J. Dairy Sci. 2009. 92:2711-2718. doi:10.3168/jds.2008-1231
© 2009 American Dairy Science Association ®
Effect of dietary ratio of Na:K on feed intake, milk production, and mineral metabolism in mid-lactation dairy cows
W. Hu and
L. Kung, Jr.1
Department of Animal and Food Sciences, University of Delaware, Newark 19716
1 Corresponding author: lksilage{at}udel.edu
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ABSTRACT
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The objective of this study was to determine the effect of altering the dietary ratio of Na:K while keeping the dietary cation-anion difference (DCAD) constant, on dry matter (DM) intake, milk production, and mineral metabolism in lactating dairy cows. Fifteen mid-lactation Holstein cows averaging 160 d in milk were used in a replicated 3 x 3 Latin square design with treatments varying in the molar ratio of Na:K (0.21, 0.53, and 1.06). Diets contained A) 0.25% Na and 2.00% K, B) 0.50% Na and 1.60% K, or C) 0.75% Na and 1.20% K (on a DM basis), and all contained the same DCAD of 33 mEq (Na + K – Cl – S)/100 g of DM. There was a quadratic effect of the ratio of Na:K on DM intake (28.4, 27.5, and 28.3 kg/d for diets A, B, and C, respectively). The ratio of Na:K did not affect milk yield (average 39.2 kg/d), milk composition (average 3.60% fat; 3.01% protein; and 8.62% solids-not-fat), or coccygeal venous plasma concentrations of HCO3– (average 29.3 mEq/L), Na+ (average 136.7 mEq/L), K+ (average 4.53 mEq/L), Cl– (average 97.5 mEq/L), Ca (average 10.06 mg/dL), and Mg (average 2.49 mg/dL), and urinary pH (average 8.38) and ratio of Cl–:creatinine (average 4.35). The ratios of urinary Na+:creatinine (1.80, 4.21, and 7.42), Ca:creatinine (0.035, 0.041, and 0.064), and Mg:creatinine (0.53, 0.60, and 0.77) increased linearly with increasing ratios of Na:K, whereas the ratio of urinary K+:creatinine decreased linearly as the ratio of Na:K increased (22.4, 15.9, and 10.3). Milk production and composition of mid-lactation cows was similar among dietary ratios of Na:K with the same DCAD of 33 mEq/100 g of DM.
Key Words: sodium • potassium • dietary cation-anion difference • lactation
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INTRODUCTION
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Altering the levels of DCAD can affect the metabolism and performance of lactating dairy cows. It appears that DCAD improves acid-base balance and is positively related to productive performance in lactating dairy cows (Sanchez and Beede, 1996; Hu and Murphy, 2004). A higher DCAD can be achieved by the addition of NaHCO3 and K2CO3 (NaCO3 and KHCO3 are not considered economically feasible). These can be fed alone or in combination in a diet for dairy cows.
Sodium and potassium cations contribute significantly to the pumping mechanisms of cells, osmotic balance, acid-base equilibrium, and kidney function in the animal body system. For instance, Na+-K+ pumping of cells is often taken to illustrate the potential role of these strong ions in the metabolic process. This mechanism requires ATP and actively maintains high levels of K+ and low levels of Na+ intracellularly; excesses of one cation in relation to the other would obviously cause the pump to operate inappropriately beyond an optimal level (Block, 1994). Manipulating dietary levels of Na and K might therefore benefit physiological processes and subsequently improve production in lactating dairy cows. However, there is limited information on the effects of altering the ratio of Na:K on metabolism and production of lactating dairy cows. The responses of DMI and milk production to either high Na or K tend to be the greatest when the dietary concentration of the other cation is low (Sanchez et al., 1994). Wildman et al. (2007) reported a quadratic effect from altering the ratio of K:Na on milk production, whereas DMI was not affected by treatment. Further efforts need to be made to examine the effects of altering dietary ratios of Na:K at a fixed DCAD on DM intake, milk production, and mineral metabolism in mid-lactation dairy cows.
