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Hay to Reduce Dietary Cation-Anion Difference for Dry Dairy Cows

E. Charbonneau^*,1, P. Y. Chouinard^*, G. F. Tremblay, G. Allard and D. Pellerin^*

^* Département des Sciences Animales, Université Laval, Québec, Québec, Canada G1K 7P4
Agriculture and Agri-Food Canada, Québec, Québec, Canada G1V 2J3
Département de Phytologie, Université Laval, Québec, Québec, Canada G1K 7P4

¹ Corresponding author: edith.charbonneau{at}fsaa.ulaval.ca

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Timothy grass has a lower dietary cation-anion difference [DCAD = (Na + K) – (Cl + S)] than other cool-season grass species. Growing timothy on low-K soils and fertilizing it with CaCl₂ could further decrease its DCAD. The objective of this study was to evaluate the effects of feeding low-DCAD timothy hay on dry dairy cows. Six nonpregnant and nonlactating cows were used in a replicated 3 x 3 Latin square. Treatments were as follows: 1) control diet (control; DCAD = 296 mEq/kg of dry matter); 2) low-DCAD diet based on low-DCAD timothy hay (L-HAY; DCAD = – 24 mEq/kg of dry matter); and 3) low-DCAD diet using HCl (L-HCl; DCAD = – 19 mEq/kg of dry matter). Decreasing DCAD with L-HAY had no effect on dry matter intake (11.8 kg/d) or dry matter digestibility (71.5%). Urine pH decreased from 8.21 to 5.89 when L-HAY was fed instead of the control. Blood parameters that decreased with L-HAY were base excess (– 0.4 vs. 3.8 mM) and HCO₃^– (23 vs. 27 mM), and blood parameters that increased were Ca²⁺ (5.3 vs. 5.1 mg/dL), Cl^– (30.5 vs. 29.5 mg/dL), and Na⁺ (60.8 vs. 60.1 mg/dL). Compared with the control, L-HAY resulted in more Ca in urine (13.4 vs. 1.2 g/d). Comparing L-HAY with L-HCl, cow dry matter intake tended to be higher (11.5 vs. 9.8 kg/d), and blood pH was higher (7.37 vs. 7.31). Urine pH; total dry matter; Ca, K, P, and Mg apparent absorption; and Ca, K, Na, Cl, S, P, and Mg apparent retention were similar. Absorption as a percentage of intake of Na and Cl was lower for L-HAY as compared with L-HCl. In an EDTA-challenge test, cows fed L-HAY regained their initial level of blood Ca²⁺ twice as quickly as the control treatment (339 vs. 708 min); there were no differences between L-HAY and L-HCl. This experiment confirms that feeding low-DCAD hay is an effective means of decreasing the DCAD of rations and obtaining a metabolic response in dry dairy cows.

Key Words: dietary cation-anion difference • nonlactating cow • hay • blood calcium

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Clinical hypocalcemia, also known as milk fever, is a widespread metabolic disorder that affects about 5.2% of dairy cows at calving (USDA, 2002). This condition can be fatal if not treated, and cows with milk fever are susceptible to other metabolic disorders, including retained fetal placenta, displaced abomasums, and mastitis (Curtis et al., 1983; Gröhn et al., 1989), as well as decreased milk yield (Block, 1984). The subclinical form of milk fever affects 50% of all cows at calving and 66% of those in their third lactation or greater (Beede et al., 1992). Up to 22% are still deficient in Ca 10 d after calving (Goff et al., 1996). Subclinical hypocalcemia may also increase the risk of dairy cows developing other metabolic disorders (Huber et al., 1981; Oetzel et al., 1988).

Decreasing the DCAD of precalving rations can reduce the incidence of milk fever (Dishington, 1975; Charbonneau et al., 2006). Anionic salts (Vagnoni and Oetzel, 1998), commercial products (Vagnoni and Oetzel, 1998), and HCl (Goff and Horst, 1998; Goff et al., 2004) have all proven effective at reducing the DCAD of dry cow diets. Horst and Goff (1997) and Horst et al. (1997) suggested that decreasing the amount of K in forage fed to cows before calving can also prevent hypocalcemia. Tremblay et al. (2006), in comparing 5 cool-season grasses, concluded that timothy had the lowest DCAD. Fertilizing with chloride could further decrease timothy hay DCAD (Pelletier et al., 2007).

