Effects of Dry or Ensiled Feeds and Protein Percent on Milk Production and Nitrogen Utilization by Lactating Cow

Effects of Dry or Ensiled Feeds and Protein Percent on Milk Production and Nitrogen Utilization by Lactating Cows D. G. G R I E V E , E. E L I Z A B E T H WHEELER, Y. YU, and G. K. M A C L E O D

Department of Animal and Poultry Science University of Guelph Guelph, Ontario, N1G 2W1 ABSTRACT

In two 2 x 2 factorial experiments, 28 cows in first and later lactation (TriaI 1) and 28 cows in second and later lactation (Trial 2) were fed rations containing either dry (low nitrogen solubility) or ensiled (high nitrogen solubility) feeds with low (14%) or high (16%) concentrations of crude protein. Rations were formulated with hay or hay crop silage and dry shelled corn or high moisture shelled corn in a 50:50 ratio (Trial 1) or 40:60 ratio (Trial 2) of forage to concentrate in the dry matter. Soybean meal was used to alter the protein content. Protein percent had no effect on milk yield or milk composition in either trial, although nitrogen digestibility was greater on high protein diets. Solids-corrected milk yield and milk fat percent were lower when nitrogen solubility of the ration was increased in Trial 2 but not in Trial 1. Analysis o f nitrogen balance data indicated higher conversion of nitrogen of feed to nitrogen of milk for low solubility diets in Trial 1 and high solubility diets in Trial 2. INTRODUCTION

Amino acids supplied to the lower gut by microbial protein are supplemented by dietary protein that escapes microbial fermentation in the rumen. A low supply of amino acids reaching absorptive sites in the lower gut may be limiting milk production even in diets containing recommended quantities of protein (5). Responses of production to postruminal infusion o f amino acids or protein (9) suggest

Received November 13, 1979.

that inadequate amounts of absorbable amino acids may be limiting dairy cattle from increased lactation yields. Modulation of the extent of protein degradation in the rumen is one method of influencing the amino acid supply to the lower gastrointestinal tract (7). Since degradation of nitrogenous material in the rumen is closely and positively correlated with the nitrogen solubility of feedstuffs in mineral buffers (10), formulation of rations with low nitrogen solubility should result in more protein escaping ruminal degradation and subsequent increases in milk production. Majdoub et al. (17) observed higher milk yield when nitrogen solubility was 22% compared to 42%, althouth feed ingredients differed and no attempts were made to equalize amino acid intake. Rations formulated with similar ingredients, but with the ingredients stored as either dry or ensiied feedstuffs, should have similar amino acid composition but markedly different nitrogen solubility due to solubilization of nitrogen during the ensiling process (3). High producing cows in early lactation should be more responsive to additions of dietary protein than cows in late lactation. Some studies have indicated a response in milk yield by cows in early lactation to increasing concentrations of dietary protein (11, 22, 23). Objectives were to investigate effects of dietary protein content and ration solubility in dry or ensiled feeds on milk production of cows during early lactation.

MATERIALS AND METHODS Trial 1

Effects of nitrogen solubility (in dry versus ensiled rations) and protein percent (low versus high) on lactation response were investigated in a 2 × 2 factorial experiment of 28 Holstein cows. Seven animals (four multiparous cows

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1980 J Dairy Sci 63:1282-1290