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MATERIALS AND METHODS
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This experiment was conducted at the University of Delaware dairy research farm, Newark, between June 19 and August 20, 2006. All experimental protocols used in the trial were approved by the University of Delaware, College of Agriculture and Natural Resources, Agricultural and Animal Care and Use Committee.
Experimental Design and Animal Care
Fifteen multiparous Holstein cows averaging 160 ± 21 DIM were assigned randomly to replicated 3 x 3 Latin squares with the following dietary treatments: molar ratios of Na:K of 0.21 [0.25% Na (10.87 mEq/100 g) and 2.00% K (51.15 mEq/100 g) in the diet on DM basis], 0.53 [0.50% Na (21.75 mEq/100 g) and 1.60% K (40.92 mEq/100 g)], and 1.06 [0.75% Na (32.62 mEq/100 g) and 1.20% K (30.69 mEq/100 g)] at a fixed DCAD of 33 mEq (Na + K – Cl – S)/100 g of DM. The fixed DCAD was within the optimal range established for lactating dairy cows (Hu and Murphy, 2004). The concentrations of Na and K were designed to meet or exceed the nutrient requirements, but to be far less than the maximum tolerable concentrations in the diets for dairy cows (NRC, 2001). Concentrations of Cl and S in the diets were kept constant in all treatments. There were 3 experimental periods (periods 1 to 3 as lactation period proceeded), with each consisting of a 2-wk adjustment period followed by a 1-wk collection period. Cows were housed in a barn with Calan gates (American Calan, Northwood, NH) and sand-bedded freestalls. Feed offered was adjusted daily, and 110% of the previous day’s consumption (as-fed basis) was provided once daily at 0700 h. Water was available for ad libitum consumption. Cows were milked twice daily at approximately 0600 and 1600 h.
The TMR was composed of 48.3% concentrate mix (Table 1), 40.0% corn silage, and 10.0% alfalfa hay on a DM basis that met nutrient requirements (NRC, 2001). The ratio of Na:K was varied by using NaHCO3 and K2CO3; these minerals were supplemented and mixed in the TMR (Table 2).
Sample Collection and Analysis
Feeds and Orts.
Feed intake and refusals for each cow were recorded daily during each experimental period. Weekly samples of corn silage, alfalfa hay, concentrate mix, and the TMR were collected. Dry matter contents of these feeds were determined by drying in a forced-air oven at 60°C until constant weights were obtained. Diets were adjusted weekly based on DM content of feeds. Weekly samples of corn silage, alfalfa hay, concentrate mix, and the TMR were stored at –15°C until the end of each experimental period and then composited and pooled for later analysis. Nutrient contents of corn silage, alfalfa hay, concentrate mix, and the TMR were analyzed by wet chemistry for CP, ADF, NDF, and minerals (Cumberland Valley Analytical Services, Maugansville, MD). Briefly, CP was calculated as N x 6.25 after analyses of N (AOAC, 2000) using a Leco FP-528 Nitrogen Combustion Analyzer (Leco, St. Joseph, MI). The ADF (AOAC, 2000) and NDF (Van Soest et al., 1991) contents of feeds were determined with the modification that Whatman 934-AH glass micro-fiber filters with 1.5-µm particle retention were used in place of fritted glass crucibles. In the analysis of NDF content of feeds, heat-stable amylase and sodium sulfite were added in the heating process before filtration (Van Soest et al., 1991). Mineral content of feeds (AOAC, 2000) was also determined using Perkin Elmer ICP (Perkin Elmer, Shelton, CT). In addition, energy concentration was calculated (Cumberland Valley Analytical Services). The nutrient composition of CP, NEL, ADF, NDF and minerals is presented in Table 2 and was calculated on the basis of nutrient content analysis of corn silage, alfalfa hay, and concentrate mix supplemented with the minerals.
Urine.
