Alfalfa Silage or Hay Versus Corn Silage as the Sole Forage for Lactating Dairy Cows

Alfalfa Silage or Hay Versus Corn Silage as the Sole Forage for Lactating Dairy Cows

Alfalfa Silage or Hay Versus Corn Silage as the Sole Forage for Lactating Dairy Cows G L E N A. B R O D E R I C K US Dairy Forage Research Center Univ...

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Alfalfa Silage or Hay Versus Corn Silage as the Sole Forage for Lactating Dairy Cows G L E N A. B R O D E R I C K US Dairy Forage Research Center University of Wisconsin 1925 Linden Drive West Madison 53706 ABSTRACT

In Trial 1, three rations were fed to 21 cows in a 3 × 3 Latin square: 60% alfalfa silage, 60% corn silage, and 79% corn silage (dry matter basis) with the balance from corn and soybean meal. Acid detergent fiber measures indicated alfalfa and corn silage were of excellent quality. Milk production was similar on 60% forage rations but lower on 79% corn silage. Milk fat was reduced on 60% corn silage. In Trial 2, four rations were fed to 16 cows in a 4 x 4 Latin square: 63% alfalfa silage, 60% alfalfa hay, 60% corn silage, and 76% corn silage. Alfalfa forages were higher in acid detergent fiber but corn silage was similar to Trial 1. Dry matter digestibility was highest on 60% corn silage, intermediate on 63% alfalfa silage and 76% corn silage, and lowest on 60% alfalfa hay. Milk production was similar on the diets containing 60 and 63% forage and lower on 76% corn silage. Milk protein concentration was reduced on the alfalfa diets. Highest protein secretion and feed conversion was supported by 60% corn silage. In both trials, potentially digestible neutral detergent fiber from alfalfa was more digestible than that from corn silage, and concentrations of urea in milk and blood were highly correlated. Results indicate high quality alfalfa silage is comparable to corn silage for milk production. INTRODUCTION

There is substantial reduction in digestibility of energy at the high rates of feed intake

required by lactating dairy cows. The classic work of Wagner and Loosli (33) showed that the degree of depression of digestible energy was greater for concentrates than forages. Tyrrell and Moe (30), from feeding studies with corn silage (CS) and corn-based concentrates, reported energy digestibility declined 4 percentage units per multiple of intake above maintenance; this relationship has been adopted for National Research Council (NRC) feeding standards (19). Work reported by Joanning et al. (15) indicated that about half of the depression in digestibility for CS was due to reduced starch digestibility. Mertens (personal communication) stated that the rate of ruminal starch digestion is sufficiently slow to limit its utilization at high intake. Moreover, other research (31) suggested that due to smaller heat increment losses, metabolizable energy from legumes yields a higher proportion net energy than grass forages of comparable quality. Corn silage represents a mixture of corn grain and grass forage and would be expected to display some of these same reductions in digestibility in lactating dairy cows. Although recently revised feeding standards [e.g. (9)] have adjusted for lower effective energy values for corn grain and CS, the current dairy NRC bulletin (19) does not distinguish between feeds when discounting digestibility at high intakes. The purpose of these studies was to compare the relative value of good quality alfalfa, principally alfalfa silage (AS), to CS as the sole forage in the diet of lactating dairy cows. These comparisons were made on two bases: equal dry matter from each forage and equal energy (as estimated from NRC tables). MATERIALS AND METHODS Trial 1

Received April 22, 1985. 1985 J Dairy Sci 68:3262-3271

Twenty-one Holstein cows, averaging 633 kg, lactation number 2.9, 68 d in milk, and 3262