There is limited research on the absorption and retention of minerals by dry cows in relation to DCAD. Most studies examine Ca, Mg, and P absorption and retention and do not relate marked differences to DCAD (Wang and Beede, 1990; Kume et al., 2001). Schonewille et al. (1994b) evaluated the apparent absorption of Ca, Mg, and P for diets with high (276 mEq/kg) and low DCAD (– 170 mEq/kg) and found an increase in Ca absorption, and a tendency for increased Mg absorption, in cows fed a low-DCAD diet. Two studies (Leclerc and Block, 1989; Delaquis and Block, 1995) reported the apparent absorption and retention of Ca, Mg, P, Na, K, Cl, and S. In those experiments, all diet treatments had a positive DCAD. Leclerc and Block (1989) measured the apparent absorption of minerals over a long duration (pre- and postcalving). Delaquis and Block (1995) tested a slight variation in DCAD (481 vs. 327 mEq/kg). In both studies, only small differences were observed in apparent absorption and retention of the minerals. Further research is needed to determine how mineral absorption and retention relates to DCAD difference in dry cow rations.

The objective of our study was to evaluate the effects of a low-DCAD timothy hay diet on DMI, acid-base metabolism, and apparent absorption and retention of minerals in dry dairy cows. Our hypothesis was that timothy hay fertilized with CaCl₂ would be as effective as HCl at lowering the DCAD of dry cow rations and preventing hypocalcemia. Dry dairy cows were fed normal- or low-DCAD timothy hay to evaluate the effects of low-DCAD hay on DMI and blood and urine components. The low-DCAD timothy hay treatment was also compared with a positive control treatment that used HCl to decrease DCAD. The apparent digestibility of fiber, N, and minerals, as well as the apparent retention of N and minerals, was evaluated for all 3 treatments.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Hay Production
Two types of timothy hay were produced on 2 different fields. Low-DCAD hay was produced on a field with low soil K content (101 kg/ha), and high-DCAD hay was produced on a field with a soil K content of 289 kg/ha. Before the start of spring growth, both fields received 80 kg of N/ha, and, based on the recommendations of Pelletier et al. (2007), the field for low-DCAD hay received 140 kg of Cl/ha as CaCl₂. Hay from spring growth was cut at the early heading stage and conserved in small bales in a barn equipped with a hay drier.

Cows, Diets, and Experimental Design
Six multiparous nonpregnant and nonlactating Holstein cows were used in a replicated 3 x 3 Latin square design with 2-wk periods. All cows were fed, as TMR, chopped timothy hay, ground corn, corn gluten meal, and a mixture of vitamins and minerals. Treatments were (Table 1) as follows: 1) high-DCAD diet (control); 2) low-DCAD diet, using only low-DCAD hay as forage (L-HAY); and 3) low-DCAD diet, using HCl to decrease the DCAD of the control diet (L-HCl). To prevent excessive DMI depression, rations should not exceed a maximal DCAD diminution of 2,300 mEq/d that can be achieved using anionic salts (Oetzel and Barmore, 1993). For that reason, low-DCAD hay had to be mixed to high-DCAD timothy hay in the control and L-HCl diets (Table 1); diets formulated with only the high-DCAD timothy hay obtained a DCAD too high to be decreased with HCl alone to the same level as L-HAY. Content differences between hay were also diminished using this mixture, which made the control and L-HAY treatments more comparable in terms of chemical composition (Table 2). Concentrated HCl was diluted with water and molasses (acid:water:molasses; 10:10:4.5, vol/vol/vol) twice a week and added daily to the L-HCl diet. Cows fed control and L-HAY received the same proportion of water and molasses (water:molasses; 10:4.5, vol/vol) in their diet as the cows on L-HCl. Diets were formulated based on NRC (2001) recommendations for transition cows; they all provided a similar level of NE_L (1.48 Mcal/kg) and CP (14.6%). Rations were fed once a day, in the morning, to provide 10% orts on an as-fed basis according to the intake of the previous day. Before the experiment began, a 2-wk acclimation period was set aside for cow adaptation to the experimental feeds. The experimental protocol was approved by the Laval University Animal Care Committee, and animals were cared for according to the guidelines of the Canadian Council of Animal Care (1993).