PROTEIN UTILIZATION IN DRY OR ENSILED RATIONS and three heifers in first lactation) were assigned randomly, within age, to each of four rations at parturition. A control ration (Table 1) was fed ad libitum for 7-wk standardization beginning at parturition. A 5-wk experimental period followed, during which rations (Table 1) were fed to multiparous cows at 100% of NRC (18) requirements for total digestible nutrients and to heifers at 120% of NRC (18) TDN requirements to allow for growth. Requirements for TDN were based upon age, body weight, milk production, and milk fat percent of individual animals during the last 2 wk of standardization. Daily feed intakes and weekly body weights were recorded during control and experimental periods. Rations were fed as complete mixed feeds with a 50:50 ratio of forage to concentrate (dry matter). Hay and hay crop silage in dry and ensiled rations, respectively, were prepared for the same crop. Rations were sampled weekly and composited every 4 wk for nutrient analyses. Feed composites were analyzed for dry matter (DM) and crude protein (CP) by standard procedures (2). Acid detergent fiber (ADF) was measured according to Van Soest (30). Gross energy (GE) was measured by bomb calorimetry. ~ Milk yield was recorded twice daily, and samples were obtained twice daily for 5 consecutive days during the last 2 wk of control and treatment periods. Milk samples were analyzed for fat, protein, and lactose by the Infra Red Milk Analyzer (4), and solids-corrected milk (SCM) was calculated by the equation of Tyrrell and Reid (28). Blood samples were taken 2 to 3 h after feeding on the last day of control and treatment periods for analysis of urea nitrogen of plasma (PUN) on an autoanalyzer. 2. Rumen fluid samples were taken via stomach tube 2 to 3 h after feeding on the last day of the treatment period. Strained and centrifuged rumen fluid samples were analyzed for pH and ammonia nitrogen (NH3-N) (20). Volatile fatty acids

1Parr Adiabatic Calorimeter, Parr Instrument Company, Moline, IL. 2Technicon Instruments, Tarrytown, NY. 3Varian Aerograph Series 2100, Varian Corp., Walnut Creek, CA. 4 Bard Hospital Division, C. R. Bard Inc., Murray Hill, NJ.

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(VFA) were extracted by methanol, then steam distilled in a sealed tube, and separated by gas-liquid chromatography 3 (21). Total feces and urine (by Bardex Foley catheters 4 ) were collected from all experimental animals during a 7-day digestion and nitrogen balance trial immediately following the experimental period. Feed samples and a constant proportion of voided feces and urine were obtained daily and frozen pending analyses of the 7-day composite. Feed and feces were analyzed for DM, CP, ADF, and GE as described, and crude fiber and ether extract by standard procedures (2). Nitrogen free extract (NFE) was determined by difference. Nitrogen (N) solubility of ration ingredients, as a percentage o f total N, was determined in Burroughs' (6) mineral buffer diluted to 10% with distilled water. Ration solubilities were calculated as the sum of weighted proportions of each ingredient according to ration composition (Table 1). Milk samples were from each milking during the collection period for component analyses (4). Data were analyzed as a 2 × 2 factorial arrangement in a completely randomized design (26) with the effect of age of animals removed from the error variance. Where applicable, treatment means were adjusted by covariance by means from the control period. Treatment means with significant interactions were tested by Tukeys' test (26). Significant differences in main effects and interactions were at .05 probability. Trial 2

A similar experiment to Trial 1 was with 28 Holstein cows in their second or later lactations. During a 6-wk control period beginning at parturition, a standard ration (Table 2) was fed ad libitum. Cows were blocked according to calving date and milk production during the control period and assigned within blocks to one o f four treatment rations (7 cows per treatment). Animals producing less than 27.5 kg milk per day at 6 wk postpartum were not in the experiment. Rations (Table 2) were fed for 5 wk according to NRC requirements (18) based upon the average body weight, production, and milk fat percent of the blocks. Requirements for TDN were adjusted for intake above maintenance (18). Journal of Dairy Science Vol. 63, No. 8, 1980

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TABLE 1. Ingredient and chemical composition of control and experimental rations (Trial 1). Control Protein Ingredients (% of DM) Hay Hay crop silage Corn silage Dry shelled corn High moisture shelled corn Soybean meal

Composition by analyses (DM basis) Dry matter, % Crude protein, % Acid detergent fiber, % Energy, MJ/kg Nitrogen solubility, %c

Dry

...

Low

Ensiled High

... 25 25 ... . . . . . . . . . . .

50 50 . . . . . . . . . . . . 50 43 . . . . 7

50 a

100 b

50.71 15.62 22.24 18.94 ND d

85.69

13.51 23.38 18.82 32.41

100b

86.59 16.26 23.09 18.69 30.53

Low

.

