Use of Anionic Salts with Grazing Prepartum Dairy Cows

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Soder and Holden The Professional Animal Scientist 15:278–285

UsePrepartum of Anionic Salts with Grazing Dairy Cows K. J. SODER1, PAS, and L. A. HOLDEN2 Department of Dairy and Animal Science, The Pennsylvania State University, University Park, PA 16802

gestation and again for 10 d during wk 4 and 12 postpartum for estimation of Twenty multiparous and four primigravid intake. Cows calved on pasture and were then integrated into the regular milking Holstein cows were utilized in a completely random design to characterize the herd and fed a total mixed ration (TMR). influence of decreasing prepartum dietary Daily milk yield and weekly milk samples were collected through wk 14 of cation-anion difference (DCAD) from lactation. Prepartum and postpartum +388 to +183 meq/kg diet on DMI, prepartum blood profiles, and postpartum DMI, milk yield and composition, and milk yield and composition of dairy cows plasma minerals were not affected by treatment. No clinical cases of milk fever grazed during the prepartum period. were observed for either treatment group. Treatments began on wk –4 prepartum, Reducing prepartum DCAD from +388 to continued through calving, and consisted +183 meq/kg DM did not improve of 1) pasture and grain pellet without anionic salts (control; +388 meq/kg) or 2) prepartum blood profiles or postpartum milk yield or composition; therefore, this pasture and grain pellet containing anionic salts (AS) (+183 meq/kg). Prepar- type of supplementation was not economical. tum cows were rotationally grazed as a single group and individually fed pellets (Key Words: Cation-Anion, Grazing, twice daily at a rate of 0.5% of BW/d. Prepartum, Intake, Dairy.) Blood and urine samples were collected on wk –4, –2.5, and –1 prepartum and analyzed for Ca, Mg, K, Na, and Cl concentrations. Urine samples were also Management and nutrition of the analyzed for pH. Chromic oxide was dosed twice daily during the last 4 wk of prepartum dairy cow can significantly affect postpartum health, including incidence of ketosis, milk fever (parturient paresis), retained 1Current address: USDA/ARS, Pasture Systems placenta, displaced abomasum, and and Watershed Mgmt. Research Lab., Univer- metritis (8, 22). Incidence of these sity Park, PA 16802. disorders can significantly affect the 2To whom correspondence should be ad- profitability of the dairy enterprise by dressed: [email protected] decreasing milk yield, increasing Reviewed by L. A. Brown, F. A. Martz, and M. L. veterinary treatment costs, and Westendorf. decreasing cow longevity (5). Under-

Abstract

Introduction

standing the metabolic changes that occur in the dairy cow during this period is crucial in the development of nutrition and management strategies to minimize the incidence of health problems and increase profitability. Dietary cation-anion difference, defined as the balance of positively and negatively charged non-metabolizable ions in the diet (38), affects Ca metabolism (4, 8, 26) and systemic acid-base status (34) of prepartum dairy cows. Balancing prepartum rations for DCAD with AS has successfully decreased incidence of milk fever and improved subsequent milk yield of prepartum dairy cows fed stored feeds (8, 26). Anionic salts have proven effective in confinement feeding situations; however, many dairy producers in the northeastern United States have expressed interest in grazing prepartum dairy cows. Research on the nutritional requirements of these grazing animals in the United States is limited. Mineral content of pasture differs from stored forages; in particular, cations such as K tend to be more highly concentrated in pasture than in stored forages (23). High K concentrations in the prepartum diet can alter the DCAD, predisposing cows to milk fever at parturition by decreasing Ca resorption from bone (7, 9,

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30, 36). The effect of high K concentrations in pasture on prepartum dairy cows is unknown. In addition, the effectiveness of AS fed to prepartum dairy cows consuming pasture diets has not been evaluated. Considering the extremely high K concentrations commonly found in pasture, it would be nearly impossible to balance the prepartum DCAD to the recommended concentration of –50 to –100 meq/kg dietary DM (15). In addition, AS is a relatively expensive feed supplement. However, there may be some benefit to decreasing the prepartum DCAD, even by a fraction of the original concentration. Therefore, the objective of this experiment was to evaluate the effects of decreasing the prepartum DCAD from +388 to +183 meq/kg on DMI, prepartum blood and urine profiles, and postpartum milk yield and composition of Holstein cows grazed during the prepartum period.

