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* Periparturient Diseases of Cattle Research Unit, National Animal Disease Center, USDA-Agricultural Research Service, Ames, IA 50010-0070
Agronomy Department, Iowa State University, Ames 50011
2 Corresponding author: jesseg{at}westcentral.net
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Key Words: dietary cation-anion difference • chloride • alfalfa • milk fever
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Ration formulation for dairy cows just prior to parturition must control the DCAD if hypocalcemia and milk fever are to be avoided. One key to reducing hypocalcemia is to avoid incorporation of high K forages into the ration. Dairy rations must incorporate some proportion of forage (usually at least 40%) to provide the effective fiber vital to the function of the cow’s rumen (Mertens, 1997). Alfalfa and cool season grasses utilizing the C3 photosynthetic system, such as orchardgrass and brome, are often used in dairy rations. Because these plants will accumulate K well beyond their needs if K is present at high concentrations in the soil, plant tissue K levels can reach 4% DM (Lanyon and Smith, 1985). The excessive K content of these forages can cause metabolic alkalosis in the cow and subsequently increases the risk of hypocalcemia and milk fever. Reducing K content of forages can be achieved by restricting K application so that soils do not support excessive accumulation of K by the plants (Lanyon and Smith, 1985; Kelling and Matocha, 1990). More than half of the dairy cows in the United States are now fed late gestation rations that utilize low K forages to limit K intake (NAHMS, 2002). If, in addition to decreasing forage K, the producer can also increase the Cl content of the forages, the resulting DCAD will be more favorable for the late gestation cow. Thomas et al. (1998) and Pehrson et al. (1999) demonstrated the chloride content of grasses can be increased markedly by addition of chloride to the soil. These findings may also be applicable to legumes such as alfalfa.
This study tests the hypothesis that application of chloride to alfalfa will increase plant tissue chloride levels and reduce DCAD of the forage. The objective of the experiment was to fertilize alfalfa with CaCl2 or NH4Cl or a combination of both and to measure the concentrations of the major cations and anions in the alfalfa tissue and to monitor forage yield.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Chloride treatments were applied once at the beginning of the 2001 growing season. Chloride fertilizer was applied as NH4Cl, CaCl2, or an equal mixture of NH4Cl/CaCl2 (based on Cl concentration) at 0, 56, 112, or 168 kg of Cl/ha. Chloride treatments were applied as 20% (wt/vol) salt solutions, and the volume of each applied to the plots was adjusted to achieve the desired chloride application rate. The alfalfa was harvested 4 times during 2001, and a fifth cutting was made in May 2002. At each harvest, plots were harvested with a sickle bar-type harvester equipped with an electronic weigh system. Prior to harvest, subsamples were taken to adjust plot yields for moisture content and to analyze mineral content. Subsamples were dried at 60°C for 4 d, after which they were ground to pass through a 1-mm screen.
Mineral content of the soil in each plot was determined in April, prior to application of the treatments, and again in July of 2001, after the second cutting was harvested. Exchangeable cation concentrations were determined following ammonium acetate extraction (Warncke and Brown, 1998). Soil chloride and sulfur concentrations were determined on deionized water extracts of the soil, which were then analyzed by the same methods used on plant tissues. Mineral concentrations in plant tissues were determined by Dairyland Labs (Arcadia, WI). The Na, K, Ca, Mg, and P determinations were made by inductively coupled plasma-mass spectrometry on samples that had been dry-ashed and redissolved in 1 N HCl. Sulfur determinations were done by inductively coupled plasma-mass spectrometry on samples that had undergone microwave digestion in nitric acid. Chloride was determined using a Corning Chloride Analyzer (Corning Medical and Scientific, Medford, MA), which is based on precipitation of Cl as silver chloride in samples blended with water. The DCAD of each forage sample was calculated as mEq of (Na+1 + K+1) – (Cl–1 + SO4–2)/kg of DM.
Statistical Analysis
Mineral content of the plant tissues is expressed as a % (wt/wt) of DM. Data from both Ames and Nashua plots were combined in an initial factorial ANOVA using location (Ames or Nashua), treatment (untreated control, and 3 Cl sources at 3 doses), and cutting (1 to 4 of the year 2001), and their interactions as main effects in the model. Location proved to have a great impact on plant mineral composition. Cutting and location x cutting interactions were also significant effects on plant mineral content.
A factorial ANOVA model that included treatment, cutting, and treatment x cutting interaction was performed using the GLM procedure of SAS for each location—Ames and Nashua (SAS User’s Guide, 1999). This analysis allowed comparison of treatment effects with those of the control plots. When ANOVA suggested a significant effect, post hoc comparisons were made.
The data were also analyzed to examine the effect of the rate of Cl application (0, 56, 112, or 168 kg of Cl/ha), regardless of the chemical source of the Cl, across the 4 cuttings harvested in 2001 from each location. Data were analyzed using the REPEATED method of the MIXED procedure of SAS (SAS User’s Guide, 1999). This model included rate of Cl application, cutting, and rate x cutting interactions. The variation between plots was specified with a RANDOM statement consisting of plot nested within rate of Cl application. Autoregressive covariance was determined to be the most applicable covariate structure used for all repeated measures analyses (Littell et al., 1998). Each cutting sampled represented a repeated measure of each plot.