ALFALFA VERSUS CORN FOR LACTATION 29:8 kg/d milk were randomly assigned to three treatments in a 3 x 3 Latin square. A m o u n t and source of forage in each diet was 60% of the dry matter (DM) from first-cutting alfalfa silage (60% AS), equal DM from welleared CS (60% CS), or equal net energy for lactation (NE 1) from CS (79% CS). Silages were from the 1982 crop year and were chopped to a nominal length of 1 cm. The balance of each ration was a concentrate mix based on corn grain and soybean meal. Diets were fed for periods o f 5 wk before switching (total 15 wk); the 1st wk of each period was transitional and production data were collected over the last 4 wk. Milk was measured at each twice daily milking. Milk was sampled at both milkings 2 d each week, and proportiona/ composites were prepared and analyzed for fat by infrared analysis 1 and protein by Kjeldahl nitrogen (N) x 6.38 (1). Milk was deproteinized by mixing with an equal volume of 25% vol/vol trichloroacetic acid (TCA). The high-speed (31,000 x g, 15 min, 2°C) TCA supernatants were stored at - 2 0 ° C until analyzed for lactose (26), urea (27), and N (1). True protein was estimated as TCA-insoluble N x 6.38. Cows were weighed on 3 consecutive d at the start of the trial and at the end of each period. Diets were fed ad libitum twice daily as total mixed rations (TMR). Cows had access to feed 24 h/d. A weekly composite of each TMR and forage was collected from daily samples of about .5 kg and stored frozen. Feed refusals were determined daily, and subsamples of refusals from each diet were composited and stored in the same manner. Forage content of as-fed rations was adjusted periodically based on DM estimated at 60°C (48 h). The actual proportion of dietary DM from each forage was c o m p u t e d from DM determined by toluene distillation (11) and at 105°C (1) for forages and concentrates, respectively. Aqueous extracts of each silage were analyzed for lactic acid (2). As-fed samples of diet ingredients were also analyzed for Kjeldahl crude protein (CP) and ash (1), neutral detergent fiber (NDF), acid detergent fiber (ADF), acid detergent lignin (ADL), and acid detergent insoluble N (ADIN) by t h e methods of Van Soest and coworkers

1Wisconsin Dairy Herd Improvement Cooperative, 5301 Tokay Blvd., Madison, WI 53711.


(13, 23). Composite samples of TMR were analyzed for potentially digestible NDF (DNDF) (10) and acid insoluble ash (AIA) (32). Samples of TMR and feed refusals were analyzed for CP (1) and DM (60°C, 48 h); CP intake and DM intake (DMI) are reported on this basis. Forage and ration compositions are in Tables 1 and 2, respectively. A single fecal grab sample was taken from each cow on d 33 or 34 of each period and dried (60°C, 72 h). Dried fecal samples were ground (1 mm) and analyzed for NDF (23) and AIA (32). Total tract digestibilities of DM, NDF, and DNDF were computed using marker ratio techniques (32, 35) with A I A as internal marker. At 4 h after feeding on the same day, a 3-ml blood sample was taken from each cow by venipuncture from the tail artery or vein. Blood was heparinized and stored at 2°C until analyzed the next day for urea (27) and glucose (14). On d 33 or 34 of each period, a rumen turnover study was conducted with 6 cows fitted with ruminal cannulae. Animals were pulse-dosed just before feeding with 200 ml of a solution of the chromium (Cr) chelate of ethylenediaminetetraacetic acid containing 20,000 pg/ml Cr (3). Samples of strained rumen fluid (SRF), taken from the ventral sac at 0 (just prior to dosing), 2, 4, 6, 8, and 10 h after dosing were prepared by straining through 2 layers of cheesecloth. Grab samples of 250 to 400 g of whole rumen contents (WRC) were taken from the ventral sac at 0, 2, 6, and 10 h. The SRF was preserved b y adding 1 ml 50% vol/vol sulfuric acid per 50 ml o f SRF (12) and stored at - 2 0 ° C until analyzed for Cr (8), ammonia (4), and volatile fatty acids (VFA). The V F A were determined with s-ethyln-butyrate as internal standard (W. C. Ellis, personal communication) using the SP-1200 column of Ottenstein and Bartley (22), which does not resolve 2-methyl butyrate and 3methyl butyrate. The WRC were dried (60°C, 72 h), ground (1 mm) and analyzed for NDF (23) and DM (105°C; 1). Liquid fractional turnover rates were slopes from linear regression of the natural logarithm of Cr concentration on time. Liquid volume was computed by dividing the Cr dose by the antilog of the regression intercept, and the DM and NDF fills from the liquid volume and mean DM and NDF in each set of four WRC samples. Journal of Dairy Science Vol. 68, No. 12, 1985


3264 TABLE 1. C o m p o ~ t i o n o f ~rages. *

Trial 1

Trial 2







Dry matter, % Crude protein, % DMB 2 Lactic acid, % DMB Neutral detergent fiber, % DMB Digestible neutral detergent fiber, % NDF 3 Acid detergent fiber, % DMB Acid detergent lignin, % DMB Acid detergent insoluble nitrogen, % N 4 Ash, % DMB NEI, Mcal/kg DMB s

44.1 18.3 4.0

40.2 8.2 4.4

46.1 20.1 3.7

86.5 18.0 ...

