Diurnal Variation in Milk and Plasma Urea Nitrogen in Holstein and Jersey Cows in Response to Degradable Dietary Protein and Added Fat

Diurnal Variation in Milk and Plasma Urea Nitrogen in Holstein and Jersey Cows in Response to Degradable Dietary Protein and Added Fat

Diurnal Variation in Milk and Plasma Urea Nitrogen in Holstein and Jersey Cows in Response to Degradable Dietary Protein and Added Fat L. A. RODRIGUEZ...

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Diurnal Variation in Milk and Plasma Urea Nitrogen in Holstein and Jersey Cows in Response to Degradable Dietary Protein and Added Fat L. A. RODRIGUEZ,1 C. C. STALLINGS, J. H. HERBEIN, and M. L. McGILLIARD Department of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg 24061-0315

ABSTRACT

INTRODUCTION

Four Holstein and four Jersey cows fitted with ruminal and duodenal cannulas were used in two 4 × 4 Latin squares to investigate the effects of varying protein degradability and supplemental fat on diurnal changes in plasma and milk urea N. Dietary dry matter contained 16.2% crude protein with two concentrations of ruminally undegradable protein (RUP) that were obtained by substituting blood meal for a portion of soybean meal. Treatments were 1 ) 29% RUP and 0% added fat, 2 ) 29% RUP and 2.7% added fat (Ca soaps of fatty acids), 3 ) 41% RUP and 0% added fat, and 4 ) 41% RUP and 2.7% added fat. Dry matter of the total mixed diet fed at 1000 and 1400 h consisted of 30% corn silage, 29% alfalfa haylage, and 41% concentrate. Ruminal ammonia, plasma urea N, and milk urea N were measured every 4 h over a 24-h period. Dry matter intake was depressed 6.7% by added fat. Ruminal ammonia was 25 to 45% lower when the 41% RUP diets were fed. Overall, the concentration of plasma urea N and milk components were not influenced by diet. However, milk urea N was higher in Holsteins than in Jerseys. Both plasma and milk urea N increased within 2 h after the 1000-h feeding followed by a decline at 6 h after the 1400-h feeding. In this short-term study, fat supplementation had no effect on milk production or yields of milk components. The inclusion of blood meal, however, increased the yields of milk components. Plasma and milk urea N did not differ among dietary treatments but varied throughout the day in relation to the time of feeding. ( Key words: protein, fat, milk components, milk urea nitrogen)

Dietary sources and amounts of energy, CP, and fat are associated with variation in milk production and the distribution of N fractions of milk (7, 8, 23). The three primary N fractions of milk are casein N, whey protein N, and NPN, which constitute approximately 77.9, 17.2, and 4.9% of the total milk N ( 4 ) . Distribution of the N fractions of milk can be affected by environmental temperature, disease, parity, stage of lactation, breed, and nutrition ( 7 ) . The primary contributor to milk NPN is urea ( 8 ) . Concentrations of NPN in milk increased from 29 to 40 mg/dl, and milk urea N ( MUN) expressed as percentage of NPN increased from 20 to 45%, as the concentration of dietary CP increased from 12.2 to 17.6% of DM (16). Plasma urea N ( PUN) and MUN were positively correlated, probably because of the passive diffusion of urea from plasma to milk ( 6 ) . In their review, DePeters and Ferguson ( 8 ) suggested three sources of urea in milk: end products of protein and NPN digestion, AA catabolism in the liver, and a small fraction contributed by Arg catabolism in the mammary gland. Sources of variation in MUN concentration include BW, parity, stage of lactation, and diurnal variation (10). Gustafsson and Palmquist ( 1 1 ) found that MUN was equilibrated with serum urea following a lag of 1 to 2 h when the rate of change of urea N in serum was 3 to 6 mg/dl per h. Those researchers ( 1 1 ) found that high producing cows experienced a peak in serum urea concentration that was 70 to 85% higher than the lowest concentration of serum urea. Ruminal ammonia ( RA) peaked at 1 h after feeding followed by a decline to the baseline value by 6 h; serum urea concentration peaked at 1.5 to 2.0 h after the RA peak. Overall effects of the wide range of dietary intakes of CP, RDP, and RUP on PUN and MUN were described and used to predict the yield of milk N fractions ( 8 ) . To what extent these relationships are influenced by other factors, such as breed, dietary fat supplementation, and diurnal patterns in RA that are associated with feeding schedule when dietary CP content is held constant, has not been determined.