Feces.
Total fecal excreted was weighed and sampled twice daily: 10 h postfeeding and at feeding the next morning on d 8, 9, and 10. The first sample was kept at 4°C until the second sample was taken. The 2 samples from the same cow in a 24-h period were pooled in proportion to excretion and were immediately frozen at – 20°C until further analysis.

Urine.
Bardex catheters (24 mm; 75-mL balloon) were installed into the bladder of each cow on d 7. At feeding on d 8, catheters were connected to a container using polyvinyl chloride tubing. Light mineral oil (50 mL) and 1 g of thymol were added in urine containers to prevent deterioration of urine (Delaquis and Block, 1995). Total collection of urine was done on d 8, 9, and 10. Urine was weighed twice a day, 10 h after feeding and at feeding the next morning. Urine was sampled under light mineral oil through polyvinyl chloride tubing. Volume mass was determined by weighing 1 L of urine. Two 50-mL samples per cow per sampling were immediately frozen at – 20°C until further analysis.

Blood Samples and Infusion of EDTA.
On d 11 of each experimental period, cows were administered 20 mg of i.v. xylazine tranquilizer (Rompun, Bayer Inc., Toronto, Canada), and catheters were introduced into both jugular veins. Starting at feeding on d 12, hypocalcemia was induced by i.v. infusion of sterile 7% (wt/vol) Na₂-EDTA·2H₂O (Laboratoire Mat Inc., Beauport, Québec) solution (pH 7.4) at a rate of 0.6 mL of solution/h per kilogram of BW by the means of a peristaltic pump (Micro Macro Plum XL3, Abbott Laboratories, Chicago, IL). The solution was made by mixing 70 g of Na₂-EDTA·2H₂O in sterilized saline solution (0.9% NaCl) to obtain a final volume of 1 L. Sodium hydroxide solution (5 N) was used to stabilize pH at 7.4 during the process. The solution was sterilized by filtration (0.2 µm). Infusion of Na₂-EDTA·2H₂O solution was stopped when cow blood Ca²⁺ had reached approximately half the initial level as determined using an automated microblood gas analyzer (ABL 77, Radiometer, Copenhagen, Denmark). The cows were allowed to recover spontaneously after the infusion, and blood Ca²⁺ was monitored until the Ca²⁺ level reached the initial concentration. Complete recovery was considered at 95% of initial blood Ca²⁺, because, on average, a 5% variation in blood Ca²⁺ could be observed between samples during baseline measurements. Blood samples were taken at catheter installation, before the infusion, every 10 min during the EDTA infusion and every 30 min after the infusion. Two blood samples were taken to confirm the initial blood Ca²⁺, the level to stop the infusion, and the total recovery. Blood samples were taken with heparinized syringes balanced for electrolytes (PICO 50, London Scientific Limited, London, Canada) for immediate analysis of blood pH, partial CO₂ and O₂ pressures, base excess, and whole-blood concentration of HCO₃^–, Na⁺, K⁺, Cl^–, and Ca²⁺ with an automated microblood gas analyzer (ABL 77, Radiometer). Volume of solution infused, time to decrease Ca²⁺ to half the initial concentration, and time for complete recovery were monitored during the experiment.