45.6 7.7 4.0






50.6 33.6 6.8

74.0 24.7 3.2

52.5 39.1 8.0

54.5 41.0 8.3

70.4 23.3 2.9

7.1 9.5 1.35

5.8 3.6 1.60

12.5 13.7 1.23

7.8 8.6 1.13

7.4 3.9 1.60

a AS = Alfalfa silage, CS = corn silage, and AH = alfalfa hay. 2 Dry matter basis. 3 As a percent of total neutral detergent fiber. *Ag a percent of total nitrogen. SNet energy for lactation (NE I) values from NRC feed composition tables (20) for forages with similar acid detergent fiber contents.

TABLE 2. Composition of diets. Dietary forage (Trial 2) 1

Dietary forage (Trim 1) ~ Component

60% AS

60% CS

79% CS

63% AS

60% AH

60% CS

76% CS

(% dry matter) Alfalfa silage Corn silage Alfalfa hay Corn grain Soybean meal Monosodium phosphate Limestone Dicalcium phosphate Trace mineral salt Vitamin premix 2 Chemical composition Crude protein Neutral detergent fiber Acid detergent fiber Acid detergent lignin Estimated NE1, Mcal/kg ~

59.70 ... . . . 34.70 4.53 .58 ... . . . . . .45 .04

... 60.40 . . . . 19.62 18.06 . . . 1.43 . .45. .04



62.79 . . 78163 . . . . . . . . . . 59.79 ... 33.00 35.66 18.92 3.23 3.49 . . . 57 .62 . . . . . . . . . 1.95 . . . . . . .46 .37 .40 .04 .04 .04



. .

. . 59172 75.60 . . . . . . 18.61 ... 20.03 22.63 . . . . . . 89 .64 31 .66 .40 .43 .04 .04








31.4 22.0 4.1

37.6 17.6 1.9

44.3 22.0 2.5

34.2 30.0 5.0

37.9 30.1 5.0

32.4 17.8 1.7

37.9 20.1 2.2







1 Proportion of dietary dry matter from alfalfa silage (AS), corn silage (CS), or alfalfa hay (AH). 2 Provided (per kilogram dry matter): 880 IU vitamin A, 880 IU vitamin D, and .088 IU vitamin E. 3 Computed from net energy for lactation (NE 1) in NRC tables (20) and forage NE 1 in Table 1.

Journal of Dairy Science Vol. 68, No. 12, 1985


ALFALFA VERSUS CORN FOR LACTATION Mean data were analyzed as a 3 x 3 Latin square replicated seven times (24) except for observations from the six ruminally cannulated cows, which were analyzed as a 3 x 3 Latin square replicated two times (24). Where significant F-values were detected due to diet (at least P<.05 for all 21 cows and P<.10 for the six cows), mean separation was by least significant difference (7). Trial 2

Sixteen Holstein cows, including 4 with rurnen cannulae, averaging 631 kg, lactation number 3.9, 88 d in milk, and 31.1 kg/d milk were randomly assigned to four dietary treatments in a 4 x 4 Latin square. The four diets differed in source of forage with 63% of the dietary DM from AS (63% AS), 60% from alfalfa hay (60% AH) or CS (60% CS), or 76% from CS (76% CS). Silages were harvested during 1983 as described in Trial 1 ; b o t h alfalfa forages were from second cutting. As in Trial 1, the balance of the rations were concentrates based on corn and soybean meal (Table 2). Diets were fed for periods of 3 wk (total 12 wk); the 1st wk was transitional and production data collected over the last 2 wk of each period. Measurement of milk production and composition and b o d y weights were as described for Trial 1 except that milk true protein was not determined. Diet feeding, sampling, and analyses were as described in Trial 1 except that samples of TMR and feed refusals were also analyzed for ADF in determining nutrient intake. Alfalfa hay was chopped (1.0 cm nominal length) prior to mixing in TMR. Fecal grab samples and blood samples were taken on d 19 or 20 of each period and analyzed as described except that deproteinized blood plasma was prepared and stored at - 2 0 ° C (6) before determination of glucose and urea. Also on d 19 or 20, 4 cows fitted with rumen cannulae were used in a rumen turnover study to determine liquid turnover, liquid volume, DM, and NDF fills as described for Trial 1. Mean data were analyzed as a 4 × 4 Latin square, replicated four times (24), except for observations from the 4 ruminally cannulated cows analyzed as a single 4 × 4 Latin square (24). Where significant F-values were detected due to diet (at least P<.05), mean separation was by least significant difference (7).

