increment rations combined S exceeded (P<.IO) both combined U and E and combined US and ES in milk yield (17.96 and 18.09 vs. 18.88 kg). Combined U and E were highest (P<.05) among soy-increment rations in milk fat percent (2.93 vs. 2.66 and 2.57). Differences (P<.01) among treatments in tureen NH 3 concentration (Table 3) were attributable to all of the following: protein percent, linear effect of NPN, U vs. E, and U + E vs. control. Treatments did not affect significantly rumen total V F A or molar proportions of acetate and butyrate (Table 4). Interactions of protein by protein source in molar proportion of rurnen propionate (P<.10) and C~/C3 ratio (P<.05) were attributable to lower relative propionate and higher C~/C 3 with U than with E among basals and the reverse among soy-increment rations. DISCUSSION

Unlike previous results with rations containing 25% bagasse pellets together with 35% or J o u r n a l of D a i r y S c i e n c e Vol. 58, No. 8

ET AL.

e~

+

z" t~ Z,

~3 E

E '7'

~.~.

"2.

8 e~

.g e~

q v

<

E

~4

<

+..=

SUPPLEMENTAL SOURCES OF NITROGEN

1113

TABLE 4. Mean rumen fluid VFA contents and proportions determined in period 3.

Ration

Total VFA

C2

C3

All

(#M/ml) 104

61

114 102 106 115 97 113 106 93 102 101 104

C4

C2/C3

17

22

3.6

62 61 59 65 57 57

16 16 16 18 19 21

22 23 25 17 24 22

3.9 3.8 3.7 3.6 3.0 2.7

58 62 59 62 58 64

17 21 18 17 18 14

25 17 23 21 24 22

3.4 3.0 3.3 3.6 3.2 4.6

(%)

Basals (see Table 1) U

US S (in U squares) E ES S (in E squares) Soy-increment (see Table 1) U US S (in U squares) E ES S (in E squares)

99

m o r e citrus pulp (14, 22), low milk fat percents were evident t h r o u g h o u t the experiment. Rum e n V F A data f r o m Period 3 did n o t provide an explanation. Mean molar acetate percentages of 56 to 61 (similar to those of present study) were associated with depressed milk fat (12), and 65 and over with normal fat (4, 5). However, mean values have been as low as 58 w i t h o u t milk fat depression (13). Present C 2 / C 3 ratios should have been c o m p a t i b l e with milk of normal fat c o n t e n t (4, 5, 13). Linear regression analyses failed to reveal any significant relationship between r u m e n V F A and milk fat percent. Control rations gave r u m e n NH 3 concentrations comparable to those in absence of NPN feeding in cows (5, 12) and comparable (7) or lower (8, 9) than r e p o r t e d for steers. A d d i t i o n of NPN at lower amounts (US and ES rations) evoked m o r e response in r u m e n NH 3 than similar NPN percentages fed elsewhere (5, 9, 12) while higher NPN (U and E rations) resulted in rumen NH 3 increases greater than in some reports with similar NPN (5, 12) b u t not different from others (7, 8). Lowering of r u m e n NH 3 c o n c e n t r a t i o n due to E as c o m p a r e d with U was expected, b u t mean difference did n o t exceed 5 or 6 mg % N H a - N and c a n n o t be d e e m e d of practical importance since performance was not affected. Observations of Bartley et al. (1) on a ground grain-urea p r o d u c t

processed similarly (in an e x t r u d e r w i t h o u t added moisture) were that this p r o c e d u r e was n o t effective in slowing urea release as judged by b l o o d NH 3 and signs of t o x i c i t y w h e n placed in evacuated r u m e n ; also, distinctive crystalline areas in p r o d u c t e x a m i n e d with a scanning electron m i c r o s c o p e suggested intact urea. Rations US and ES, which resulted in m e a n daily urea intakes of 170 to 189 g daily, were not consistently inferior to controls in animal performance, which agrees with suggestions that up to 200 g urea can be fed effectively in concentrates c o n s u m e d twice daily (13, 20). Superiority o f controls over rations U and E (331 to 374 g urea consumed) in milk production was consistent enough to be significant, hut was n o t great. Holter et al. (12) reported no effects on ration intake or utilization by substituting as m u c h as 300 g urea daily for plant protein in high-quality concentrates, b u t their highest urea ration c o n t a i n e d a b o u t 13.3% p r e f o r m e d protein in DM, which m a y (10) or m a y not (19) be below o p t i m u m . Over 300 g urea c o n s u m p t i o n daily in bagasse c o m p l e t e rations was c o m p a t i b l e with good milk production and feed efficiency w h e n e x t r a protein synthesized f r o m urea clearly was needed (18). On the other hand, Van Horn and J a c o b s o n (21) and Van H o r n et al. (22) f o u n d limited usefulness of urea as a s u p p l e m e n t to basal Journal of Dairy Science Vol. 58, No. 8

1114

RANDEL ET AL.