On d 19 of each experimental period, cows were manually stimulated to urinate at 0900 h, and midstream urine was collected in a 50-mL plastic container. Urine pH was measured immediately. Urine samples (30 mL for each cow) were stored at –15°C until further analyzed. Urine concentrations of Na+, K+, and Cl– were determined using an ion selective electrode; urine concentrations of Ca, Mg, and creatinine were measured spectrophotometrically. All these assays were performed on the Hitachi 917 analyzer (Roche, Indianapolis, IN) using Roche diagnostic reagents.
Urinary mineral excretions (Na+, K+, Cl–, Ca, and Mg) were expressed as ratios of minerals to creatinine concentration to overcome variations in urine volume among animals and as fractional clearance (FC) to indicate the renal clearance of electrolytes. The FC of electrolytes was calculated using the equation (Fleming et al., 1992)
where FCx = fractional clearance of X (%); Ux = urine concentration of X (mg/dL or mEq/L); Px = plasma concentration of X (mg/dL or mEq/L); PCreatinine = plasma concentration of creatinine (mg/dL); and UCreatinine = urine concentration of creatinine (mg/dL).
Blood.
On d 19 of each experimental period, coccygeal venous blood (there might be a slight chance for artery blood to be included in the blood sample) was collected at 0900 h using a Vacutainer (Becton Dickinson, Franklin Lakes, NJ) containing lithium heparin, placed on crushed ice, and immediately centrifuged at 1,500 x g for 15 min. Plasma was transferred to 5-mL plastic tubes and frozen at –15°C until analyzed.
Coccygeal venous plasma concentration of HCO3– was measured spectrophotometrically. Coccygeal venous plasma concentrations of Na+, K+, Cl–, Ca, Mg, and creatinine were determined using the same analytical methods as for the urine. All of these assays were performed on the Hitachi 917 analyzer (Roche, Indianapolis, IN).
Milk Production and BW.
Milk production was recorded twice daily at 0600 and 1600 h. On d 17 and 18 of each experimental period, milk samples were collected consecutively, and analyzed for milk fat, true protein, lactose, SNF, and SCC using a MilkoScan System 4000 (Foss North American, Eden Prairie, MN) by an infrared method (Dairy One Milk Laboratory, Ithaca, NY). Although BW was determined at the start of the trial and weekly thereafter, only BW measured during the last week of each experimental period was included in the statistical analyses.
Statistical Analysis
Dry matter intakes from d 15 to 20 and milk yields from d 15 to 21 in each experimental period were averaged for each cow. Mean milk composition from d 17 to 18 in each experimental period was calculated after being weighed for the proportion of milk produced at each milking. Urine pH was converted to free H+ concentration ([H+], assuming an activity coefficient of 1) and subjected to statistical analysis (Murphy, 1982). The resulting mean [H+] could still be transformed for convenience and reported as pH. Therefore, each mean was presented as both [H+] and pH; because of the asymmetric standard error of pH resulted from transformation, the larger number was presented as the standard error of pH (Murphy, 1982).
Data were analyzed with the GLM procedure in SAS (SAS Institute, 2004) according to the model for a replicated Latin square design:
where µ = overall mean; Ci = effect of cow i within square m (i = 1, 2, 3); Pj = effect of experimental period j (j = 1, 2, 3); Tk = effect of treatment k (k = 1, 2, 3); Sm = effect of square m (m = 1, 2, 3, 4, 5); and eijkm = error term.
Significance was defined as P 
0.05, and 0.05 < P 0.10 was considered a trend. Two single degree-of-freedom orthogonal contrasts were constructed to infer the linear and quadratic effects of dietary ratio of Na:K.
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RESULTS AND DISCUSSION
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The environmental conditions during the lactation trial are presented in Table 3. Mean maximum and minimum daily temperature averaged 30.9 and 19.4°C across the 3 experimental periods. The minimum, mean, and maximum temperature-humidity index were close to or above the critical values of 64, 72, and 76 for Holstein cows, respectively (Igono et al., 1992), indicating that heat stress could have been induced in the cows throughout the trial.