.

. . . . . . 50 . . . . . . . . . . 50 ... 100 b

55.11 14.09 22.97 19.07 60.68

High

50 43 7 100 b

56.20 16.82 22.69 18.94 53.21

aplus 50% of dry matter as Elora 101 dairy ration containing (%): corn 45; barley 25; soybean meal (44%) 22.5; molasses 5; limestone .6; dicalcium phosphate .75; trace mineral salt 1.0 (96.5% salt; .4% zinc; .16% iron; .12% manganese; .033% copper; .007% iodine; .004% cobalt); vitamin A, D, E premix .15 (4,400,000 IU/kg vitamin A; 1,1OO,O00 IU/kg vitamin D; 7,700 IU/kg vitamine E). bExperimental rations topdressed with 400 g/day of a mineral mix containing (%): dicalcium phosphate 21; monosodium phosphate 36; trace mineral salt 36 (as above); vitamin A, D, E premix 7 (as above). CNitrogen solubility determined on ingredients of rations. riND, not determined.

Rations were fed as c o m p l e t e mixed feeds with a 4 0 : 6 0 ratio o f forage to c o n c e n t r a t e (dry matter). Feed intake was recorded daily. Body weights were recorded on 2 consecutive days initially and again at the end o f each period. Rations were sampled weekly and c o m p o s i t e d every 2 wk for n u t r i e n t analyses as described in Trial 1. Sampling procedures for milk and b l o o d were as described. Following the e x p e r i m e n t a l period, four cows per t r e a t m e n t were in a digestion and nitrogen balance trial. A 7-day period of total collection o f feces and urine was as described. Data were analyzed as a 2 × 2 factorial arrangement in a r a n d o m i z e d block design (26). T r e a t m e n t means were adjusted by covariance analyses with control period as covariate. Significant differences in main effects were at the .05 probability. RESULTS A N D DISCUSSION Trial 1

Chemical

composition

of

control

Journal of Dairy Science Vol. 63, No. 8, 1980

and

experimental rations is in Table 1. Crude protein contents (%) of low and high protein rations were 13.80 and 16.54. Ensiled rations had a higher N solubility than dry rations (Table 1). F e r m e n t e d feedstuffs generally have a high N solubility due to proteolysis by plant enzymes and N solubilization during storage

(3). In analyses o f data, few interactions between ration t y p e and protein were significant. The main means, rather than t r e a t m e n t means, are reported. There were no significant differences in adjusted means for b o d y weight, dry m a t t e r intake, or milk p r o d u c t i o n due to ration t y p e or protein percent (Table 3). A trend was towards increased intake and milk p r o d u c t i o n by animals fed the high protein rations, but differences were not significant. No response in milk yield f r o m increasing c o n c e n t r a t i o n o f dietary protein in early lactation was shown in (8, 29). Since 12 of the 28 animals in this e x p e r i m e n t were heifers, which are less respon-

PROTEIN UTILIZATION IN DRY OR ENSILED RATIONS

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TABLE 2. Ingredient and chemical composition of control and experimental rations (Trial 2). Control Protein Ingredients (% of DM) Hay Hay crop silage Corn silage Dry shelled corn High moisture shelled corn Soybean meal

Dry

...

Low

20.0 . . 20.0 ... 49.8 10.2

.

.

40.0 . .

.

6010 . . . ...

1O0a Composition by analyses (DM basis) Dry matter, % Crude protein, % Acid detergent fiber, % Energy, MJ/kg Nitrogen solubility, %c

Ensiled

1O0b

55.78 15.89 19.11 19.03 ND

87.01 14.00 18.96 18.78 25.20

High

Low

40.0 .

. . . 51.5

.

.

. 8.5 1O0b

87.01 17.73 19.23 18.90 26.70

.

.