Chemical Analysis
Forage, TMR, and Orts.
Forage, TMR, and orts samples were freeze-dried, ground in a Wiley mill (Arthur H. Thomas Co., Philadelphia, PA) through a 1-mm screen, and pooled proportionally to their original quantity by cow by period. Subsamples of ground forages, TMR, and orts were analyzed for ADF and NDF using the Ankom (Ankom²⁰⁰ Fiber Analyzer, Fairport, NY) immersion method (Van Soest et al., 1991). Subsamples of ground forage, TMR, and orts were mineralized using a mixture of sulfuric and selenious acid as described by Isaac and Johnson (1976). Crude protein concentration was determined using total N (method 13-107-6-2-E; Lachat Instruments, 2005) that was measured using a Lachat QuikChem 8000 flow injection analysis system (Zellweger Analytics Inc., Lachat Instruments Division, Milwaukee, WI). Total P (method 15-115-01-4-A; Lachat Instruments, 2005) was measured simultaneously with N. Flame emission was used to determine K concentration. The same extract as N, P, and K was used to determine Mg concentration by atomic absorption spectrometry (Perkin Elmer 3300, Überlingen, Germany). Subsamples of ground forages, TMR, and orts were mineralized at 500°C for 4 h. Ashes were dissolved with 1.0 N HCl (Miller, 1998). Concentrations of Na and Ca were determined by atomic absorption spectrometry (Perkin Elmer 3300). Subsamples of ground forages, TMR, and orts were mixed with 20 mL of 0.0007 M sulfuric acid (Liu, 1998) for 60 min, centrifuged at 32,570 x g for 30 min, and the Cl concentration of the supernatant was determined by conductivity on a Dionex DX500 equipped with a AS11HC column (Dionex Corporation, Sunnyvale, CA). Subsamples of ground forages, TMR, and orts were digested in nitric acid (Mills and Jones, 1996). Organic and inorganic forms of S were converted to a sulfate form that was precipitated with acidified barium chloride, suspended in a colloidal form, and analyzed by turbidimetry (adaptation of method 10-116-10-1-G; Lachat Instruments, 2005) on a Lachat QuikChem 8000 flow injection analysis system (Zellweger Analytics Inc., Lachat Instruments Division). Daily intake of ADF, NDF, CP, K, Na, Cl, S, Ca, P, and Mg was calculated by subtracting ort nutrients from TMR nutrients. Ort and TMR nutrients were calculated by multiplying chemical composition with the corresponding amount of orts or TMR.

Feces.
Fecal samples were freeze-dried. Once dried, they were ground in a Wiley mill (Arthur H. Thomas Co.) through a 1-mm screen. Subsamples were pooled by cow and period, proportionally to daily fecal excretion. Feces were then analyzed for ADF, NDF, CP, K, Na, Cl, S, Ca, P, and Mg, using procedures previously described for forages, TMR, and orts. Daily excretion of ADF, NDF, CP, and macrominerals was then calculated by multiplying fecal excretion by its nutrient concentration. Apparent digestion of ADF, NDF, CP, and the apparent absorption of macrominerals were calculated by subtracting nutrient daily excretion from daily intake.

Urine.
Urine pH was taken immediately after sampling (Oakton pH 10 Series, Vernon Hills, IL). Thawed subsamples of urine were pooled by cows and period proportionally to the urine excreted. Analysis of N, K, Na, Cl, S, Ca, P, and Mg were then completed without any mineralization, using procedures previously described for forages, TMR, and orts.