Abbreviation key: CSFA = Ca soaps of fatty acids, MUN = milk urea N, PUN = plasma urea N, RA = ruminal ammonia.

Received September 9, 1996. Accepted July 11, 1997. 1Present address: Department of Animal Science, Michigan State University, Anthony Hall, East Lansing 48824-1225. 1997 J Dairy Sci 80:3368–3376

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Therefore, the objective of this study was to evaluate the effects of supplemental dietary fat and two ratios of dietary RDP to RUP on diurnal variation of components in the rumen, blood, and milk of Holstein and Jersey cows. MATERIALS AND METHODS Experimental Design and Diets Four Holstein and four Jersey cows (31 to 61 DIM) fitted with ruminal and duodenal cannulas were used in two 4 × 4 Latin squares, one for each breed, balanced for residual effects (each treatment followed each other treatment the same number of times). During the four 15-d periods, cows were housed in tie stalls to determine individual DMI. Ingredients and chemical composition of the diets are shown in Table 1. Diets were formulated ( 1 8 ) to contain 17% CP and 29 or 41% estimated RUP. In situ disappearance from dacron bags indicated that diets contained 23.3 and 40.8% of total protein as RUP (data not shown). Because the CP content of alfalfa silage decreased throughout all periods, the mean CP percentage in

dietary DM was 16.2%. Blood meal was substituted for soybean meal on a protein equivalent basis to increase RUP. Each level of RUP was formulated with and without 2.7% added fat. A portion of shelled corn was replaced with Ca soaps of fatty acids ( CSFA) (Megalac; Church & Dwight Co., Inc., Princeton, NJ). Corn gluten meal was added to supply the equivalent amount of CP from the corn that was removed. All diets were formulated to contain 31% NDF and 19% ADF. A sufficient amount of the total mixed diet was fed at 1000 and 1400 h to allow 5 to 10% orts. Cows were moved to a parlor for milking at 0100 and 1300 h daily, except on d 15 of each period. Measurements and Sampling All procedures for this study were approved by the Virginia Tech Animal Care Committee. The DMI was determined on d 11 through 15 of each period. Forages were sampled weekly, and concentrates were sampled once each period. Milk production was recorded daily. A jugular vein catheter was installed on d 14 at 1400 h and removed on d 15 at 2200 h.

TABLE 1. Ingredient and chemical composition of diets. Treatment1 Composition Ingredient, % of DM Corn silage Alfalfa silage Corn grain Soybean meal Blood meal Corn gluten meal Ca Soaps of fatty acids Mineral and vitamins2 Chemical CP, % of DM NEL,3 Mcal/kg of DM ADF, % of DM NDF, % of DM NFC,3,4 % of DM RUP,3 % of CP EE,5 % of DM Ca,3 % of DM P,3 % of DM