High

... 40.0 4010 . . . . . . . . . . . 56.4 47.9 3.6 12.1 1O0b

47.82 12.23 20.06 19.15 43.40

1O0b

49.00 15.88 20.44 19.24 39.20

acontrol ration topdressed with 400 g/day of a mineral mix containing (%): calcium phosphate 71; trace mineral salt 25 (as in a in Table 1); magnesium oxide 4 (54% magnesium). b Experimental rations topdressed with 400 g/day of a mineral mix (as in footnote b in Table 1). C

-

•

•

Nitrogen solubdlty determined on ingredients of rations.

dND, not determined.

sive t h a n o l d e r cows t o increased p r o t e i n (11, 2 3), t h e i r inclusion in this e x p e r i m e n t m a y have a f f e c t e d the d a t a such t h a t d i f f e r e n c e s due to p r o t e i n p e r c e n t could n o t b e d e t e c t e d . Also, p r o d u c t i o n r e s p o n d s w h e n c o w s are fed high p r o t e i n rations f r o m p a r t u r i t i o n (22), w h e r e a s

t r e a t m e n t s were i m p o s e d after 7 w k o f lactation. Results o f the digestion and n i t r o g e n balance trial (Table 4) i n d i c a t e d significantly l o w e r N r e t e n t i o n in dry rations with low N solubility t h a n in ensiled rations w i t h high N solubility. R e t e n t i o n o f N increased (1, 17, 25) w h e n

TABLE 3. Effect of ration type and protein percent on adjusted means for body weight, intake, and yield (Trial 1). Ration type a

No. of cows Body weight, kg Dry matter intake, kg/day Milk yield, kg/day Fat, % Protein, % Lactose, % Solids corrected milk, kg

Protein percent a

Dry

Ensiled

Low

High

13 571.7 18.4 25.1 3.39 3.00 5.09 23.0

14 571.3 18.1 24.9 3.55 3.06 5.10 23.3

14 569.8 18.0 24.7 3.45 3.01 5.10 22.8

13 573.1 18.5 25.3 3.49 3.05 5.10 23.6

SD b

15.78 1.64 1.68 .22 .08 .10 3.62

aNo main effects were significant P>.05. bstandard deviation of adjusted means. Journal of Dairy Science Vol. 63, No. 8, 1980

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TABLE 4. Effect of ration type and protein percent on mean a nitrogen utilization and apparent digestibility coefficients (Trial 1). Ration type

Protein percent

Dry

Ensiled

Low

High

SD

481.2 197.5 283.7 183.1" 123.7 -23.1" 26.0* 45.5*

483.4 186.5 296.9 159.5 111.1 26.3 23.2 38.6

404.5* 177.6" 226.9* 126.1" 109.1 -8.3 27.0* 48.5*

566.2 207.1 359.2 219.1 125.8 14.2 21.9 34.8

64.2 26.1 44.8 14.8 20.3 38.9 2.7 6.4

Apparent digestibility (%) Dry matter Nitrogen Acid detergent fiber Nitrogen free extract Crude fiber Ether extract Gross energy

63.8 58.2* 52.9 70.7" 54.3* 57.2* 62.4

65.3 61.0 50.2 73.7 45.7 65.4 63.9

63.8

56.0* 50.7 72.0 49.1 60.7 62.7

65.5 63.6 52.4 72.5 50.7 62.3 63.7

2.27 3.18 5.04 2.36 6.05 5.68 2.90

Total digestible nutrients, %

65.8

65.6

65,4

66.0

3.25

Nitrogen (N) utilization (g/day) N Intake N Feces N Absorbed N Urineb N Milk N Retained Milk N (% of intake) Milk N (% of absorbed)

aMean of 13 cows for dry or high protein rations and 14 cows for ensiled or low protein rations. bsignificant ration type by protein interaction, P<.05. Treatment means by ration type, protein were: dry, low 132.6; dry, high 242.2; ensiled, low 119.7; ensiled, high 199.3 * Main effect significant P<.05.