Statistical Analysis
Mixed model procedures from SAS 9.1 (SAS Institute, 2002) for a replicated 3 x 3 Latin square design were used to evaluate the effect of treatments on parameters. Raw data were transformed (square root, x², x³, 1/x, 1/x³) when it was deemed appropriate to meet homogeneity of variance criteria. Cows were defined as a random effect, and Akaike’s information criterion was used to select the best covariance structure among compound symmetry, first-order autoregressive, and unstructured. Orthogonal contrasts were defined a priori and used to compare the following: 1) control vs. L-HAY treatments and 2) L-HAY vs. L-HCl treatments. Differences between treatments were declared significant when P-values were < 0.05 and a tendency was noted when 0.05 < P < 0.10.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
The DCAD of L-HAY was lower than control (P < 0.001) but did not differ from the L-HCl treatment (P = 0.89; Table 2). Compared with control, L-HAY had no effect on BW or BW variation, but average BW was higher with L-HAY than with L-HCl. Dry matter intake and DMI/BW were similar between L-HAY and control, but DMI had a tendency to be lower when cows were fed L-HCl as compared with L-HAY (Table 3). Based on the mean DMI of 9.8 kg/d, cows on L-HCl ingested on average 293 mL of concentrated HCl (12 N) per day. The patterns of DMI during the 14-d experimental periods were similar for cows fed L-HAY and control diets, but the DMI of cows fed L-HCl decreased initially before returning to comparable levels after an average of 7 d (Figure 1). Dry matter of feces and total fecal production were similar between L-HAY and control, whereas DM of feces was higher and total fecal excretion was lower for cows fed L-HCl compared with L-HAY (Table 3). Urine volume of cows did not vary between L-HAY and control nor between L-HCl and L-HAY. Urine pH decreased from 8.21 to 5.89 when L-HAY was fed instead of control (Table 3), but there was no significant difference between L-HCl (5.78) and L-HAY.

View larger version (11K): [in this window] [in a new window]   Figure 1. Variation in DMI during the 14 d of the experimental period for cows fed diets with different levels of DCAD obtained by using a low-DCAD hay or HCl. = control; = L-HAY (low-DCAD treatment using low-DCAD hay); = L-HCl (low-DCAD treatment using HCl). Error bar = standard error.

Because the low-DCAD timothy hay contained more Cl (Table 2), Cl intake (Table 7) was higher when cows were fed L-HAY as compared with the control diet. Although Cl concentration was higher in L-HCl than in the L-HAY diet (Table 2), Cl intake was only numerically higher for L-HCl compared with L-HAY (Table 7), because DMI was lower for that treatment (Table 3). Apparent absorption of Cl, expressed in grams per day or as a percentage of intake, was higher for cows fed

L-HAY than control (Table 7). Excretion of Cl in feces was lower for L-HCl than for L-HAY, which resulted in a higher Cl apparent absorption expressed as a percentage of intake, but only a numerical increase was observed in the total amount of Cl apparently absorbed (Table 7). When fed L-HAY, cows had higher Cl excretion in urine than the control but a similar amount of apparently retained Cl, expressed in grams per day or as a percentage of intake, compared with control or L-HCl treatments (Table 7).

No differences were observed between L-HAY and control for S intake, S excreted in feces and urine, and S absorbed or retained. Intake of S was lower for L-HCl compared with L-HAY, which resulted in lower S excreted in feces and apparently absorbed. However, no significant difference was observed for S urinated or retained between these 2 treatments (Table 7).

The L-HAY treatment had no effect on Ca intake, Ca excreted in feces, and Ca apparently absorbed compared with control (Table 8). Lower Ca intake and Ca excreted in feces were observed for L-HCl compared with L-HAY, but similar amounts of Ca were apparently absorbed. Cows fed the L-HAY treatment excreted more Ca in urine than when they were on control but a similar amount when they were on L-HCl. Cows excreted more Ca than they absorbed for all treatments, which resulted in a negative value for apparently retained Ca. When they were on L-HAY, cows tended to retain less (lose more) Ca (g/d) than when they were fed the control treatment but retained more (lost less) Ca than when they received the L-HCl diet.

Magnesium intake and Mg excreted in feces were higher for L-HAY as compared with both control and L-HCl treatments (Table 8). As well, apparently absorbed Mg, expressed in grams per day, tended to be higher and Mg excreted in urine was higher for L-HAY when compared with control but not with L-HCl. Apparently absorbed Mg, expressed as a percentage of intake, and apparently retained Mg, expressed in grams per day or as a percentage of intake, did not vary when cows were fed L-HAY instead of control or L-HCl treatments.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Effect of Low-DCAD Hay to Decrease the Ration DCAD
Comparison between L-HAY and control treatment indicates that low-DCAD hay can induce a compensated metabolic acidosis as shown by decreases in urinary pH and blood HCO₃^– and base excess. This acidification capacity resulted in a tendency for increased blood Ca²⁺ and a greater ability to increase Ca²⁺ availability when cows are stressed, like during the EDTA challenge. Differences in the absorption of some minerals were observed when cows were fed L-HAY instead of the control diet, but there was no difference in DMI and digestion of OM, ADF, NDF, and N, which should not be altered during the critical period of transition. These results support the interest of using a low-DCAD hay before calving.