BW, DMI, and Milk Yield and Composition
Body weight was not affected by treatments. A quadratic effect of the dietary ratio of Na:K was detected for DMI (P = 0.03; Table 4). The DMI was the lowest in the diet with a ratio of Na:K of 0.53 compared with ratios of 0.21 and 1.06. In experimental period 2, the daily mean temperature-humidity index was 75.2, which greatly exceeded the critical value of 72 (Igono et al., 1992). Severe heat stress could dramatically decrease DMI and milk production. However, such decreases in DMI and milk production did not occur in experimental period 2. Decreasing trends of DMI and of milk production as lactation period proceeded were observed (data not shown).
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Table 4. Least squares means of DMI, BW, milk yield, 4% FCM, and milk composition for lactating cows fed experimental diets with 3 ratios of Na:K
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Milk yield and 4% FCM were similar for mid-lactation cows fed diets with different ratios of Na:K (Table 4). The same was true for milk fat, true protein, lactose, SNF percentages and yields, and SCC.
Previous research has focused on the effect of Na or K cation source, rather than on the effect of the ratio of Na:K in diets of lactating cows. O’Connor et al. (1988) did not detect any difference in DM intake or milk production of mid-lactation Holstein cows fed 2 dietary concentrations of Na (0.24 vs. 0.62%) or K (1.14 or 1.59%). West et al. (1992) also reported that there was no effect on DMI and milk production when either Na or K was used as the cation source. It should be noted that, when Na or K ion source was compared in these studies, the concentration of the other ion or DCAD was not equalized (O’Connor et al., 1988; West et al., 1992), because there are potential interaction effects between the strong ions or between each strong ion with DCAD on lactating cows.
Sanchez et al. (1994) reported that, based on the empirical model analysis from a large data set, the interaction between Na and K influenced DMI and 4% FCM yield, with the greatest DMI and 4% FCM yield occurring in either high Na or K when the concentration of the other ion was relatively low. Wildman et al. (2007) examined the effect of dietary ratios of K:Na of 2:1, 3:1, and 4:1 (on DM percentage basis) on milk production in cows averaging 188 ± 59 DIM and showed that milk yield and ECM yield were lower (quadratic, P < 0.05) for a ratio of 3:1 K:Na (26.2 and 27.1 kg/d) compared with ratios of 2:1 and 4:1 (27.7 and 28.1; 29.3 and 28.7 kg/d, respectively), whereas DMI was similar across treatments. Interestingly, the lowest values of DMI in the present experiment and milk yield and ECM yield of Wildman et al. (2007), and the lowest values of both DMI and 4% FCM of Sanchez et al. (1994) occurred at the moderate, rather than the high or low, dietary ratio of Na:K. The underlying mechanism responsible for these observations remains unclear.
Coccygeal Venous Blood Minerals and Acid-Base Measures
Coccygeal venous plasma concentrations of Na+ and K+ were not altered by the ratios of Na:K even though dietary contents of Na or K were much higher for cows receiving diets with ratios of Na:K of 1.06 or 0.21, compared with that for cows with a ratio of 0.53. This is in agreement with Tucker et al. (1988), who found that concentrations of Na and K in the serum were not influenced by concentrations of these minerals in the diet. However, Roche et al. (2005) observed a quadratic increase in concentration of blood serum Na and a linear decrease in the concentration of blood serum K in early lactating cows with increased concentration of Na and same concentration of K in the diets, respectively, as DCAD increased. In addition, concentrations of other coccygeal venous plasma minerals (Cl–, Ca, and Mg) were not affected in the present experiment (Table 5).