ration N solubility decreased. While total N intake and fecal N were not affected by ration N solubility, excretion o f urine N was higher by animals fed dry rations than by those fed ensiled rations. Wohlt et al. (31) found, in contrast, a correlation (.82) o f dietary soluble protein intake with urine N excreted. Rapid d e a m i n a t i o n o f soluble f e e d s t u f f protein by r u m e n microorganisms, with subsequent losses of N in the urine, is to be e x p e c t e d (25). A significant interaction of ration type by protein percent was in urine N excretion. At high protein, t r e a t m e n t means (Table 4, f o o t n o t e b) were significantly higher in dry rations than in ensiled rations, but no differences due to ration t y p e were d e t e c t e d at low protein. These results m a y be explained by factors o t h e r than N solubility that influence the e x t e n t of degradation of ruminal protein. Such factors include r u m e n r e t e n t i o n time and rate of passage of digesta, feed intake, particle size (7), and rate o f c o n s u m p t i o n (25), all o f which may have been influenced by feeding ensiled as c o m p a r e d Journal of Dairy Science Vol. 63, No. 8, 1980

to dry rations. In addition, the e x t e n t of N recycling to the rumen via saliva increases the efficiency of N utilization and lowers the a m o u n t of N e x c r e t e d in the urine when low protein rations are fed (19). The efficiency of utilization o f feed N or absorbed N for milk N (Table 4) was significantly higher by animals fed rations with low N solubility than by those fed rations with high N solubility. Nitrogen retention (Table 4) was not affected by dietary protein concentration, although intake and excretion of N (via feces and urine) were greater by animals fed high dietary protein. There were no differences in milk N due to ration protein. Feeding rations containing low protein resulted in a more efficient conversion of feed N to milk N (Table 4) than when high protein rations were fed. Similar results in efficiency of N utilization were shown by Kwan et al. (16). A p p a r e n t digestibility (Table 4) o f nitrogen, N F E , and ether extract was higher by animals

PROTEIN UTILIZATION IN DRY OR ENSILED RATIONS fed ensiled rations than by those fed dry rations. Digestibility of crude fiber was increased by feeding dry versus ensiled rations. Digestibilities of DM, ADF, and GE were not affected by ration type, and TDN contents of dry and ensiled rations were remarkably similar. Apparent nitrogen digestiblity increased with increase in CP in the ration (Table 4), which agrees with

(12). Rumen ammonia did not differ from ration type or protein (Table 5). Animals fed rations with high N solubility tended to have higher concentrations of tureen NH3-N than those fed rations with low N solubility, but differences were not significant. A high correlation between dietary protein solubility and rumen ammonia concentrations has been reported (14, 31). Rumen pH was not affected by dietary treatment (Table 5). Urea nitrogen in plasma (Table 5) was not affected by ration type or protein concentration. Reports indicated a direct effect of N solubility (31) and dietary crude protein (16) on concentrations of urea nitrogen in blood. Concentrations of V F A (Table 5) signific a n d y increased for butyrate, isobutyrate, and isovalerate with increasing ration N solubility. Acetate, propionate, or total VFA concentrations did not differ with ration type. An increase in butyrate when N solubility is increased (31) is attributed to a high proportion of glutamic acid in the soluble ration protein and possible conversion of

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glutamic acid to butyrate without prior formation of acetic acid. Concentrations of V F A were n o t affected by ration protein. Trial 2

Changes in procedure from Trial 1 included use of only cows in second and later lactation, blocking o f cows by pretreatment milk production to remove effects of feed intake, and use o f a 40:60 ratio o f forage to concentrate. As hay crop silage was lower in CP than hay, additional soybean meal was required in the ensiled rations (Table 2). The composition of the rations (Table 2), however, differed in CP content of dry (15.87%) and ensiled (14.06%) rations. Ensiled rations were higher in soluble N than dry rations (Table 2), although differences in solubility due to ration type were less pronounced than in Trial 1. Dry matter intake (Table 6) of dry rations was significantly higher than intake of ensiled rations in spite of attempts to reduce variation in feed intake across treatment by blocking. Cows fed dry rations had similar milk production but higher milk fat percent than cows fed ensiled rations. Inclusion of some hay in silage-based rations generally increases dry matter intake (15) and milk fat percentage by the fiber effect (27). Increased fat and lactose percent in milk resulted in a higher yield of SCM by cows fed dry rations than by those fed

TABLE 5. Effect of ration type and protein percent on rumen and blood measures (Trial 1).