29% RUP

29% RUP + Fat

41% RUP

41% RUP + Fat

30.0 29.0 25.7 13.1

30.0 29.0 21.7 13.1

30.0 29.0 29.9 5.5 3.4

2.2

0.6 3.4 2.2

2.2

30.0 29.0 25.9 5.5 3.4 0.6 3.4 2.2

16.1 1.57 19.3 29.6 44.3 29.0 2.5 0.98 0.56

16.2 1.68 19.3 29.3 41.6 29.1 5.2 1.28 0.55

16.2 1.59 18.7 29.0 45.4 40.7 2.5 0.97 0.53

16.3 1.68 18.7 28.6 42.8 40.6 5.2 1.27 0.52

1Supplemental

fat was provided as Ca soaps of fatty acids, which contained approximately 80% fat. 18% NaHCO3, 16% Ca, 6.5% P, 3.5% K, 2.2% Mg, 3.2% S, 5.8% Cl, 0.027% Fe, 0.013% Cu, 0.0003% Co, 0.11% Mn, 0.13% Zn, 0.002% I, 0.0005% Se, 110,000 IU of vitamin A/kg, 44,000 IU of vitamin D/kg, and 550 IU of vitamin E/kg. 3Estimated using DAIR4 (18). 4Nonfiber carbohydrates. 5Ether extract. 2Contained:

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Three to 5 min before catheterization, cows were given 0.75 ml of Bonamine (Schering-Plough Animal Health Corp., Kenilworth, NJ) intravenously (10 mg/ ml) as an analgesic. The jugular vein catheter was composed of Tygon tubing (Fisher Scientific Co., Pittsburgh, PA) with a 0.02-mm i.d. and a 0.06-mm o.d. After the 1300-h milking in the parlor on d 14, cows were milked in tie stalls with a bucket milking system. The initial milking began at 1600 h on d 14, after which cows were milked at 4-h intervals. Blood, and fecal samples and samples from the rumen and duodenum also were collected every 4 h. Sampling times started at 2000 h on d 14 and ended at 2000 h on d 15. One of two 50-ml milk samples was preserved with Bronopol (The Boots Co. PLC, Nottingham, England) and stored at 2°C until completion of sampling; the milk samples was then analyzed by a DHIA laboratory (Blacksburg, VA) for fat, SNF, and lactose. The duplicate sample was analyzed for total N, casein N, and MUN. Blood samples (10 ml) were collected via the jugular catheter. Blood samples were combined with 100 ml of saline containing 286 U of sodium heparin (Sigma Chemical Co., St. Louis, MO) and centrifuged (Beckman J2-21; Beckman Instruments, Columbia, MD) at 3220 × g for 15 min to isolate plasma for storage at –20°C. Ruminal fluid was collected by siphoning through the ruminal cannula using a tube fitted with a suction strainer inside a plastic pipe with 1.2-cm holes to allow the passage of ruminal liquid. From a 50-ml sample, 5-ml aliquots were preserved with 1 ml of 25% H3PO4 for ammonia analysis or with 1 ml of 25% H3PO4 plus 1 ml of 30 mM isocaproic acid (internal standard) for VFA analysis and stored at –20°C. The remaining portion of each sample was used to determine pH (Accumet pH meter; Fisher Scientific, Raleigh, NC). Analyses Concentrate and forage DM were determined by drying overnight at 60°C in a forced-air oven. Dried samples were ground through a 1-mm screen in a Cyclotec mill (Tecator 1093; Tecator Inc., Ho¨gana¨s, Sweden) and analyzed for total N ( 2 ) , ether extract ( 2 ) , ADF ( 9 ) , and NDF (20). Nonfiber carbohydrates were calculated as 100 – (NDF + CP + ether extract + ash). Milk samples were analyzed for fat and lactose by infrared spectroscopy (Fossomatic 605, B filter; Foss Electric, Hillerød, Denmark) and for total milk N by Kjeldahl analysis ( 2 ) . Journal of Dairy Science Vol. 80, No. 12, 1997