Because urinary pH is directly related to DCAD (Vagnoni and Oetzel, 1998; Charbonneau et al., 2006), it was expected that the low-DCAD hay would diminish urinary pH. Lowering the DCAD of a ration to – 20 mEq/kg is within the recommended levels of 0 mEq/kg or lower (Horst and Goff, 1997; NRC, 2001; Charbonneau et al., 2006), but this resulted in a urinary pH that was lower than the recommended level of 7.0 (NRC, 2001; Charbonneau et al., 2006). Low urinary pH is associated with too strong a metabolic acidosis for the cow (Charbonneau et al., 2006). The great difference we observed in urinary pH in response to the DCAD value of the ration in this trial emphasizes the importance of testing urinary pH to evaluate the reaction of each cow to low-DCAD rations.

Lower blood HCO₃^– and base excess observed with the L-HAY treatment indicate a compensated metabolic acidosis resulting in a higher level of blood Ca²⁺ (Joyce et al., 1997; Rose and Post, 2001; Goff et al., 2004), even without Ca deficit. The lack of measurable difference for partial blood pressure of CO₂ and O₂ indicates that the acidosis did not result in respiratory mechanisms to compensate for the metabolic acidosis (Rose and Post, 2001). Higher blood Cl^– probably comes from the increase in Cl apparently absorbed, whereas the tendency for higher blood Na⁺ might come from the close regulation of Na with Cl (Shills, 1997).

Because blood Ca²⁺ tended to be higher when cows were fed L-HAY compared with control, cows on this treatment might have shown more resistance to hypocalcemia than those fed the control diet, as demonstrated by Wang and Beede (1990) with an EDTA infusion. In our EDTA infusion challenge, however, this increase in resistance was not confirmed. The difference in response between both experiments could be due to different protocols. Wang and Beede (1990) stopped their EDTA infusion when 1 of the cows in the group showed signs of milk fever, whereas we stopped our infusion when individual cows reached 50% of their initial blood Ca²⁺ level. As well, the lack of response for time of infusion and volume infused, as opposed to the results of Schonewille et al. (1999b), could probably be explained by the EDTA solution being more concentrated in our experiment (7 vs. 5%) with a similar rate of infusion. In our experiment, all cows responded similarly to the infusion. The lower concentration of EDTA solution used in Schonewille et al. (1999b) may have allowed a wider range of responses from the different treatments. The decrease in blood Ca²⁺ recovery time after the EDTA challenge observed in cows fed L-HAY or L-HCl is usual for low-DCAD treatments (Schonewille et al., 1999b) and confirms that a low-DCAD timothy hay is effective in increasing Ca²⁺ availability when cows are in need.

An increase in Ca absorption is usually associated with Ca regulation when cows are in hypocalcemia (Horst, 1986). In the present experiment, nonpregnant dry cows were used, and Ca requirements were far lower than those required for colostrum production at calving; this could explain the lack of difference in apparent absorption of Ca. As well, the low apparent absorption of Ca (negative for 2 of the 3 treatments) and Ca retention as a percentage of intake (negative for all treatments) were similar to those observed by Wang and Beede (1990) and Schonewille et al. (1994a) and would mostly come from endogenous Ca excreted into the gut lumen (Moodie, 1960; Wang and Beede, 1990). Endogenous excretions have more effect on multiparous cows, because they excrete as much as younger animals but do not absorb Ca as efficiently (Moodie, 1960; Wang and Beede, 1990). An increase in Ca excretion in urine is typical of a low-DCAD ration (Gaynor et al., 1989; Van Mosel et al., 1993; Schonewille et al., 1994a). The increase of Ca in urine (Table 8) and the tendency for higher blood Ca (Table 5) with no increase in Ca apparent absorption indicates that Ca mobilization from reserves is responsible for the increase in available Ca with L-HAY compared with control. The increase in Ca available without absorption difference supports the hypothesis that cows, even without Ca deficit, are mobilizing Ca from bone when fed a low-DCAD ration.