Dry Rurnen volatile fatty acid concentration (mM/1) Acetic Propionic Butyric Isobutyric Isovaleric Total VFA

Ration type Ensiled

Protein percent Low High

SD

31.9 8.9 4.1" .4* .6* 45.8

34.1 9.6 5.2 .5 .9 50.3

32.0 8.7 4.6 .4 .7 46.3

34.2 9.9 4.7 .5 .8 50.1

8.39 3.88 1.26 .38 .38 13.06

Rumen pH

6.9

6.9

6.9

6.9

.29

Rumen NH3-N, mg/lO0 rnl

8.8

10.6

9.7

9.7

7.96

Plasma urea-N, mg/lO0 mg

16.2

16.0

16.1

16.0

3.62

amain effect significant, P<.05. Journal of Dairy Science Vol. 63, No. 8, 1980

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GRIEVE ET AL.

TABLE 6. Effect of ration type and protein percent on adjusted means for body weight and yield (Trial 2). Ration type

No. of cows Body weight, kg Dry matter intake, b kg/day Milk yield, kg/day Fat, % Protein, % Lactose, % Solids corrected milk, kg

Protein percent

Dry

Ensiled

Low

High

SD

13 622 20.7* 31.7 3.45* 3.00 5.02* 29.0*

14 620 19.5 30.5 3.21 3.01 4.92 27.0

14 620 20.0 30.2 3.41 3.01 4.95 27.3

13 622 20.2 32.1 3.25 2.99 4.98 28.7

27.3 .9 b 2.4 .26 .16 .12 2.2

astandard deviation of adjusted means. bObserved means, standard deviation of observed means. *Main effect significant, P<.05.

ensiled rations. Previously, feeding of rations with low N solubility had a beneficial effect on milk production as compared with high soluble N rations but did not affect milk composition (17). Ration protein had no effect on milk yield or component composition (Table 6), although a trend towards higher milk production was evident when cows were fed high protein. Reports indicate increase in milk production (11, 22, 23) or no response in milk production (8, 29) when ration protein increased. Differences in ration protein and DM intake resulted in higher N intake by cows fed dry rations than by those fed ensiled rations (Table 7). More N was absorbed, and urine N excretion was higher (P<.05) by cows fed dry rations than by those fed ensiled rations although N balance was not affected by ration N solubility. Milk N, as a percentage of N intake (Table 7), was significantly greater when cows were fed rations with high N solubility than when cows were fed less soluble rations. These results oppose those in Trial 1 and indicate increased efficiency of nitrogen utilization for milk production by cows fed rations high in soluble N. There were large variations in milk N (percent of absorbed) and no differences from ration type. Increases in N intake, N absorbed, and N urine were significant due to increases in ration protein (Table 7). Higher ration protein tended to increase milk N, but differences from protein were not statistically significant. As in Trial 1, Journal of Dairy Science Vol. 63, No. 8, 1980

N retention was not affected by protein, although the trend of increased N retention on high protein diets was similar in both experiments. Apparent digestibility (Table 7) of DM and CP were not affected by ration type but were significantly higher when ration protein was increased. Total digestible nutrients (%)were higher and GE digestibility was significantly greater in high protein rations than in low protein rations as reported by Kwan et al. (16). Urea nitrogen of plasma (Table 7) followed trends similar to urine N suggesting a correlation between these. The PUN was significantly higher in cows fed dry as opposed to ensiled rations or high as opposed to low protein in the ration. Wohlt et al. (31) reported increases in PUN with increasing ration N solubility. Higher crude protein in the dry rations than in the ensiled rations (Table 2) may have masked the effect of N solubility as PUN increased with increasing ration protein content as in (12, 16). In summary, two trials involving 28 cows in early lactation examined the effects of low N solubility (dry feeds) versus high N solubility (ensiled feeds) diets on cow performance and N utilization. Increasing N solubility from approximately 30 to 56% in Trial 1 did not affect cow performance, although utilization of N for milk was higher for dry rather than for ensiled rations. In the second trial, intake of dry rations was greater than intake of ensiled rations resulting in higher milk fat percent and SCM yield by cows fed dry rations. Efficiency