Finally, to discern differences among the 3 sources of Cl on alfalfa mineral composition and yield, the data from each location were analyzed by repeated measures ANOVA (SAS Institute, 1999). The data obtained from control plots were eliminated from these analyses because there was not a set of control plots designated for each of the 3 Cl sources. This model included source of Cl, rate of application, cutting, source x rate, rate x cutting, source x cutting, and source x rate x cutting interactions as main effects. The variation between plots was specified with a RANDOM statement consisting of plot nested within source of Cl and rate of Cl. Autoregressive covariance was the covariate structure used for all repeated measures analyses (Littell et al., 1998). Each cutting sampled represented a repeated measure of each plot. This analysis was done twice—once with just the 4 cuttings of 2001 and a second time, which included data from the fifth alfalfa cutting of spring 2002 to determine if treatment effects were carried over the winter.
Mean separations, using the method of Tukey-Kramer, were computed when significant effects were observed (P < 0.05) in the F-test.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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). After application of the chloride, the chloride content of the soil increased with increasing dose of chloride fertilizer when sampled in July 2001. In Ames, fertilizing with 56, 112, or 168 kg of Cl/ha increased soil chloride content to 9.5, 14.1, and 23 mg/kg. The same treatments applied to Nashua plots resulted in soil with water-extractable chloride content of 5.2, 8.5, and 15.6 mg/kg of soil. This suggests differences exist in the soil chloride binding capacity of the 2 soils. There were no significant differences in plot soil chloride content as affected by source of chloride. Application of chloride did not significantly change the soil content of any other minerals.
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Across both locations, and within each location, the application of Cl increased alfalfa Cl content when compared with control plots (Tables 2 and 3). Application of chloride from NH4Cl, CaCl2, or the mixture of NH4Cl and CaCl2, increased alfalfa Cl content equally well. Repeated measures ANOVA using rate of Cl application, cutting, and rate by cutting as main effects demonstrated application of 112 kg of Cl/ha from any source was able to increase alfalfa Cl more than application of 56 kg of Cl/ha. However, no further increase in alfalfa Cl was observed in alfalfa from plots treated with 168 kg of Cl/ha. Although Cl was applied to the plots just once in the spring, there was no indication that there was a graded diminution of alfalfa Cl content with each succeeding harvest in the 2001 growing season. However, the first cutting of alfalfa made in the spring of 2002 was significantly lower in Cl than any of the cuttings made in 2001, suggesting depletion of Cl from the soil over the winter. Chloride treatments had no effects on alfalfa Na, S, K, Mg, Ca, or P content.
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). Lower doses of CaCl2, and all other Cl treatments, did not affect yield significantly. However, the 168 kg of Cl/ha dose, regardless of source, tended to depress yield, especially at first cutting of alfalfa in both locations. Cutting had a major effect on yield, but a cutting x treatment effect on yield was not observed.
Across both locations, chloride application decreased alfalfa DCAD when compared with the control plots (Table 2). This effect was greatest in the plots in Nashua (Table 3), probably because DCAD in Ames was already considerably lower in control plots, as a result of lower K and higher Cl concentration than in control plots in Nashua.
Feeding high DCAD forages to dairy cows in late gestation is a well-documented means of inducing hypocalcemia and milk fever in dairy cows (Ender et al., 1971; Goff and Horst, 1997). Potassium concentration in alfalfa tissues can often range from a low of 1.2 to 1.6% up to 3.6 to 4.2% (Lanyon and Smith, 1985). Because K is the major cation contributing to high DCAD diets, an obvious solution is to limit K application to avoid luxury consumption of K by the forage crop. However, alfalfa containing less than 2.1 to 2.3% K may have reduced yield and winter survival (Lanyon and Smith, 1985; Kelling and Matocha, 1990). Thus, producing alfalfa with less than 2.1% K is not profitable, especially in northern regions.
The high DCAD of alfalfa and many grasses has caused a shift toward increased reliance on corn silage as the forage source for dry cows. Unfortunately the physically effective fiber of corn silage can vary, and lack of effective fiber can lead to increased incidence of displaced abomasum. Feeding low DCAD dry hay in the close-up ration could benefit rumen and abomasal functions. Using the results of alfalfa plots in Nashua as an example, the potassium content of the alfalfa was 24 g/kg, a level that would be expected to support maximal yield. The DCAD in the control plots was +390 mEq/kg of DM. Feeding 4 kg/d of control alfalfa hay to enhance effective fiber would alter the DCAD of the ration by a net +1,560 mEq. If instead we use the high Cl alfalfa from plots receiving 112 kg of Cl/ha, we would be adding just +1,068 mEq cation to the ration. About half an equivalent less anion would need to be added from anionic salts to achieve the same final DCAD in the total ration. We believe anions coming into the diet through the forage would be more palatable than anions supplied from sources such as ammonium chloride or calcium chloride. However, studies have not been conducted at this time to verify this.