3265 R ESU LTS

Trial 1

There were no differences in DMI due to diet (Table 3). It was intended that intake of CP be equal across diets. However, CP was higher (P<.05, Table 3) with 60% AS due to higher CP of alfalfa silage during the trial than during standardization when the concentrates were formulated. Although the TDN computed from estimated NE1 [Table 2, (20)] was about 6% higher on 60% CS, total tract digestibility of DM and NDF estimated using AIA internal marker was not different among diets (Table 3). Digestibility of DNDF was higher on 60% AS (Table 3). Effects of diet on b o d y weight and milk production are summarized in Table 4. Body weight was essentially maintained on 60% AS and 79% CS, but there was a gain o f over .5 kg/d on the 60% CS diet. Generally, production o f milk and milk components was simitar on both diets containing 60% forage DM and significantly lower on 79% CS, which contained NE 1 (Table 2) similar to the 60% AS diet. However, fat content was lower on 60% CS, and cows fed both corn silage diets yielded less fat and produced milk slightly lower in lactose. Effective NE1, computed from NE 1 requirements (19) to support maintenance plus observed production and weight change, are shown in Table 4. Milk protein production, whether estimated from total N or T e A precipitable N, followed essentially the same pattern as actual milk. Concentrations of urea in milk and blood were lower (P<.05) with feeding AS than CS (Table 5). Rumen ammonia was significantly higher with feeding of AS than CS (Table 5), which probably reflects higher CP content of AS. The influence of diet on V F A concentrations and molar proportions in SRF was generally nonsignificant (Table 5). Exceptions were lower butyrate and higher isobutyrate on the 60% AS diet. Molar proportions of acetate and propionate and acetate:propionate were not altered by forage source despite the milk fat depression observed with 60% CS (Table 4). Liquid volumes in rumen were 62.3,. 55.9, and 66.9 L on 60% AS, 60% CS, and 79% CS with that on 60% CS being significantly (P<.05) lower. Dry matter fill, NDF fill, and liquid turnover averaged 13.9 kg, 9.4 kg, and Journal of Dairy Science Vol. 68, No. 12, 1985


BRODERICK .165 h - 1 , respectively, and were not affected by diet. Trial 2














2 <



Journal of Dairy Science Vol. 68, No. 12, 1985

Although DMI was not affected by diet, significantly more CP was consumed on the 63% AS and more A D F was consumed on both alfalfa-based diets (Table 3). This reflected principally differences in diet composition, but feed refusals from cows fed 60% AH were lower in CP and higher in ADF (P<.01) than the diet. However, this changed estimated CP and ADF intake less than 1% from that calculated from diet composition alone. Significant selection effects did not occur with the silage-based diets. Intake of NDF was not determined from direct analysis of TMR and refusals. In this trial, total tract digestibility of DM was higher on CS, while digestibility of DNDF was higher on AS and particularly AH (Table 3). Effects of diet on body weight and milk production are summarized in Table 4. Body weight was maintained on 60% AH, but gains were positive on the other three diets. Generally, production of milk, fat, and lactose were similar on the three diets containing about 60% forage DM and higher than that on 76% CS (Table 4). However, efficiency (milk/DMl) and protein production were significantly higher on 60% CS, and protein content was higher on both CS diets. Again, there was a trend for higher lactose on the alfalfa-based diets• Effective NE I were also computed for rations fed in this trial (Table 4). Concentrations of urea in milk and plasma were highest on 76% CS, intermediate on 60% CS, and lowest on the alfalfa diets (Table 5). Glucose in plasma was slightly higher with feeding of corn silage versus alfalfa forages, and rumen pH had the opposite pattern (Table 5). Rumen ammonia was not affected by diet, but there were several significant trends in rumen VFA. Acetate was higher, propionate and butyrate lower, and acetate:propionate higher in cows fed the alfalfa-based diets (Table 5). Although typically associated with milk fat depression, no pattern of reduced milk fat was observed on corn silage diets in Trial 2 (Table 4). Isobutyrate was higher on the 63% AS diet, and isovalerate and valerate were lower on the 60% AH diet. Rumen liquid volume, DM fill, NDF fill, and liquid turnover averaged 73.1 L,