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TABLE 7. Effect of ration type and protein percent on mean a nitrogen utilization, apparent digestibility coefficients and urea nitrogen of plasma (Trial 2). Ration type

Protein percent

Dry

En siled

Low

High

SD

481.2" 181.2 300.0* 162.2" 134.0 -4.8 28.0* 45.7

413.5 176.3 237.2 109.2 140.3 2.0 34.4 69.5

389.2* 183.5 205.8*

505.5 174.1 331.5 176.7

Apparent digestibility (%) Dry matter Nitrogen Acid detergent fiber Nitrogen free extract Crude fiber Ether extract Gross energy

63.3 60.0 43.2* 68.6* 45.2* 59.0 61.7

63.8 59.8 27.5 74.3 30.3

Total digestible nutrients, %

62.8

Plasma urea nitrogen, mg/lO0 ml

13.0*

Nitrogen (N) utilization (g/day) N Intake N Feces N Absorbed N Urine N Milk N Retained Milk N (% of intake) Milk N (% of absorbed)

128.6 -12.0 33.4* 71.1

145.7

9.1 28.9 44.1

29.6 49.4 40.5 15.3 15.9 23.1 2.6 25.9

63.0

61.2" 54.2* 33.4 69.5 35.7 58.0 59.8*

65.9 65.6 37.2 73.4 39.8 62.1 64.9

2.8 2.8 4.5 3.7 4.4 6.3 2.9

64.9

61.7"

66.0

2.5

7.6

7.5*

13.1

2.8

61.1

94.7*

aMean of 8 observations per main effect. * Main effect significant, P<.05.

o f N u t i l i z a t i o n f a v o r e d ensiled r a t i o n s in t h e l a t t e r trial. In view o f c o n f l i c t i n g results, effects o f d r y o r ensiled diets o n cow p e r f o r m a n c e are f r o m factors o t h e r t h a n i n h e r e n t N s o l u b i l i t y such as p a l a t a b i l i t y a n d n u t r i e n t digestibility. A s s e s s m e n t of o u r diets w i t h t h e s y s t e m prop o s e d b y R o y e t al. (24) s h o w e d all p r o v i d e d adequate tureen degradable and undegradable p r o t e i n to m e e t t h e a n i m a l ' s m i c r o b i a l a n d tissue p r o t e i n needs. This p r o b a b l y explains t h e lack of c o n s i s t e n t r e s p o n s e t o e i t h e r N s o l u b i l i t y o r r a t i o n p r o t e i n . Failure to r e s p o n d t o increased d i e t a r y p r o t e i n p e r c e n t a b o v e 13% also m a y be because r a t i o n s were i m p o s e d at 6 to 7 w k p o s t p a r t u m r a t h e r t h a n at p a r t u r i t i o n , as r e s p o n s e t o h i g h e r p r o t e i n d i e t s is m o r e likely t o o c c u r early in l a c t a t i o n w h e n feed i n t a k e is least. In a d d i t i o n , s o m e animals in first l a c t a t i o n were in Trial 1. These animals have b e e n less responsive t o i n c r e a s e d r a t i o n p r o t e i n c o n c e n t r a t i o n in early l a c t a t i o n t h a n o l d e r cows (11, 23). T h e r e were n o i m p o r t a n t i n t e r a c t i o n s b e t w e e n r a t i o n t y p e a n d p r o t e i n in this factorial experiment.

ACKNOW LEDGMEN3"S

This research was s u p p o r t e d in p a r t b y t h e Ontario Ministry of Agriculture and Food and b y t h e N a t i o n a l R e s e a r c h Council, Canada.

REFERENCES

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