Plasma urea N, MUN, and RA were determined according to the method of Chaney and Marbach ( 5 ) as modified by Weatherburn ( 2 1 ) with the exception that urease concentration was 10-fold higher than recommended. Milk casein N was determined using the procedure of Aschaffenburg and Drewry ( 1 ) . Ruminal fluid VFA concentration was determined by GLC (Varian Instruments, Palo Alto, CA) as described by Wonsil et al. (22), but temperatures measured by the column, injector, and detector were 115, 170, and 180°C, respectively. Statistical Analyses Data were analyzed by ANOVA and repeated measures using the general linear models procedure of SAS (13). Data are presented as least squares means and standard errors of the means. Variation in the dependent variable, Y, was described as follows: Yijklmn = m + Bi + C( i) j + Pk + Rl + Fm + ( R × F ) lm + ( B × R ) il + ( B × F ) im + ( B × R × F ) ilm + (error A ) ijklm + Tn + ( T × all effects) + (error B ) ijklmn where dependent variable; overall mean of Y; effect of breed ( i = 1 or 2); effect of cow within breed ( j = 1, 2, 3, or 4); Pk = effect of period ( k = 1, 2, 3, or 4); Rl = effect of concentration of dietary RUP ( l = 1 or 2); Fm = effect of dietary fat ( m = 1 or 2); ( R × F ) lm = interaction of RUP and fat; ( B × R ) il = interaction of breed and RUP; ( B × F ) im = interaction of breed and fat; ( B × R × F ) ilm = interaction of breed, RUP, and fat; (error A ) ijklm = random residual error (among cows); Tn = effect of time ( n = 1, 2, 3, 4, 5, 6, or 7); ( T × all effects) = interaction of time and all other effects; and (error B ) ijklmn = random residual error (within cows). Yijklmn m Bi C( i) j

= = = =

Breed differences were tested using error A. Breed contrast represents the comparison of least squares means for Holsteins versus Jerseys averaged across treatments. Contrasts for fat, RUP, and their interaction were calculated within each breed. Contrast for fat represents the comparison of diets without added

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fat versus those with added fat and average RUP. The RUP contrast represents the comparison of 29% RUP diets versus 41% RUP diets with average fat. Models for DMI, milk production, and milk component content and yield excluded time; however, models for diurnal variation of milk components, plasma, and ruminal parameters included time. There were six degrees of freedom for the error term to test breed, 15 degrees of freedom for treatments and their interactions with breed, and 90 degrees of freedom for time of testing and its interaction with treatments. RESULTS AND DISCUSSION Dry matter intake decreased 7.7% for Holsteins and 5.4% for Jerseys when fat was added to the diets (Table 2). Adaption to fat was allowed on d 1 through 10 of each period, and DMI was measured on d 11 through 15. Dry matter intake between breeds was not different statistically. Expressed as a percentage of BW, DMI decreased when fat was supplemented. Similar reductions in DMI have been observed when CSFA were fed (15). However, intake of estimated NEL was similar when diets either without or with added fat were fed, 31.3 versus 30.6 Mcal/d for Holsteins and 25.8 versus 26.0 Mcal/d for Jerseys. The reduction in DMI caused by fat supplementation might have been the result of metabolic control of energy intake (12).

Milk production was increased 8.0% when Holsteins were fed the 41% RUP diets. Milk production expressed as 4% FCM also was greater when Holsteins were fed the 41% RUP diets. Lack of a significant response to added fat in this experiment might have been due to the length of the periods (15 d ) or to the small number of cows. Tomlinson et al. ( 1 9 ) concluded that effects of CSFA on milk production were not expressed until wk 4, but protein effects were expressed by wk 3 of feeding. Ruminal pH did not differ among diets except at 1600 h when Jerseys that were fed the 41% RUP diets had a greater pH than did Jerseys that were fed the 29% RUP diets and at 2000 h when Holsteins that were fed added fat had a lower pH than did Holsteins that were not fed added fat (data not shown). Contrary to this result, Rodriguez et al. (15), using the same diets, reported greater ruminal pH when Holsteins were fed the 41% RUP diets. Differences between the studies may be due to different techniques in sampling the ruminal fluid (esophageal tube vs. ruminal cannula) and to the number of cows used. Jerseys had a greater ruminal pH before feeding than did Holsteins (Figure 1). Overall, the response of pH across time included an increase during the 6 h prior to the 1000-h feeding followed by a decrease during the next 6 h. Robinson and McQueen observed a similar reduction in pH after feeding that might have been due to increased availability of fermentable substrate after feeding. The subsequent increase in pH