TABLE 4. Body weight change, production of milk and milk components, and effective net energy for lactation (NE 1) of diets. Dietary forage (Trial 1) 1

Dietary forage (Trial 2) 1


60% AS

60% CS

79% CS


63% AS

60% AH

60% CS

76% CS


Weight change, kg/d Milk, kg/d Milk/dry matter intake 4% FCM, 2 kg/d Fat, % Fat, kg/d Total protein, % Total protein, kg/d True protein, kg/d Lactose, % Lactose, kg/d

.02 b 26.3 a 1.26 a 25.1 a 3.72 a .97 a 3.16 .83 a

.54 a 26.1 a 1.25 a,b 24.1 a 3.50 b .91 b 3.18 .83 a .78 a 4.67 b 1.22 a

-.01 b 23.9 b 1.20 b 22.9 b 3.74 a .89 b 3.21 .76 b .72 b 4.66 b 1.11 b

.17 .34 .02 .36 .04 .02 .03 .01 .01 .02 .02

.47 a'b 29.8 a 1.24 b 28.3 a 3.68 1.09 a'b 3.11 b .92 b . . 4.89 a 1.46 a

-.02 b 29.4 a'b 1.24 b 28.0 a'b 3.70 1.08 a'b 3.11 b .91 b . . 4.88 a 1.44 a'b

.90 a 30.3 a 1.36 a 29.2 a 3.86 1.14 a 3.32 a .99 a . . 4:83 a'b 1.47 a

.63 a 28.0 b 1.18 b 26.9 b 3.84 1.05 b 3.33 a .91 b

.21 .58 .04 .46 .11 .02 .04 .02

;~ t-" ;> <~

4:80 b 1.35 b

:03 .03

2; O




. ..

Effective NEI, Mcal/kg DM 3

.78 a 4.77 a

1.25 a



r~ 1.38










a'bMeans in rows within each trial having different superscripts differ (P<.05).


1 Proportion of dietary dry matter from alfalfa silage (AS), alfalfa hay (AH), or corn silage (CS).


2 Fat-corrected milk. W

3 Effective net energy for lactation (NE 1) computed from NE 1 required for maintenance, production, weight change [Table 2, (19)], and average DM intake (Table 3).

o_ ox

o0 Z o

0o kn




-7 TABLE 5. Concentration of m e t a b o l i t e s in milk, blood, and rume n fluid. < o OX 00

Dietary forage (Trial 1) 1 Item

60% AS

60% CS

79% CS

4.47 c

5.25 b

5.66 a

4.86 b 61.1 a

5.45 a 58.8 a'b

5.91 17.9 a 120.5 61.5 20.7 12.7 b 1.29 a 2.08 1.83 3.01

Dietary forage (Trial 2) l SE

63% AS

60% AH

60% CS

76% CS



5.25 c

5.18 c

5.89 b

6.41 a


5.90 a 57.9 b

.16 .81

5.27 c 64.4 a,b

5.54 c 62.9 b

6,61 b 67.9 a

7.52 a 66.5 a'b

.20 1.33




6.58 a

6.55 a

6.21 b

6.35 b


14.2 b 115.5 60.0 20.1 14.8 a 1.07 b 2.19 1.79 2.99

13.6 b 110.2 59.9 20.8 14.7 a .99 b 2.00 1.66 3.13

12.0 105.7 70,3 a 16.5 c 8,9 c 1.23 b 1.50 b 1.54 b 4.27 a

12.0 109.6 57.1 b 25.8 a 12.1 a 1.20 b 2.09 a 1.78 a 2.40 b

11.4 109.0 59.9 b 23.4 a'b 11.5 a'b 1.29 b 2.15 a 1.76 a 2.65 b

.62 5.32 1.54 1.98 .43 .08 .37 .07 .25

oz ~o

Milk urea, mM Blood Urea, mM Glucose, mg/dl R u m e n fluid pH A m m o n i a nitrogen, mg/dl Total VFA, 2 mM Acetate, molar % Propionate, molar % Butyrate, molar % Isobutyrate, molar % lsovalerate, molar % Valerate, molar % Acetate:propionate