Thomas (1996) and Thomas et al. (1998) have previously demonstrated that applying chloride to grass species could reduce DCAD of grass hays. They fertilized reed canarygrass with 56 kg of Cl/ha, and the grass Cl content rose from 7.2 g/kg in control plots to 14.2 g/kg in Cl fertilized plots. Pehrson et al. (1999) explored the effects of K2SO4 and KCl added to soil on grass DCAD. Addition of SO4 to the soil did not alter plant sulfur or DCAD content. Application of Cl increased plant chloride content and reduced DCAD.
Pelletier et al. (2007) recently examined the influence of increasing rates of chloride and nitrogen application on DCAD of timothy grass. In these studies addition of chloride increased plant chloride content and reduced DCAD in a quadratic fashion that plateaued at a certain rate of chloride application dependent on soil and season of plant growth. They used both ammonium chloride and calcium chloride to supply the chloride. The 2 sources of chloride were equally effective in reducing DCAD of the timothy hays. They also found a small but significant reduction in DCAD of timothy hays could be achieved by nitrogen application alone if soil K was low or moderate, but not if soil K was high. This treatment also increased plant yield. This study found that the economically optimal application rate of chloride fertilizer was between 78 and 123 kg of Cl/ha, depending on soil K and Cl content prior to application.
The results of the present study extend these findings by demonstrating that adding Cl to the soil can alter alfalfa tissue Cl concentration. At Nashua, the addition of 56 kg of Cl/ha more than doubled alfalfa Cl content, but the increase was not as dramatic at Ames (Table 3). The pretreatment soil water-extractable Cl was higher at Ames than Nashua (3.06 vs. 2.26 mg/kg), and a large difference in alfalfa Cl content from the control plots in the 2 sites was evident as well (2.9 g/kg in Nashua vs. 7.7 g/kg in Ames). In both locations, increasing the dose of Cl from 56 to 112 kg of Cl/ha further increased alfalfa Cl content, but increasing Cl application from 112 to 168 kg of Cl/ha did not significantly increase Cl content of alfalfa any further. This observation is difficult to understand. In Ames, the assumption might be made that application with 112 kg of Cl/ha met or exceeded the soil Cl level required to maximize alfalfa Cl content (about 1% Cl in Ames). In Nashua, despite the lower soil Cl and lower Cl content of alfalfa from control plots, 112 kg of Cl/ha also maximized alfalfa Cl uptake. About 0.8% Cl was the maximal Cl content achieved in Nashua. The same alfalfa cultivar was planted in each location, and yet maximal achievable Cl content differed in the 2 locations. Differences in rainfall or soil anion-binding capacity could be responsible for these differences.
Interestingly, soil water-extractable S content in Nashua was 13-fold greater than in Ames. Though alfalfa S was significantly higher in alfalfa from Nashua the differences were small, averaging 0.28% S in Nashua and 0.24% in Ames. Pehrson et al. (1999) and E. D. Thomas (personal communication, Miner Institute, Chazy, NY) have observed that utilizing S application as a means of adjusting forage DCAD does not hold great promise.
The DCAD was not as greatly affected by application with Cl as was the alfalfa Cl content. The formula for DCAD is calculated as the mEq/kg (Na + K) – (Cl + S). The concentration of K, Na, and S in the plants varied among cuttings, and these differences introduced sufficient variability into the DCAD equation to overshadow the effects that increased Cl content had on DCAD calculations.
Although Cl-treated plots tended to yield less than the control plots, the differences are, with a few exceptions at the highest Cl application rate, not statistically significant. Analysis of data combined across locations indicated that the Cl application had the largest effect on yield at the first cutting of alfalfa and at the highest doses of Cl. The alfalfa plants had begun their spring growth at the time the Cl treatments were applied, and within a few days of application, we observed browning of some of the emerged tissue. Earlier application of the Cl to the soil, before the crowns had emerged, may avoid burning the plants, and may not affect yield as greatly. Autumn application of Cl following the final harvest of the year might also be considered. Our data suggest that Cl application once in the spring increased Cl content of alfalfa harvested during that growing season, but did not carry over very well into the next spring’s harvest. Whether the harvested alfalfa depleted the soil of the Cl or winter snowmelt leached the Cl is unknown.
Future experiments will need to determine whether bringing anions into the ration within forages proves more palatable to the cow than current efforts to reduce diet DCAD by addition of anion supplements directly to the ration. If chloride entering the ration within forages does not result in a significantly more palatable ration than when chloride sources are added directly to the ration, it is unlikely to be an economical means of preventing hypocalcemia in cows fed TMR. Chloride applied to forage in fields may still prove useful in management of late gestation dairy cows fed pasture.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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FOOTNOTES |
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Received for publication January 31, 2007. Accepted for publication July 16, 2007.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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