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Table 5. Least squares means of coccygeal venous plasma acid-base measures, and mineral concentrations for lactating cows fed experimental diets with 3 ratios of Na:K
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No difference in the concentration of coccygeal venous plasma HCO3– was found among the dietary ratios of Na:K. This is similar to that of Wildman et al. (2007). It is known that concentration of blood HCO3– increases quadratically with increasing DCAD (Hu and Murphy, 2004). The lack of an effect on the concentration of coccygeal venous plasma HCO3– is probably because the DCAD was held the same for these dietary treatments, supporting the allegation that the DCAD, and not the individual element of Na or K, has the most influence on the status of systemic acid-base balance (Tucker and Hogue, 1990). Similarly, the coccygeal venous plasma cation-anion difference [expressed as mEq (Na + K – Cl)/L] and anion gap were not affected by dietary ratios of Na:K.
Urine pH and Minerals
Urinary excretion of creatinine is relatively constant (De Groot and Aafjes, 1960; Albin and Clanton, 1966; Asai et al., 2005); thus, changes in urinary concentrations of creatinine indicated changes in urine concentration. Total urine output was not measured for cows in the present experiment. Therefore, the concentration of creatinine in the urine was used as an index to estimate excretion of minerals. Urine concentration of creatinine tended to increase linearly with increasing ratios of Na:K (P = 0.06); higher concentrations of creatinine in cows with a higher ratio of Na:K (Table 6) suggested that urine was more concentrated and, consequently, total daily urine volume was less than that for cows with the lower ratio of Na:K. The mineral load, particularly Na and K that needs to be excreted, largely determines the urine volume (Bannink et al., 1999). In the kidney, the ratio of reabsorption to filtration of K+ was lower than that of Na+, and the reduction in the ratio of reabsorption to filtration in response to increased intake was much higher for K+ (Maltz and Silanikove, 1996). The different volumes of urine (indicated by urinary concentration of creatinine in the present experiment) resulting from different dietary ratios of Na:K suggested that urine volume might be more closely related to the need for urinary excretion of K.
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Table 6. Least squares means of urine pH and concentrations of urine components for lactating cows fed experimental diets with 3 ratios of Na:K
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The fractional clearance of minerals is a measure of the percentage of minerals in the urine versus the minerals reabsorbed by the kidney. It is measured in terms of plasma and urine minerals, rather than by urinary mineral concentration alone, as urinary mineral concentration can vary with water resorption. Therefore, fractional clearance compares the plasma and urinary mineral concentrations to obtain a more accurate picture of renal clearance. The measured fractional clearance of minerals (Table 6) is consistent with urinary mineral concentration expressed as the ratio of mineral:creatinine, suggesting that both of them are good indicators of the renal excretion of minerals.
There was no difference in urine pH across the different ratios of Na:K (Table 6). The pH of urine proved to be a very sensitive and practical means of assessing the degree of acidification following supplementation with acidogenic salts in dairy cows (Vagnoni and Oetzel, 1998; Charbonneau et al., 2006; Hu et al., 2007). In contrast, a relatively small increase in urine pH (from 8.01 to 8.23) occurred as DCAD increased from 22 to 47 mEq/100 g of DM for dairy cows in early lactation (Hu et al., 2007). In the present experiment, the DCAD was 33 mEq/100 g of DM across dietary treatments, and was probably the reason that urine pH was similar among treatments (Table 6), regardless of different ratios of Na:K. The unchanged pH of urine together with similar concentrations of coccygeal venous plasma HCO3–, as discussed above, indicated that the acid-base status of the cows was unaffected by the different ratios of Na:K in the diets.
The ratios of Na:K were manipulated by the addition of NaHCO3 and K2CO3. Adding NaHCO3 and K2CO3 increased the excretion of urinary Na and K. In the present experiment (Table 6), excretion of Na+, as the ratio of Na+:creatinine (P < 0.01) or FC of Na+ (P < 0.01), increased linearly, whereas excretion of K+, as the ratio of K+:creatinine (P < 0.01) or FC of K+ (P < 0.01), decreased linearly with increasing dietary ratios of Na:K. These changes reflected their dietary contents of Na and K (Table 2).