TABLE 2. Daily DMI, milk production, and efficiency of milk production in response to dietary fat and RUP. Treatment1 29% RUP

Item and breed

29% RUP + Fat

41% RUP

Contrast 41% RUP + Fat

SEM

Fat

RUP

INT2

Breed

P DMI kg/d Holstein Jersey % of BW Holstein Jersey Milk production, kg/d Holstein Jersey 4% FCM, kg/d Holstein Jersey FCM/Mcal of NEL, kg Holstein Jersey

19.1 16.4 3.62 3.99

18.4 15.2 3.45 3.77

20.5 16.3 3.80 4.03

18.1 15.7 3.41 3.88

0.3

0.01 0.02

NS3 NS

0.02 NS

NS

0.08

0.01 0.03

NS NS

NS NS

NS

29.2 22.6

31.4 21.7

32.7 22.8

32.8 23.8

0.7

NS NS

0.01 NS

NS NS

0.05

27.8 27.0

31.2 26.4

32.0 27.2

31.5 28.6

1.0

NS NS

0.01 NS

NS NS

NS

0.06

NS NS

NS NS

NS NS

NS

0.93 1.06

1.01 1.04

0.99 1.05

1.03 1.09

1Supplemental 2Interaction 3P

fat was provided as Ca soaps of fatty acids, which contained approximately 80% fat. of fat and RUP.

> 0.05. Journal of Dairy Science Vol. 80, No. 12, 1997

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Figure 1. Effect of time on ruminal pH in Jerseys ( ÿ) and Holsteins ( o) . *Difference ( P < 0.05) according to breed at 0800 h.

might have been due to a reduction in substrates and a shift in fermentation to fiber digestion before the next feeding. Total VFA were not affected by diet or breed except at 0800 h when Jerseys had a lower total VFA concentration than did Holsteins (data not shown). However, total VFA concentration changed across time, decreasing before feeding (0800 h ) and increasing until 0000 h (Figure 2). The VFA response to

Figure 2. Effect of time on total ruminal VFA ( o) and the ratio of acetate to propionate ( ÿ) in Holstein and Jersey cows. Journal of Dairy Science Vol. 80, No. 12, 1997

Figure 3. Effect of dietary RUP on ruminal valerate concentrations in Holsteins fed the 29% RUP diets ( o) or the 41% RUP diets ( ÿ) and in Jerseys fed the 29% RUP diets ( ◊) or the 41% RUP diets ( ⁄) . aJerseys differed ( P < 0.05) because of dietary RUP content at the indicated times. bHolsteins differed ( P < 0.05) only at 0000 h.

feeding indicated decreased fermentable substrate before feeding followed by an increased availability of fermentable substrate after feeding. The ratio of acetate to propionate was not affected by diet or breed; however, the ratio changed with time. The ratio increased before feeding and decreased after feeding (Figure 2), which was a result of both increased acetate concentrations and decreased propionate concentrations before feeding. Across breeds, valerate concentration was less when the 41% RUP diets were fed (Figure 3). Lower concentrations of valerate caused by the 41% RUP diet occurred in Holsteins at 0000 h and in Jerseys from 2000 to 0800 h. Lower valerate concentrations also were observed by Rodriguez et al. ( 1 5 ) when the 41% RUP diets were fed. Reduced concentrations of butyrate and valerate in response to high RUP diets also were observed by Seymour et al. (17), which might indicate decreased ruminal fermentation of AA. In contrast to the diurnal pattern in ruminal valerate concentrations, isobutyrate concentrations were elevated prior to feeding (Figure 4). However, in Holsteins fed the 41% RUP diets compared with Holsteins fed the 29% RUP diets, isobutyrate concentrations were lower from 0000 to 0800 h. Jerseys had a greater isobutyrate concentration than did Holsteins at 0400 and 0800 h. No interactions of protein and fat occurred for valerate or isobutyrate.