The lower K intake for L-HAY could explain the lower K apparent absorption, because K is mostly absorbed by diffusion (NRC, 2001). The similar apparently retained K but lower K intake for the L-HAY treatment compared with the control confirms that K is regulated mainly through urination.

Both L-HAY and control treatments resulted in a low Na apparent absorption, expressed as a percentage of intake. Sodium is considered to be easily absorbed in the digestive tract with absorption at around 90% (NRC, 2001); it is regulated mostly through renal mechanisms. In this experiment, cows fed the control diet had low Na apparent absorption (43% of intake). The Na apparent absorption was better for cows fed the L-HAY diet (60%), but this level is still far below the expected absorption of 90%, especially given that the Na of both diets was in large part from salt (NaCl), which is considered to be absorbed at nearly 100% (NRC, 2001). Studies specifically on dry cows have reported variations in apparent absorption of Na between 55 and 83% of intake (Leclerc and Block, 1989; Delaquis and Block, 1995). Results from the current study and previous ones suggest that Na apparent absorption is lower for nonlactating dairy cows as compared with lactating cows. Because Cl regulation is mainly through urine, and Cl is mostly absorbed in the gut (Underwood and Suttle, 1999), the difference in Cl apparent absorption would typically come from differences in intake.

Potassium is known to affect Mg absorption (Schonewille et al., 1999a), but the variation in K concentration between diets was not sufficient to have had an effect. The variation in K concentration between both forages was smaller than that observed by Schonewille et al. (1999a), who reported a decrease of Mg absorption when forage with high K (4.75% of DM) was fed instead of forage with low K (2.75% of DM). The difference of Mg concentration in feces and urine could be due to increased Mg intake, which resulted in more Mg excretion from intestinal and renal regulation. Magnesium homeostasis is known to be affected by both intestinal and renal regulation (Shills, 1997). The urine excretion results are in accordance with Wang and Beede (1990), who reported an increase in urine Mg excretion associated with an increase in protein and Mg concentration in the diet. Renal regulation of Mg is not fully understood and could also be influenced by NaCl reabsorption (Rose and Post, 2001); this may also partly explain the increased Mg excretion with L-HAY compared with control, because L-HAY also increased Cl excretion.

Comparison of Low-DCAD Treatments
Comparison between L-HAY and L-HCl shows that decreasing DCAD with a low-DCAD hay can be as effective as decreasing it with HCl. There was little difference between the 2 treatments for urinary pH, blood components, and in the EDTA challenge. Considering the length of time it took for cows to adapt to the L-HCl diet, and the tendency of reduced DMI to result in lower absorption of ADF, NDF, and N when L-HCl was fed instead of L-HAY, low-DCAD hay might be a better choice than dietary HCl addition for transition cows given their high nutritional needs. The DCAD of L-HCl ration had already been lowered by the addition of 23% of low-DCAD hay. Even with the mixture of forages used, the amount of HCl (±293 mL, which represent 2.5 Eq/d) required to achieve similar DCAD then L-HAY was apparently enough to decrease DMI. The tendency for lower DMI for L-HCl compared with L-HAY confirms that a small augmentation over the 2.3 Eq/d identified from Oetzel and Barmore (1993) can effectively result in lower DMI. These results suggest that using low-DCAD hay instead of HCl to decrease DCAD of the ration by the same magnitude does not have the same effect in DMI.

The difference in DMI could explain the variations in BW observed. Two-week periods are short to determine BW variations, but results were still interpreted, because the L-HCl treatment had a very strong effect on DMI, which resulted in differences in BW. The higher DM concentration in feces with the L-HCl ration suggests there is an increase in water absorption when HCl is used to decrease the DCAD value of a ration.