Materials and Methods Cows and Dietary Treatments. This experiment was approved by the Institutional Animal Care and Use Committee of The Pennsylvania State University. Twenty multiparous Holstein cows and four primigravid Holstein cows were assigned to treatment according to parity, BW, and previous milk yield (projected milk yield for primigravid cows) at wk –4 prior to the expected calving date. The mean cow BW at the beginning of the experiment (wk –4 prior to the expected calving date) was 700 kg. Cows were rotationally grazed during the prepartum period on pasture containing orchardgrass (Dactylis glomerata L.), with lesser amounts of smooth bromegrass (Bromus inermis L.) and Kentucky bluegrass (Poa pranteneis), at a stocking rate of 1.6 cows/ha beginning at wk –4 prior to expected calving date, i.e., June 10, 1996, and continuing through calving. Cows grazed each of 12 1.21ha paddocks for 2 d so that the rest interval between grazing paddocks was 22 d. Water was available on all paddocks. Cows began grazing

pasture at dry off (beginning at approximately wk –8 prior to the expected calving date) and were supplemented with free-choice trace mineral salt blocks (Morton IOFIXT TM salt block; contained 98% NaCl, 0.35% Zn, 0.28% Mn, 0.175% Fe, 0.035% Cu, 0.007% I, 0.007% Co; Morton International, Chicago IL). At approximately wk –4 prior to the expected calving date, cows were assigned to treatment based on previous milk yield (projected milk yield for primigravid cows), BW, and body condition. Treatments consisted of 1) pasture and grain pellet without AS (control; +388 meq/kg dietary DM) or 2) pasture and grain pellet containing AS (+183 meq/kg dietary DM; Table 1). The DCAD was calculated as milliequivalents of [(Na+ + K+) – (Cl– + S2–)]/kg total dietary DM [pasture plus supplement; (15)]. The nutrient content of pellets was similar except for anions (Cl– and S2–). Cows were rotationally grazed as a single group and individually fed pellets twice daily at a rate of 0.5% of

BW/d. All cows consumed all pellets offered, so orts were not recorded. Cows calved on pasture and then were integrated into the regular milking herd and fed a TMR containing corn silage, legume silage, chopped legume hay, and a grain and mineral mix (Table 2). Experimental Measures. Intake was estimated twice prior to parturition during wk –3 and –1 prepartum using Cr2O3 as an indigestible fecal marker. Cows were dosed twice daily (at pellet feeding time) for 4 wk with 2 g Cr2O3 via gelatin capsule. Fecal grab samples were collected once daily during the morning pellet feeding for 5 consecutive d each period. Pellets and pastures were sampled beginning 1 d prior to the start of fecal collections for 5 consecutive d. Pasture samples were plucked by hand from 20 randomly selected areas of the paddock to the approximate height to which the cows grazed (14). Blood samples were collected once after the morning pellet feeding

TABLE 1. Ingredient composition of control and anionic salt (AS) grain pellets (DM basis). Ingredient

Control

AS pellets (%)

Dry shelled corn Wheat middlings Monosodium phosphatea Limestone Calcium sulfate gypsum Salt Se premixb (0.06% Se) Penn State trace mineral premix #4®,b,c Vitamin Dd Magnesium sulfate Calcium chloride Magnesium oxide Liquid molasses Pellet binderb

50.51 31.10 0.61 10.70 ... 1.05 0.17 0.22 0.33 ... ... 0.83 3.53 0.95

49.44 30.46 0.57 7.24 0.92 1.03 0.11 0.23 0.34 2.43 2.85 ... 3.46 0.92

aContained 26% P. bAgway, Inc. (Syracuse, NY). cContained 25.3% Ca, 5.8% S, 5455 ppm Cu, 20,202 ppm Fe, 0.1 ppm I, 17,172

ppm Mn, and 54,545 ppm Zn. dContained 227,273 IU/kg vitamin D.