In contrast with previous research (Wildman et al., 2007), there was an effect of the dietary ratio of Na:K on the concentration of Ca in urine in the present experiment (Table 6). Excretion of Ca, as indicated by the ratio of Ca:creatinine (P < 0.01) or FC of Ca (P < 0.01), increased linearly with increasing dietary ratios of Na:K, although no difference was observed in concentrations of coccygeal venous plasma Ca among dietary treatments. Sodium, in the form of NaCl, increases urinary excretion of Ca; however, Na consumed as a salt with a metabolizable anion such as bicarbonate appears to exert no effect on excretion of Ca (Massey and Whiting, 1996; Heaney, 2006). Decreased excretion of Ca with a lower ratio of Na:K might be attributed to the hypocalciuric effect of supplemental K in the diet. O’Connor et al. (1988) reported that urinary excretion of Ca decreased as dietary K increased from 1.14 to 1.58% in lactating dairy cows. Moreover, the significant role of dietary K to increase renal reclamation of Ca2+, and consequently reduced excretion of Ca2+ in urine, is well documented in humans (Morris et al., 2006). Although the total amount of Ca recovered by reducing urinary excretion of Ca is considered relatively small (Goff, 2006), reducing excretion of Ca in urine might have a biological significance for cows when the Ca demand is very high.
All diets contained the same amount of Mg, but differences in the excretion of Mg by dairy cows fed the 3 diets were observed. Excretion of Mg, as indicated by the ratio of Mg:creatinine (P < 0.01) or FC of Mg (P < 0.01), increased linearly with increasing dietary ratios of Na:K (Table 6). This is in contrast with previous research (Wildman et al., 2007) in which a quadratic response of excretion of Mg to ratio of K:Na was observed. Urinary excretion of Mg reflects changes in the absorption of Mg and is considered a more reliable index of absorption of Mg than the difference between intake of Mg and fecal excretion of Mg (Jittakhot et al., 2004). In the present experiment, urinary excretion of Mg increased with increasing dietary ratios of Na:K; thus, more Mg was probably absorbed into the body as the ratio of Na:K increased. Active transport of Mg across the rumen wall is a Na-linked transport process. Addition of Na to the diet can improve the transport of Mg across the rumen when the dietary concentration of Na is low (Goff, 2006). Khorasani and Armstrong (1990) reported that the concentration of Na in the diet per se was not an important factor affecting Mg metabolism, but the ratio of dietary Na:K correlated positively with the net absorption of Mg before the small intestine in sheep. It was shown that the addition of K to the diet of ruminants inhibits the absorption of Mg (Ram et al., 1998; Schonewille et al., 1999), and consequently, reduces the urinary excretion of Mg (Khorasani and Armstrong, 1990; Jittakhot et al., 2004). In addition, the normal range of Mg in cow plasma is 1.8 to 2.4 mg/dL (Goff, 2006). The concentration of Mg in the plasma in the present experiment (Table 5) indicated that the homeostasis of Mg was maintained in the diets of dairy cows fed these ratios of Na:K.
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CONCLUSIONS
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The DMI of mid-lactation cows responded quadratically to different dietary ratios of Na:K, with the lowest DMI occurring at a ratio of Na:K of 0.53 compared with the ratios of 0.21 and 1.06. Milk yield and composition and coccygeal venous plasma concentrations of HCO3–, Na+, K+, Cl–, Ca, and Mg were similar among the treatments. Changes in urinary excretion of Na+ and K+ were consistent with changes in their dietary contents. The excretion of Ca and Mg in the urine increased linearly with increasing dietary ratios of Na:K. The dietary ratio of Na:K did not affect milk production in mid-lactation dairy cows fed with the same DCAD of 33 mEq/100 g of DM.
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ACKNOWLEDGMENTS
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The authors thank Richard Morris of the University of Delaware Dairy Research Farm for care and feeding the cows. We also thank the following graduate students in our laboratory: Candice Klingerman, Renato Schmidt, and Erin McDonell, for assistance in blood and urine sample collection.
Received for publication April 3, 2008.
Accepted for publication January 24, 2009.
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ABSTRACT
INTRODUCTION