rations, which consisted mainly of corn grain and corn silage (11.4% CP) in the first case and of corn grain, citrus pulp, and pelleted bagasse (9.8 or 10.2% CP) in the second. The latter basals already contained 1.2% (21) or .4% (22) urea. Milk production increased in both trials when these same basals were supplemented with SBM. Supplemental nitrogen sources were compared in our study holding feed intakes as nearly constant as possible, but this was not true of our comparison between total nitrogen; soy-increment rations were consumed in significantly greater amounts. Basal rations did not result in maximum intake and production and also were inferior to soy-increment rations in gross feed efficiency (feed:milk, 1.12 vs. 1.05%; feed:SCM, 1.41 vs. 1.30). With total protein evidently limiting, one would expect milk yield to be correlated closely with protein intake. Five criteria of the latter were calculated: CP (analyzed crude protein), DP (estimated digestible protein assuming urea fully digested), DP' (estimated digestible proteins assuming no contribution from either urea or bagasse), MP' (estimated metabolizable protein assuming urea fully metabolizable), and MP (estimated metabolizable protein assuming only urea up to UFP limit metabolizable). Linear correlations, calculated from ration means of milk, SCM, and milk protein yields on each protein intake variable and on feed intake (representing available energy limitation) are in Table 5. Also included are data from two other experiments mentioned above (21, 22) in which NPN was added to basal rations suboptimal in protein content (Table 5). Among protein-intake criteria, DP' and MP, which discount urea completely in the first case and beyond UFP limit in the second, were most highly correlated with lactational responses in two previous studies, but MP', which places no limit on urea contribution, was more so in present data. This again points to a difference in urea effectiveness between present and two previous trials. Linear correlations show that feed intake also was an important variable in all three trials (Table 5). Therefore, standard partial regressions of each lactational variable (Y) on each protein-intake variable (X1) independent of feed intake (X2) and on feed intake independent of each protein-intake variable were calcuJournal of Dairy Science Vol. 58, No. 8

lated (Table 5). In Van Horn and Jacobson (21) data, DP' and MP were only X1 variables to give positive standard partial regressions; there were also most effective protein-intake criteria by which to adjust feed intake to reduce importance of the latter. Standard partial regressions emerging from Van Horn et al. (22) data show no consistent pattern. Among those calculated from our data, MP' followed by CP were most effective X l variables whereas MP showed little relationship with lactational responses. The latter result might be explainable in part by a too-small range of mean MP intakes. Furthermore, UFP values employed, which predict lower MP contents for US and ES than for other basals and only controls of all soy-increment rations appreciably higher than basals (Table 1), need reevaluation. The protein-intake criterion DP (which assumed urea N was fully digested) was ineffectual in all three sets of data. Further research is needed to develop a meaningful criterion for milking cows of protein status in rations containing supplemental NPN. ACKNOWLEDGMENT

The extruded corn-urea product in this experiment was generously supplied by Kuder Citrus Feed Company, Lake Alfred, FL. REFERENCES

1. Bartley, E. E., C. W. Deyoe, K. C. Behnke, G. W. Griffel, and R. M. Meyer. 1973. Toxicity of Starea or cooked grain-urea products. J. Anita. Sci. 37:336. (Abstr.) 2. Baumgardt, B. R. 1 9 6 4 . Practical observations o n the quantitative analysis of free volatile fatty acids (VFA) in aqueous solutions by gas-liquid chromatography. Dept. Bull. 1, Dairy Sci. Dep., Univ. Wisc., Madison. 3. Burroughs, W., A. H. Trenkle, and R. L. Vetter. 1972. Proposed new system of evaluating protein nutrition of feedlot cattle (metabolizable protein and urea fermentation potential [UFP] of feeds). Iowa State Univ. Coop. Ext. Serv. and Agr. and Home Econ. Exp. Sta. A. S. Leaflet R161. 4. Colovos, N. F., J. B. Holter, H. A. Davis, and W. E. Urban, Jr. 1967. Urea for lactating dairy cattle. I. Effect of concentrate fiber and urea levels on nutritive value of the ration. J. Dairy Sci. 50:518. 5. Colovos, N. F., J. B. Holter, H. A. Davis, and W. E. Urban, Jr. 1967. Urea for lactating dairy carrie. II. Effect of various levels of concentrate urea on nutritive value of the ration. J. Dairy Sci. 50:523. 6. Conrad, H. R., and J. W. Hibbs. 1968. Nitrogen utilization by the ruminant. Appreciation of its nutritive value. J. Dairy Sci. 51:276.

T A B L E 5. Linear c o r r e l a t i o n s (LC) a n d s t a n d a r d partial regressions (SPR) c a l c u l a t e d f r o m r a t i o n m e a n s o f l a c t a t i o n a l p a r a m e t e r s (Y) o n five criteria o f p r o t e i n intake (X 1) and feed i n t a k e (X2). Present e x p e r i m e n t LC V

X1

rX1Y

rX2Y

b'X1Y.X 2

Milk yield

CP DP DP' MP' MP

.86 .73 .76 .90 .70

.91 . . . . . . . .