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via gelatin capsule. Fecal grab samples were collected once daily during the TABLE 2. Ingredient composition morning feeding on d 7 to 11 of each of postpartum TMRa (DM basis). period. The TMR samples were collected daily for 5 consecutive d Ingredient beginning 2 d prior to the start of fecal collections for nutrient and (%) chemical analyses. HMb shelled corn 14.79 Following calving, cows were HMc barley 9.89 milked at 0500 and 1700 h. Milk yield MMc legume hay 3.92 was recorded daily at each milking Legume silage 17.73 through wk 14 of lactation. Milk Corn silage 25.17 samples were taken once weekly at Dry shelled corn 5.88 consecutive a.m. and p.m. milkings Dry brewers grain 2.47 for analysis of fat, protein, and milk Light distillers corn 2.47 ®,d urea N. Ren Plus 1.98 Oats 2.47 Cows were weighed once every 2 Animal fat 1.96 wk beginning on wk –4 prepartum Soybean meal (48% CP) 8.81 and continuing through wk 14 of Dicalcium phosphate 0.59 lactation. Three independent observLimestone 0.16 ers scored cows for body condition Magnesium oxide 0.04 based on a five-point scale [1 = thin Salt 0.49 to 5 = fat; (40)] on the same day the 0.04 Se premixe (0.06% Se) cows were weighed. Sodium bicarbonate 0.98 Sample Analyses. Pasture, pellet, Penn State trace mineral premix #4®,e,f 0.10 and TMR samples were analyzed for Vitamins A, D, and Eg 0.04 DM, OM, and CP (2); ADF and NDF Vitamin D 650Kh 0.02 [(18); ANKOM200 fiber analyzer; ANKOM Technology Corporation, aTMR = Total mixed ration. Fairport, NY]; IVDMD [(13); ANKOM bHM = High moisture. Daisy II; ANKOM Technology CorpocMM = Mixed mostly. ration]; and mineral content using dMoyer Packing (Souderton, PA). wet chemistry techniques [(2); Cumberland Valley Analytical Services, eAgway, Inc. (Syracuse, NY). Maugansville, MD]. fContained 25.3% Ca, 5.8% S, 5455 Fecal samples were stored at –20°C ppm Cu, 20,202 ppm Fe, 0.1 ppm I, and subsequently thawed, dried at 17,172 ppm Mn, and 54,545 ppm Zn. gContained 1,360,000 IU/kg vitamin A, 55°C in a forced-air oven, and ground through a 1-mm screen using 454,545 IU/kg vitamin D, and 1363 IU/ kg vitamin E. a Wiley mill (Arthur H. Thomas, hContained 295,455 IU/kg vitamin D. Philadelphia, PA). Fecal samples were composited by cow and period and analyzed for DM, OM (2), and Cr content according to the procedure of Parker et al. (27). Fecal output was calculated by the during wk –4, –2.5, and –1 prepartum following equation: fecal output (FO) via jugular venipuncture. Urine = (g Cr dosed/d)/(g Cr/g fecal DM). samples were collected via manual Intake was estimated using the stimulation after the morning pellet equation DMI = FO/(1 – IVDMD). feeding once during wk –4, –2.5, and The IVDMD for pasture was used in –1 prepartum. the equation. The first set of calculaIntake was estimated twice after tions utilized the IVDMD from calving (while cows were consuming pasture alone. Pasture DMI was a TMR) during wk 4 and 12 postpardetermined by difference [total DMI tum using Cr2O3. Cows were dosed (estimated with Cr2O3) – actual pellet twice daily for 10 d with 3 g Cr2O3 DMI]. Once pasture DMI was deter-

mined, a weighted mean for IVDMD (weighted for pasture and pellet IVDMD) was calculated and used to correct total DMI. Therefore, pasture DMI was determined with the new total DMI from the corrected IVDMD, and only calculations from the corrected values are presented (14). Milk samples were preserved in 2bromo-2-nitropropane-1,3 diol and analyzed for fat, protein, and milk urea N by the Pennsylvania DHIA testing laboratory (infrared analysis; Foss 605B Milko-Scan; Foss Electric, Hillerød, Denmark). One 7-mL aliquot of blood was collected in tubes containing sodium heparin and 4% sodium fluoride for glucose analysis; the remainder (~15 mL) was allowed to clot at room temperature (23°C) and was centrifuged at 4000 x g for 20 min. The supernatant was retained for nonesterified fatty acid (NEFA) and mineral analyses. One 50mL urine sample was acidified with 12N HCl to reduce the pH of the urine in the collection vessel below 2.5 and was frozen (–20°C) for later analysis of creatinine concentration [(12; Sigma kit number 555-A; Sigma Chemical Co., St. Louis, MO]. Another 50-mL sample was frozen without acidification for mineral analysis. Blood and urine samples were analyzed for Ca, Mg, K, and Na concentration using a flame spectrophotometer (IL Video 22; Thermo Jarrell Ash, Franklin, MA) and for Cl concentration using an ion chromatograph (DX-500; Dionex Corp., Sunnyvale, CA). Blood samples were analyzed for glucose [(28); Sigma kit trender 500; Sigma Chemical Co.] and NEFA [(17); Wako NEFA-C kit; Biochemical Diagnostics, Edgewood, NJ] concentrations. Statistical Analyses. Data were analyzed using the GLM procedure of SAS (31). The model Y = treatment + cow(treatment) + week + treatment x week + error was used for all analyses. Cow(treatment) was used as an error term to test the effect of treatment in all models. All means presented are least squares means. Differences