1.10 4.08 1.00 .94 .44 .03 .09 .06 .16

13.2 108.9 66.4 a 17.8 b,c 10.4 b 1.54 a 1.93 a 1.95 a 3.73 a

a'b'CMeans in rows within each trial having different superscripts are different (P<.05). 1 Proportion of dietary dry m a t t e r from alfalfa silage (AS), alfalfa hay (AH), or corn silage (CS), 2 Volatile fatty acids.

ALFALFA VERSUS CORN FOR LACTATION 12.7 kg, 9.0 kg, and .168 h - 1 , respectively, and were not affected by diet. DISCUSSION

Composition of the forages fed in these trials (Table 1) indicate they were of generally good quality. At 18.3% CP and 33.6% ADF, AS fed in Trial 1 corresponded to early bloom, wilted silage with 1.35 Mcal NE1/kg (20). The CS A D F from both trials (24.7 and 23.3%) were lower than the mean of 28% given by the NRC (20) for well-eared CS, indicating they were of excellent quality. The AH fed in Trial 2 was somewhat lower in CP and higher in NDF and A D F than AS, possibly because o f leaf loss during baling. The AS from Trial 2 was almost two percentage units higher in CP than that from Trial 1 but was also about 50% higher in ADIN (Table 1), reflecting some heat damage (28). Hence, the lower A D F and ADIN of AS from Trial 1 indicate it was a higher energy forage than that from trial 2. Although ADF were determined in both TMR and refusals only in Trial 2, A D F concentrations in diets as consumed were not different from that fed for silage-containing rations. However, ADF was higher (P<.01) in refusals than in the AH ration; this had a small (1%) but significant (P<.01) effect on estimated fiber intake. Use of TMR resulted in no significant diet selection on the silage-based rations (suggesting analysis of refusal composition is not necessary), but some selection occurred when chopped hay was incorporated into TMR. Production of milk and milk components during Trial 1 (Table 4) indicated that the AS was approximately equal in energy (DM basis) to corn silage. Production of most milk components was not significantly different among the diets containing about 60% forage DM in Trial 2, but the greater productive efficiency on 60% CS reflects its higher effective energy content (Table 4). Poorer performance on alfalfa forages fed in Trial 2, relative to 60% CS, was probably due to lower quality. Also, the level of milk production averaged about 4 kg/d more during Trial 2; therefore, energy requirements were greater than in Trial 1. Digestibility of DM, estimated using A I A as internal marker, was not different among diets in Trial 1 but was lower in the alfalfa-fed cows during Trial 2 (Table 3). Digestibility of DM at