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Figure 4. Effect of dietary RUP on ruminal isobutyrate concentrations in Holsteins fed the 29% RUP diets ( o) or the 41% RUP diets ( ÿ) and in Jerseys fed the 29% RUP diets ( ◊) or the 41% RUP diets ( ⁄) . aHolsteins differed ( P < 0.07) because of dietary RUP content at the indicated times.

Figure 5. Effect of dietary RUP on ruminal ammonia concentrations in Holsteins fed the 29% RUP diets ( o) or the 41% RUP diets ( ÿ) and in Jerseys fed the 29% RUP diets ( ◊) or the 41% RUP diets ( ⁄) . aHolsteins differed ( P < 0.07) because of dietary RUP content. bJerseys differed ( P < 0.05) at the indicated times.

Holsteins fed diets with added fat had lower RA concentrations at 0000 h and higher concentrations at 1200 h than did Holsteins fed diets with no added fat. However, an interaction of protein and fat occurred at 0000 h: Holsteins that were fed added fat and 29% RUP had lower RA concentrations; RA concentrations did not differ between fat treatments when Holsteins were fed the 41% RUP diet. Increased RA in Holsteins after the first feeding of diets with added fat might have been due to the lower concentration of nonstructural carbohydrates in these diets. When dietary RUP concentrations were compared (Figure 5), RA concentration was lower overall, and, in Holsteins, the RA concentration was lower at all times except at 1600 h. Jerseys differed in response to dietary RUP only at 2000 and 0400 h. A peak in RA was observed 2 h after feeding at 1000 h, and the concentration doubled from the lowest (0800 h ) to the highest point (1200 h). When cows were fed the 29% RUP diets, the peak was 12.6% greater than when cows were fed the 41% RUP diets. A similar increase in RA concentration throughout a 24-h period and in peaks after feeding were observed for cows when soybean meal was compared with blood and corn gluten meal (14). Also, Gustafsson and Palmquist ( 1 1 ) observed a peak in RA concentration at 1 h after feeding. Although no dietary effects were observed, PUN concentration varied across time, increasing from

0000 to 0400 h and from 0800 to 1200 h and decreasing from 0400 to 0800 h and from 1600 to 2000 h (Figure 6). An apparent increase in PUN after feeding might have been partially due to increased RA concentration. Gustafsson and Palmquist ( 1 1 )

Figure 6. Effect of time on concentrations of ruminal ammonia N ( ⁄) , plasma urea N ( o) , and milk urea N ( ÿ) . Journal of Dairy Science Vol. 80, No. 12, 1997

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reported that PUN increased 2 h after feeding. Milk urea N at 1200 and 2000 h was lower when Holsteins were fed diets with added fat. Diet had no effect on MUN at any other time. However, across time, MUN concentrations tended to increase after feeding (Figure 6). A breed effect was noted at 0800 and 1200 h when Holsteins had greater MUN concentrations than did Jerseys. In a previous study (15), Holsteins had greater concentrations of MUN than did Jerseys prefeeding and postfeeding. In this study, concentrations of MUN followed very closely the pattern of PUN, although MUN concentrations were greater than PUN concentrations. In contrast, in the study of Gustafsson and Palmquist (11), when samples were collected hourly, there was a 1-h lag for equilibration between MUN and PUN, and PUN concentrations were less than MUN concentrations when PUN was declining. Roseler et al. ( 1 6 ) reported that MUN tended to be less than PUN in cows fed diets varying in RDP. Our results indicated that MUN was greater than PUN when MUN concentrations were measured in whole milk. Roseler et al. ( 1 6 ) removed the protein and fat from the milk before analysis, which perhaps may explain the difference. However, we have no other explanation why MUN was greater than PUN in the present study. Table 3 contains overall milk component yields and concentrations. Fat yield was not affected by diet or