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between treatments were declared at P < 0.05.

Results and Discussion The ingredient and nutrient compositions of the prepartum pasture and grain pellets and postpartum TMR are presented in Tables 1, 2, and 3. The nutrient content of the diets met or exceeded the NRC (23) requirements for cows at production levels observed in this experiment. Total nutrient intakes, both prepartum and postpartum, are summarized in Table 4. Prepartum pasture DMI, total prepartum (pasture plus pellet) DMI, and postpartum DMI were not affected by treatment (Table 5). Pasture prepartum DMI and total prepartum DMI were not affected by week (Table 6). As expected, postpartum DMI was higher (P=0.02) at wk 12 postpartum when compared with that at wk 4 postpartum. Response of DMI to AS in diets fed during confinement has been variable; some research (24, 35) showed decreased DMI; other studies (4, 6, 25) showed no decrease in DMI. It is important to keep in mind that each study used a different source and amount of AS; in addition, the DCAD of the anionic diets varied widely, ranging from –170 to +480 meq/kg dietary DM. Horst et al. (15) recommends a DCAD of –50 to –100 meq/kg dietary DM for optimal prevention of milk fever. Because of the extremely high K content of the pasture in the present study (3.19%), it was not feasible to decrease the DCAD to the optimal concentration because of pelleting problems, palatability concerns, and toxicity considerations. Because the DCAD in the treatment containing AS was not within this range, the metabolic acidbase status of the cows might not have been altered sufficiently to observe a DMI response. Although few data are available on DMI of grazing prepartum dairy cows, our DMI results are similar to others (3, 11) who measured DMI of prepartum cows fed stored feeds.

TABLE 3. Nutrient and chemical composition of pasture, control grain pellet, anionic salt (AS) grain pellet, and postpartum TMRa (DM basis). Item

Pasture

Control pellet

AS pellet

Postpartum TMR

DM, % CP, % NELb, Mcal/kg ADF, % NDF, % IVDMD, % Ca, % P, % Mg, % K, % S, % Na, % Cl, % Calculated DCADc, meq/kg DM

21.8 27.8 1.67 21.6 46.1 63.3 0.45 0.39 0.18 3.19 0.21 0.013 0.46 +562.77

89.3 12.0 1.80 5.50 18.2 87.9 4.05 0.55 0.74 0.62 0.35 0.63 0.85 -25.30

89.6 11.1 1.80 5.30 18.1 86.1 4.45 0.57 0.66 0.68 0.98 0.67 2.33 -803.10

48.50 16.00 1.66 21.70 34.00 76.50 0.78 0.44 0.24 1.51 0.22 0.22 ... ...

aTMR

= Total mixed ration. according to NRC (23). cDCAD = Dietary cation-anion difference; Calculated from actual nutrient composition. bEstimates

TABLE 4. Total nutrient intake of cows fed a prepartum pasture diet with either control or anionic salt (AS) pellet and postpartum TMRa. Prepartum Item

Control

CP, kg/d NEL, Mcal/d ADF, kg/d NDF, kg/d Ca, g/d P, g/d Mg, g/d K, g/d S, g/d Cl, g/d Estimated DCADd, meq/kg Calculated DCADe, meq/kg

3.47 24.94 2.56 5.70 201.22 63.11 47.41 370.04 36.01b 82.01b +388 +438

aTMR = Total mixed ration. b,cMeans within a row with

AS 3.32 24.30 2.45 5.49 217.94 62.51 44.11 358.19 59.54c 137.47c +183 +209

Postpartum TMR

SEM

3.75 39.00 5.08 7.98 186.33 102.88 56.65 355.25 51.47 ... ... ...