both levels of CS was comparable during each trial. This suggests that DM digestibility of the extra concentrate (principally corn grain) in the 60% CS diets was similar to CS alone. Tyrrell and Moe (29) reported energy digestibility of rations with 40 and 70% corn silage was similar. Colorado workers observed substantial reductions in digestibility o f mixtures of corn silage and corn grain in beef cattle (15). It is possible that impaired energy utilization on high CS diets may also be due to inefficiencies in metabolism, since effective NE 1 with 79 and 76% CS (Table 4) was lower despite equal DM digestibilities. Digestibility of NDF was similar among all diets (Table 3), even though potentially digestible NDF (DNDF) was higher in CS than AS (Table 1). Therefore, digestibility of DNDF was greater for alfalfa forages than CS in both trials (Table 3). Any potential differences in NDF digestibility could have been masked by the adverse effects of soluble carbohydrates (34) and the relative lag in initiation of digestion of CS fiber (18). Lower rumen pH on CS diets (Table 5), related to a possible buffering effect o f alfalfa forage (17), m a y have depressed microbial cellulolytic activity (25). Estimation of the relative energy of AS and CS is confounded in the present studies because most of the concentrate in the alfalfa rations was c o r n grain with which digestibility depressions have been observed at high intakes. The NRC (20) indicates that early bloom, wilted AS with 17% CP and 33% ADF has 1.35 Mcal/kg NE1, and CS with 28% A D F is assigned a NE 1 of 1.60 Mcal/kg. The CS fed in the present studies had lower ADF, and presumably, higher energy. Although the effective NE1 of rations with about 60% AS or CS (Table 4) averaged about 90% o f NE 1 computed from NRC (19, 20) data (Table 2), those with 79 and 76% CS were only about 84%. Effective NE1 computed for the alfalfa hay diet was 88% of that from NRC data. The NE1 estimated from A D F using the equations o f Mertens [cited in (9)] appear to predict somewhat more accurately energy o f these forages. These equations assign NE 1 of 1.39 and 1.56 Mcal/kg to AS and CS (Trial 1), 1.24 and 1.19 to AS and AH, and 1.57 Mcal/kg to CS (Trial 2). Based on NE I requirements and ration contents estimated from NRC tables, DM intakes averaged 3.3 and 3.8 times maintenance in Trials 1 and 2, reJournal of Dairy Science Vol. 68, No. 12, 1985



spectively. The relative depression in energy utilization, at these very high rates of intake, was probably greater for the corn silage diets (15, 29). Effects on fat are interesting in that depression of milk fat content occurred on 60% CS in Trial 1 (Table 4) without apparent change in rumen pH and VFA patterns (Table 4). However, during Trial 2, there were significantly lower pH, acetate and acetate:propionate ratios, and higher propionate and butyrate in rumen fluid on both CS diets b u t no milk fat depression. Periods were only 3 wk long in Trial 2, which may be insufficient time for occurrence of milk fat depression. However, these changes in rumen VFA suggest a potential problem in practical feeding regimens where the sole forage is CS. Milk protein concentrations were lower on both alfalfa rations during Trial 2 (Table 4), even though CP was one percentage unit higher on the AS diet. This indicates protein inadequacy, relative to the CS diets, occurred at the higher levels of production in this trial but not in Trial 1. Protein in legume forages, particularly that in AS, is very degradable in the rumen (16). Differences in CP between alfalfa forages and CS were made up by soybean meal, a protein source that may be more resistant than alfalfa to ruminal breakdown. Olmer and Wiktorsson (21) used concentrations of urea in milk to assess relative adequacy of dietary protein and energy, suggested urea in milk above 5 mM reflects protein excess and below 5 mM protein shortage relative to energy. This implied an energy inadequacy on 79% CS and a protein inadequacy on AS during Trial 1. Urea in milk was about 5.2 mM on both alfalfa diets during Trial 2, despite an apparent protein insufficiency, indicating 5 mM may not be a good standard of protein-energy adequacy under all conditions. Concentrations of urea in milk of 5.9 and 6.4 mM with CS feeding in this study imply a relative inefficiency in energy utilization. Consideration of results of both trials emphasize the need to supplement alfalfabased diets with resistant protein even though NRC requirements for CP appear to be met. As in previous reports (5, 21), urea in milk and blood were highly correlated (P<.005, r 2 = .701). CONCLUSIONS

Results of these experiments indicate that Journal of Dairy Science Vol. 68, No. 12, 1985

high quality AS is essentially equal to CS for milk production with reduced problems from milk fat depression. Increased ADF indicates a reduction in quality and decreased effective energy in alfalfa forage. Computing energy content of CS and alfalfa-based diets using data in NRC tables will underestimate the relative value of high quality alfalfa. Alfalfa-based diets may be more limiting in resistant protein than those based on CS supplemented with soybean meal. ACKNOWLEDGMENTS

The author gratefully acknowledges Len Strozinski and his coworkers for care and feeding of the cows. The excellent technical assistance of Mike Meyer, Brad Ricker and Heidi Mier is greatly appreciated. The author thanks Randy Shaver and Bob Roffler for reviewing a preliminary draft of this manuscript. REFERENCES

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Journal o f Dairy Science Vol. 68, No. 12, 1985