The lack of difference between L-HAY and L-HCl treatments during the EDTA challenge corroborates the hypothesis that low-DCAD hay can be as efficient as HCl at decreasing the risk of hypocalcemia. Because blood pH was less affected by L-HAY than L-HCl, low-DCAD hay might work as well as HCl at decreasing the risk of hypocalcemia while also not being as stressful on cow metabolism.

A higher intake of Ca for the L-HAY treatment compared with the L-HCl treatment is associated with a higher DMI for L-HAY and a higher Ca concentration in the low-DCAD as compared with the high-DCAD timothy hay. The difference in Ca intake but not in Ca absorption resulted in more Ca excreted in feces for L-HAY as compared with L-HCl. Cows on both treatments excreted more Ca in urine than their Ca intakes, which is typical of low-DCAD diets (Gaynor et al., 1989; Van Mosel et al., 1993; Schonewille et al., 1994a). More Ca was apparently retained with L-HAY compared with L-HCl. The higher Ca retained for L-HAY probably comes from the numerically higher absorbed Ca (+3.8 g/d) with L-HAY than L-HCl but a similar excretion of Ca in urine (+0.7 g/d). The lower Ca retention for L-HAY could indicate a lower Ca mobilization as compared with L-HCl, which would be consistent with the higher blood pH for L-HAY but not with the identical level of blood Ca²⁺.

Although there were higher concentrations of K in the L-HCl diet, the tendency for lower DMI for cows on this treatment resulted in no difference in K intake compared with L-HAY. Less K was excreted in feces for L-HAY as compared with L-HCl, and when linked with the numerically lower K intake, this explains the tendency for lower apparently absorbed K expressed as a percentage of intake but the lack of difference in apparently absorbed K expressed in grams per day.

When considering the processes of Na and Cl absorption, the difference between L-HAY and L-HCl in Na total apparent absorption (g/d) and Na and Cl apparent absorption as a percentage of intake could be explained, in part, by an increase in the absorption of these 2 minerals in the form of NaCl (Harper et al., 1997). A complementary mechanism to NaCl absorption for Na and Cl exists in the exchange of Na⁺ for H⁺ and Cl^– for HCO₃^– (Harper et al., 1997). Because H⁺ and HCO₃^– come from the chemical reaction of transforming the CO₂ entering the cell [CO₂ + H₂O H⁺ + HCO₃^–], the exchange of H⁺ and HCO₃^– in the intestinal cells for Na⁺ and Cl^– from the gut lumen provides neutrality. Because both diets were high in Cl, more Cl than Na would have been exchanged by this process, which must have disturbed the equilibrium between H⁺ and HCO₃^–. The absorption mechanism in the gut lumen and the mechanism in blood to maintain electrical neutrality could explain the metabolic acidosis created by both treatments.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Results from this study indicate that low-DCAD hay effectively reduces the DCAD value of dry cow diets and modifies cow metabolism as indicated by reduced urine pH, difference in blood components, and a better response to simulation of hypocalcemia when compared with diets without DCAD alteration. When compared with HCl, a product used to decrease the DCAD of a ration, the low-DCAD hay obtained similar results without the adverse effect of HCl addition on DMI. A low-DCAD hay could provide a viable alternative to preventing hypocalcemia at calving.

	ACKNOWLEDGEMENTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
This study was supported by a grant from the Action Concertée FQRNT-Novalait-MAPAQ in collaboration with Agriculture and Agri-Food Canada and a scholarship from FQRNT. We thank Annie Brégard for her help during the experiment. Also, sincere appreciation is expressed to the staff of the Centre de recherche en sciences animales de Deschambault, the institution where the experiment took place. Thanks go to Mario Laterrière from Agriculture and Agri-Food Canada, Québec, for laboratory assistance. Appreciation is also extended to Rachel Gervais and Andrée-Anne Gingras, graduate students, as well as François Bécotte, undergraduate student, at Université Laval for their help during sample collections.

Received for publication October 12, 2007. Accepted for publication December 14, 2007.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 


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