0.16 1.40 0.22 0.35 8.00 4.56 2.96 17.05 3.76 3.76 ... ...

different superscripts differ (P<0.05).

dDCAD = Dietary cation-anion difference; estimated from ration formulation results. eCalculated from actual nutrient composition and assuming 10.65 kg/d DM pasture

intake and 4 kg/d DM pellet intake.

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These studies (3, 11) reported mean DMI values of 10 to 14 kg/d for prepartum dairy cows. Because we measured DMI at specific times during the prepartum period rather than continuously, we were unable to evaluate whether DMI decreased during the last 10 d of gestation, as has been reported previously (3, 11). Mean daily milk yields, 4% fatcorrected milk, milk urea N, and percentages and yields of milk protein and milk fat were not affected by treatment (Table 5; Figure 1). Mean prepartum and postpartum BW, body condition, and their changes were also unaffected by treatment (Table 5). Anionic salts have been shown to decrease incidence of milk fever (4, 9, 24) and increase milk yield (4, 10, 26) of prepartum dairy cows fed stored feeds. However, milk yield responses have been most frequently observed when the DCAD was maintained in the recommended range of –50 to –100 meq/kg dietary DM (15). The DCAD of the diets in the present study were not within the recommended range; that, in combination with the absence of DMI response, may partially explain the lack of differences in milk yield and milk composition. Plasma NEFA, glucose, Ca, K, Na, and Cl concentrations were not affected by treatment or time (Tables 7 and 8). Plasma Mg concentrations were not affected by treatment; however, plasma Mg concentrations were higher (P=0.02) at wk –4 prepartum than at wk –2.5 or –1 prepartum. The lower Mg concentrations as calving approached might have resulted from increased lipolysis normally associated with late gestation (11). Lipolysis may be associated with hypomagnesia as mobilization of fat results in the redistribution of Mg into adipocyte cells (19). Although high pasture K concentrations have been shown to increase risk of hypomagnesemia (21), neither group of cows was hypomagnesemic at any sampling period during the experiment (1). The Ca and Mg contents of the

Soder and Holden

TABLE 5. Mean DMI, milk yield, milk composition, BW, and body condition of cows fed a prepartum pasture diet with either control or anionic salt (AS) pellet (DM basis). Item

Control

Prepartum DMIa Pasture, kg/d Total, kg/dc Total, % of BWc Postpartum DMId Total, kg/d Total, % of BW Milk yield, kg/d Yield 4% fat-corrected milk Milk composition Protein, % Protein, kg/d Fat, % Fat, kg/d Urea N, mg/dL BW, kg Prepartum Postpartum BW change, kg Prepartum Postpartum Body conditione Prepartum Postpartum Body condition change Prepartum Postpartum aMeans

AS

SEM

P

10.88 14.64 2.06

10.41 14.25 1.99

0.52 0.59 0.07

NSb NS NS

23.15 3.49

23.85 3.68

1.01 0.15

NS NS

41.75 37.17

41.59 38.85

0.97 1.24

NS NS

2.96 1.22 3.31 1.43 12.82

2.96 1.20 3.42 1.41 12.95

0.05 0.06 0.15 0.10 0.57

NS NS NS NS NS

712.1 659.8

713.6 637.8

24.70 19.86

NS NS

32.1 –32.6

25.4 –46.6

13.2 13.2

NS NS

3.43 2.80

3.38 2.98

0.12 0.12

NS NS

0.08 –0.17

0.10 –0.17

0.09 0.09

NS NS

pooled across wk –3 and –1 collection periods.

bP>0.05. cPasture plus grain pellet. dMeans eThe

pooled across wk 4 and 12 collection periods. body condition score was based on a scale of 1 to 5 [1 = thin to 5 = fat; (40)].