breed. Protein yield was not affected by added fat, but 41% RUP diets increased protein yield in Holsteins because of an increase in milk production; milk protein concentration was not affected. Contrary to this result, Rodriguez et al. ( 1 5 ) reported that Holsteins had reduced protein yield and content when the 41% RUP diets were fed. Increased lactose and SNF yields were also due to increased milk production when Holsteins were fed the 41% RUP diets. The SNF content, however, was decreased when Holsteins were fed added fat, which might have been due to a nonsignificant decrease in milk protein content when these diets were fed. The concentration of MUN was 6.5% lower when Holsteins were fed added fat, similar to the response that was previously observed by Rodriguez et al. (15). Concentrations of MUN expressed as percentages of total N, however, were not affected by diet. A greater MUN concentration was noted for Holsteins than for Jerseys (5.28 vs. 3.74 g/ 100 g of total N). In this study, milk components were sampled seven times during the day, and the mean of the last six samples was used to compare the effect of the diets. The lack of significant differences in milk component contents in this experiment compared with those of Rodriguez et al. ( 1 5 ) might have been due to the diurnal variation of the components or to an insufficient number of cows. Although overall milk protein content did not differ because of diet, some differences in response to added

Figure 7. Effect of dietary fat on milk protein content in Holsteins fed no supplemental fat ( o) or supplemental fat ( ÿ) and in Jerseys fed no supplemental fat ( ◊) or supplemental fat ( ⁄) . aHolsteins differed ( P < 0.05) because of dietary fat.

Figure 8. Effect of dietary RUP on milk protein content in Holsteins fed the 29% RUP diets ( o) or the 41% RUP diets ( ÿ) and in Jerseys fed the 29% RUP diets ( ◊) or the 41% RUP diets ( ⁄) . aHolsteins differed ( P < 0.05) because of dietary RUP content. bJerseys differed ( P < 0.05) at the indicated times.

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fat (Figure 7 ) and RUP (Figure 8 ) were evident during the 24-h period. Milk protein percentage was numerically lower throughout the day when Holsteins were fed added dietary fat compared with those fed no added fat. In contrast, Jerseys had lower milk protein content throughout the day when fed the 41% RUP diets versus the 29% RUP diets. Bartsch et al. ( 3 ) reported that milk protein content did not change over time when cows were milked and fed every 3 h in a 24-h period. However, when cows were milked every

3 h during a 24-h period during which feed was withheld, milk protein content decreased from 3.65 to 3.15%. Reduced protein content before feeding might have been due to an insufficient supply of AA to the mammary gland (23). CONCLUSIONS Lower concentrations of RA, valerate, and isobutyrate indicated increased resistance of protein

TABLE 3. Milk component yields and contents in response to dietary fat and RUP. Treatment1 Item and breed

29% RUP

29% RUP + Fat

41% RUP

Contrast 41% RUP + Fat

SEM

Fat

RUP

(kg/d) Fat Holstein Jersey Protein Holstein Jersey Lactose Holstein Jersey SNF Holstein Jersey Fat Holstein Jersey Protein Holstein Jersey Lactose Holstein Jersey SNF Holstein Jersey Casein N Holstein Jersey Urea N Holstein Jersey Casein N Holstein Jersey Urea N Holstein Jersey

Int2

Breed

P<

1.07 1.19

1.24 1.18

1.26 1.20

1.22 1.27

0.05

NS3 NS

NS NS

NS NS

NS

0.86 0.84

0.87 0.78

0.99 0.80

0.94 0.81

0.03

NS NS

0.01 NS

NS NS

NS

1.51 1.13

1.63 1.09

1.69 1.15

1.69 1.20

0.04

NS NS

0.01 NS

NS NS

0.05

2.64 2.14

2.78 2.91 2.04 2.13 (g/100 g of milk)