.39 10 26 44 16

CP DP DP' MP' MP

.89 .83 .63 .94 .61

.95 .37 . . . . 25 ... -.03 . . . . 43 . . . -.07

CP DP DP' MP' MP

.89 .77 .74 .91 .65

.84 . . . . . . . .

SCM yield

e~

t~

< o oo

Z O oo

Milk protein yield

. . . .

. . . .

. . . .

. . . .

.60 31 30 88 15

LC b'X2Y.X1

Van Horn a n d J a c o b s o n (21)

Van Horn et al. (22)

SPR

SPR

LC

t-

SPR

rX1Y

rX2Y

b'XIY-X 2

b'X2Y.X 1

rXIY

rX2Y

.60 ,83 .73 .49 .79

.32 .13 .67 .53 .83

.88 . . . . . . . .

. . . .

. . . .

.10 12 10 21 32

.86 .89 .82 .80 .63

.46 .45 .90 .65 .88

.84 --.63 . . . . 61 .68 . . . --.60 . . . . 61

1.35 1.33 .27 .81 .30

.66 .45 .97 .55 1.00

.50 .31 .74 .44 .64

.68 . . . . . . . .

. . . .

. . . .

.36 31 52 20 25

.59 .68 .31 .60 .47

.52 .51 .91 .70 .88

.88 --.55 ... --.54 . . . . 57 . . . -.54 . . . . 49

1.33 1.31 .40 1.37 .45

.37 .61 .64 .03 .74

.30 .09 .72 .50 .85

.93 . . . . . . . .

. . . .

.07 0g 12 15 28

.91 .93 .84 .87 .70

. . . . .

. . . .

. . . . .

. . . . .

. . . . .

. . . . .

b'XIY-X 2

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

b'x2Y.X 1

. . . . .

. . . . .

t~ Z t© C

©

z

g Z

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7. Davis, G. V., and O. T. Stallcup. 1964. Influence of dietary nitrogen on nitrogen metabolism in the rumen. J. Dairy Sci. 47:1237. 8. Davis, G. V., and O. T. Stallcup. 1967. Effect of soybean meal, raw soybeans, corn gluten feed, and urea on the concentration of r u m e n fluid c o m p o n e n t s at intervals after feeding. J. Dairy Sci. 50:1638. 9. Freitag, R. R., W. H. Smith, and W. M. Beeson. 1968. Factors related to the utilization of urea vs. protein-nitrogen supplemented diets by the ruminant. J. Anim. Sci. 27:478. 10. Gardner, R. W., and R. L. Park. 1973. Protein requirements of cows fed high concentrate rations. J. Dairy Sci. 56:390. 11. Harvey, W. R. 1972. Instructions for use of LSMLGP. Ohio State Univ. 12. Holter, J. B., N. F. Colovos, H. A. Davis, and W. E. Urban, Jr. 1968. Urea for lactating dairy cattle. l i e Nutritive values of rations of corn silage plus concentrate containing various levels of urea. J. Dairy Sci. 51 : 1243. 13. Huber, J. T., R. A. Sandy, C. E. Polan, H. T. Bryant, and R. E. Blaser. 1967. Varying levels of urea for dairy cows fed corn silage as the only forage. J. Dairy Sci. 50:1241. 14. Marshall, S. P., and H. H. Van Horn. 1975. Complete rations for dairy cattle. II. Sugarcane bagasse pellets as the roughage in blended rations for lactating cows. J. Dairy Sci. 58:896.

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ET AL. 15. National A c a d e m y of S c i e n c e s - National Research Council. C o m m i t t e e on Animal Nutrition. 1964. Joint United S t a t e s - Canadian tables o f feed composition, Publ. 1232, Washington, DC. 16. National A c a d e m y of S c i e n c e s - National Research Council. C o m m i t t e e on Animal Nutrition. 1970. Nutrient requirements of beef cattle, 4th revised ed., Washington, DC. 17. National A c a d e m y of S c i e n c e s - National Research Council. C o m m i t t e e on Animal Nutrition. 1971. Nutrient requirements of dairy cattle, 4th revised ed., Washington, DC. 18. Randel, P. F. 1970. Bagasse complete rations employing urea or urea plus fishmeal as supplemental nitrogen sources. J. Dairy Sci. 53:1722. 19. Thomas, J. W. 1971. Protein requirements of milking cows. J. Dairy Sci. 54:1629. 20. Van Horn, H. H., C. F. Foreman, and J. E. Rodriguez. 1967. Effect of high-urea supplementation on feed intake and milk production of dairy cows. J. Dairy Sci. 50:709. 21. Van Horn, H. H., and D. R. Jacobson. 1971. Response of lactating cows to added increments of dietary protein and nonprotein nitrogen. J. Dairy Sci. 54:379. 22. Van Horn, H. H., S. P. Marshall, C. J. Wilcox, P. F. Randel, and J. M. Wing. 1975. Complete rations for dairy cattle. IlL Evaluation of protein percent and quality and citrus pulp-corn substitutions. J. Dairy Sci. 58:1101.