prepartum diets in this study were higher than NRC (23) recommendations. Generally, Ca and Mg contents of prepartum diets are low to stimulate Ca resorption from bones prior to parturition (23). If AS are fed, the Ca and Mg contents are increased to account for decreased absorption across the intestinal epithelium (9). In the present study, dietary Ca and Mg were increased in both treatments (rather than in just the treatment containing AS, as may be expected)

to prevent a confounding effect of different dietary concentrations of Ca and Mg. Preliminary field trials suggest that increasing dietary Ca concentrations >150 g/d and increasing dietary Mg concentrations >40 g/d may result in a protective effect against milk fever (Richard Adams, The Pennsylvania State University, personal communication). Preliminary data (29, 36) support the high Ca theory, showing a possible link between

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TABLE 6. Least squares DMI means by week prepartum or postpartum (DM basis). wk prepartum Item Prepartum DMI Pasture, kg/d Total, kg/db Total, % of BWb Postpartum DMI Total, kg/d Total, % of BW

wk postpartum

–3

–1

4

12

SEM

P

10.54 14.28 2.00

10.76 14.61 2.04

... ... ...

... ... ...

0.20 0.20 0.03

NSa NS NS

... ...

... ...

22.73c 3.37c

24.27d 3.79d

0.44 0.08

0.02 0.02

aP>0.05. bPasture plus grain pellet. c,dMeans within a row with

different superscripts differ (P<0.05).

extremely high dietary Ca concentrations and decreased incidence of milk fever. The high dietary Ca and Mg concentrations in this study might have provided a protective effect against milk fever in the present study, but this theory needs further validation. Neither treatment group was hypocalcemic or hypomagnesemic, as plasma Ca and Mg concentrations were within physiologically normal ranges (32). Additionally, no clinical cases of milk fever were observed in the present study. Tucker et al. (36) reported that confined cows fed varying DCAD (+93.5 vs -34.1 meq/kg dietary DM) and 1.6% dietary Ca showed no differences in plasma Ca, Mg, or Cl

Figure 1. Mean daily milk yield during the first 14 wk of lactation of cows fed a prepartum pasture diet with either control (•n•) or anionic salt (•) pellet.

concentrations. Oetzel et al. (26) reported higher Na concentrations prior to calving in confined cows fed a diet containing a DCAD of –75 meq/kg dietary DM compared with cows fed a diet containing a DCAD of +188 meq/kg dietary DM. In

contrast, Gaynor et al. (8) found no differences among plasma Na concentrations in cows fed a diet containing a DCAD of +22, +59, or +126 meq/100 g dietary DM. However, sulfur was not included in the DCAD equation in this study. Lack of differences in blood mineral profiles in the present study might have been partially a result of the fact that the DCAD of both diets was positive; that is, the high K content of the pasture exceeded the ability to decrease the DCAD to the recommended concentration. Urinary Mg, K, and Na concentrations, expressed as milligrams of mineral per milligram of creatinine, were not affected by treatment (Table 7). As expected, cows fed AS had higher (P=0.02) urinary Ca and higher (P<0.01) Cl concentrations than cows on the control treatment. Urinary pH was lower (P<0.01) for cows on the treatment with AS than for cows on the control treatment (Table 7) and ranged from 8.14 to 8.51 for cows on the control treatment and 5.80 to 8.43 for cows on

TABLE 7. Mean prepartum blood and urine profiles in cows fed a pasture diet with either control or anionic salt (AS) pellet. Item

Control

AS

SEM

P

Plasma NEFA, µeq/L Plasma glucose, mg/dL Plasma minerals, mg/dL Ca Mg K Na Cl Urine mineral excretion, mg mineral/mg creatinine Ca Mg K Na Cl Urine pH

289.15 65.87

312.26 65.48

23.65 0.92

NSa NS

10.31 2.25 19.38 169.15 463.24

10.50 2.24 19.14 169.90 472.25

0.11 0.03 0.87 2.28 8.95

NS NS NS NS NS

0.10b 0.29 239.39 1.30 44.95b 8.35b

0.23c 0.20 186.54 1.05 86.65c 7.72c

0.04 0.05 20.12 0.16 7.78 0.10

aP>0.05. b,cMeans

within a row with different superscripts differ (P<0.05).