2.87 2.21

0.07

NS NS

0.02 NS

NS NS

NS

3.69 5.30

3.96 5.51

3.89 5.32

3.69 5.38

0.16

NS NS

NS NS

NS NS

0.01

2.98 3.72

2.75 3.65

3.05 3.56

2.90 3.45

0.10

NS NS

NS NS

NS NS

0.05

5.15 4.97

5.16 4.99

5.16 5.04

5.11 5.05

0.04

NS NS

NS NS

NS NS

NS

9.01 9.44

8.80 9.44

8.91 9.35

8.75 9.34

0.05

0.01 NS

NS NS

NS NS

0.05

0.347 0.457

0.337 0.442

0.358 0.438

0.343 0.428

0.011

NS NS

NS NS

NS NS

0.01

0.0248 0.0216

0.0226 0.0240 0.0208 0.0210 (g/100 g of total N )

0.0230 0.0214

0.0006

0.03 NS

NS NS

NS NS

NS

1.6

NS NS

NS NS

NS NS

NS

0.34

NS NS

NS NS

NS NS

0.05

74.9 78.5 5.42 3.57

78.3 77.8 5.60 3.66

75.1 78.9 5.25 3.78

75.5 78.9 4.86 3.97

1Supplemental 2Interaction 3P

fat was provided as Ca soaps of fatty acids, which contained approximately 80% fat. of fat and RUP.

> 0.05. Journal of Dairy Science Vol. 80, No. 12, 1997

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to ruminal degradation when the 41% RUP diets were fed. A major factor that influences bacterial growth in the rumen is availability of fermentable substrate (amount and time of feeding). This suggestion was supported by the fact that before feeding, concentrations of RA, total VFA, propionate, and valerate were lower, and ruminal pH and acetate concentration were higher. However, after feeding, concentrations of RA, total VFA, propionate, valerate, and isovalerate increased, and acetate and pH decreased. Differences in ruminal fermentation, especially ammonia concentration, because of the percentage of RUP, did not affect PUN or MUN. The major factor that affected RA, PUN, and MUN was time relative to feeding. Increased concentrations of PUN and MUN after feeding might have been partially due to increased RA. Some milk proteins arise from preformed plasma proteins, but the major source is synthesis from free AA in the secretory cells of the mammary gland. Milk protein changed throughout the day. Diurnal variation of milk N components might indicate variable supply of AA relative to time of feeding. REFERENCES 1 Aschaffenburg, R., and J. Drewry. 1959. New procedures for the routine determination of the various non-casein proteins of milk. Proc. 15th Int. Dairy Congr. London, England 3:1631. 2 Association of Official Analytical Chemists. 1990. Official Methods of Analysis. 15th ed. AOAC, Arlington, VA. 3 Bartsch, B. D., C. G. Beck, R. B. Wickes, and A. F. Hehir. 1981. Influence of milking interval and feeding strategy on the composition of milk and milk fat. Aust. J. Dairy Technol. 36:26. 4 Cerbulis, J., and H. M. Farrell, Jr. 1975. Composition of milk of dairy cattle. I. Protein, lactose and fat contents and distribution of protein fraction. J. Dairy Sci. 58:817. 5 Chaney, A. L., and E. P. Marbach. 1962. Modified reagents for determination of urea and ammonia. Clin. Chem. 8:130. 6 Clark, J. H., H. R. Spires, and C. L. Davis. 1978. Uptake and metabolism of nitrogen components by the lactating mammary gland. Fed. Proc. 37:1233. 7 DePeters, E. J., and J. P. Cant. 1992. Nutritional factors influencing the nitrogen composition of bovine milk: a review. J. Dairy Sci. 75:2043.

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