0.02 NS NS NS <0.01 <0.01

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TABLE 8. Least squares means by week for prepartum blood and urine profiles. wk prepartum Item

–4

–2.5

–1

SEM

P

Plasma NEFA, µeq/L Plasma glucose, mg/dL Plasma minerals, mg/dL Ca Mg K Na Cl Urine mineral excretion, mg mineral/mg creatinine Ca Mg K Na Cl Urine pH

284.11 65.66

295.79 65.83

322.06 65.53

11.15 0.57

NSa NSa

10.40 2.31b 19.92 167.06 458.61

10.49 2.19c 18.38 167.13 477.67

10.33 2.22c 19.50 174.39 466.95

0.12 0.03 0.79 2.23 9.50

NSa 0.02 NSa NSa NSa

0.04b 0.15 256.19 1.27b 29.00b 8.16

0.18c 0.37 207.85 1.41b 84.95c 8.00

0.26d 0.22 174.85 0.84c 83.46c 7.94

0.03 0.07 24.52 0.15 9.90 0.12

<0.01 NSa NSa 0.03 <0.01 NSa

aP>0.05. b,c,dMeans

within a row with different superscripts differ (P<0.05).

the treatment with AS. Urinary excretion of Mg and K was not affected by week (Table 8). Urinary Ca excretion was lowest (P<0.01) at wk –4 prepartum, intermediate at wk –2.5 prepartum, and highest at wk –1 prepartum. Urinary excretion of Na was higher (P=0.03) at wk –4 and –2.5 prepartum than at wk –1 prepartum. Urinary Cl excretion was higher (P<0.01) at wk –2.5 and –1 prepartum than at wk –4 prepartum. Urinary pH was not affected by week. Urinary mineral excretion in the present study was similar to values reported by Tucker and Hogue (35), with the exception of K; however, dietary K was significantly higher in the present study (3.2 vs 1.3% K). In addition, Tucker and Hogue (35) utilized lactating cows in their study, and milk yield might have provided an additional outlet for K excretion. Gaynor et al. (8) and Tucker et al. (37) reported increased urinary Ca losses in cows fed an anionic diet but implied that renal Ca conservation may not be directly responsible for

improved Ca balance observed in metabolic acidotic ruminants. Rather, the increased urinary Ca probably reflects excretion of excess Ca as a result of increased intestinal absorption and Ca bone resorption (8). A low DCAD diet increases the flow of Ca through the readily exchangeable Ca pool in the body (33) and increases the concentration of ionized Ca in the blood (26). However, Wang and Beede (39) reported that 10% of Ca filtered by the kidneys was excreted by cows fed an anionic diet compared with 2% excreted by control cows (fed a cationic diet). The authors suggested that the kidneys of the control cows might have reabsorbed Ca more efficiently. Dietary AS in the present study might have increased renal Ca absorption, increased intestinal Ca resorption, or increased Ca bone resorption, even though the DCAD was not in the recommended range. Decreased urinary Na excretion at wk –1 prepartum supports the findings of Maltz and Silanikove (20),

who reported an increase in Na reabsorption in prepartum dairy cows as calving approached. Tucker et al. (37, 38) and Tucker and Hogue (35) reported higher urinary Cl concentrations in cows fed an anionic diet compared with that in cows fed a cationic diet. Urinary Cl increased linearly with increasing dietary Cl in the present study, suggesting that urinary Cl concentrations reflected Cl content of the diet. Jardon (16) reported that urine pH should range between 6 and 7 for consistent results in preventing milk fever. However, the author (16) also stated that urine pH cannot be used to predict whether individual animals will become hypocalcemic. Rather, urinary pH can be used to monitor the overall effectiveness of an AS program on a whole-herd basis. The urine pH values in this study were higher than the optimum range in both treatments and varied widely, suggesting that AS would not have been effective in preventing milk fever.

Conclusions Decreasing the DCAD of the diet of grazing prepartum dairy cows from +388 to +183 meq/kg dietary DM did not affect DMI, prepartum blood mineral profiles, or postpartum milk yield or composition. The pellet with AS cost $1.08/d per cow, and the control pellet cost $0.92/d per cow. With no observed improvement in production or metabolic parameters, these supplements were not economically practical for grazing cows; however, grass pasture may be a viable alternative forage source for prepartum dairy cows.

Acknowledgments The authors gratefully acknowledge the expert technical assistance of M. Long; the able animal care provided by The Pennsylvania State University farm staff, including M. Amsler, R. Hoffman, and H. Wiggin; and the many students involved with

Prepartum Grazing and Anionic Salts

the feeding of animals and laboratory analyses.

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