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a Agriculture and Agri-Food Canada, Soils and Crops Research and Development Centre, 2560 Hochelaga Blvd, Sainte-Foy, QC G1V 2J3, Canada
b Département de Phytologie, Faculté des sciences de l'agriculture et de l'alimentation, Université Laval, Sainte-Foy, QC G1K 7P4, Canada
c Dep. of Plant Science, Macdonald Campus, McGill Univ., 21111 Lakeshore Rd., Sainte-Anne-de-Bellevue, QC H9X 3V9, Canada
d Agriculture and Agri-Food Canada, Soils and Crops Research and Development Centre, Research Farm, 1468 St. Cyrille Street, Normandin, QC G8M 4K3, Canada
* Corresponding author (tremblaygf{at}agr.gc.ca)
Received for publication May 25, 2005.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DCAD, dietary cation anion difference • DCADL, DCAD calculated using a long equation • DCADS, DCAD calculated using a short equation • DM, dry matter
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The target DCAD of the ration for milk fever prevention is around –50 mmolc kg–1 DM when using the DCADS equation or around +150 mmolc kg–1 DM when using the DCADL equation (Goff and Horst, 2003).
The addition of anions to the ration to reduce DCAD is limited because of problems with the palatability of the anionic salts commonly used (Oetzel and Barmore, 1993; Horst et al., 1997). Blood pH and, therefore, the incidence of milk fever can also be reduced by removing cations from the ration. Goff and Horst (1997) provided the first evidence that cows fed a ration low in K or Na have less milk fever than those on rations high in these cations. The cation present in highest amounts in the commonly fed ruminant rations is K+ derived from forages. Dietary K is significant in determining the susceptibility of dairy cows to milk fever (Goff and Horst, 1997; Horst et al., 1997). Dry cow rations contain a high proportion of forage and, therefore, forages fed to dry cows 2 to 4 wk prepartum should have a low DCAD.
Forage species with a lower K concentration could be used to reduce the K content of the dry cow ration. Forage grasses are known to have a lower K concentration than forage legumes (Adams, 1975). Differences in K concentration also exist among forage grass species. The K concentration varied between 2.4 to 40.4 g kg–1 DM among the 352 grass forage samples analyzed by the Pennsylvania State forage testing laboratory between 1969 and 1973 (Adams, 1975). Thill and George (1975), in Iowa, reported that the K concentration of nine grass species ranged from 20.3 g kg–1 DM in Kentucky bluegrass (Poa pratensis L.) to 39.7 g kg–1 DM in orchardgrass and tall oatgrass [Arrhenatherum elatius L. (Mert. and Koch)]. Fairbourn and Batchelder (1980) observed that orchardgrass had a higher K concentration than tall fescue and smooth bromegrass during spring growth; tall fescue and orchardgrass also consistently produced forage with Mg concentration >2.0 g kg–1 DM. Sanchez et al. (1998), comparing two pasture grass species commonly used in dairy farms of Costa Rica, reported that higher DCADS, K, and Na concentrations, and a lower S concentration in Setaria anceps than the control pasture Cynodon nlemfuensis could be key nutritional risk factors increasing the susceptibility of dairy cows to milk fever incidence. It is therefore expected that differences in the mineral composition of cool-season grasses commonly grown in eastern Canada will result in different DCAD values.
Differences in Na and K concentrations between cultivars of orchardgrass, timothy, and tall fescue were reported (ap Griffith et al., 1965; ap Griffith and Walters, 1966). Gross and Jung (1978) also reported large differences in K, Ca, and Mg concentrations between cultivars of timothy and orchardgrass. Forbes and Gelman (1981) observed that three cultivars of orchardgrass differed in Ca, P, Na, and Mg concentrations. Saiga et al. (1992) found significant differences among four orchardgrass cultivars in concentrations of N, P, Ca, and Mg, but not K. Mika (1982), however, analyzed 12 cultivars of orchardgrass and reported strong varietal differences only in Na concentration. Cultivar differences in mineral composition could therefore affect the DCAD of grass species.
Growth period can influence the forage mineral composition, as shown for tall fescue (Hojjati et al., 1977). Pehrson et al. (1999) observed that the lowest K concentration in grass-dominated swards was found in the second cutting from the crop, which had not been fertilized with K. Grunes et al. (1992) reported that cation and anion concentrations, and the difference between total cations and anions (the sum of K, Mg, Ca, and Na minus the sum of Cl, SO4–S, P, and NO3–N) of smooth bromegrass grown in a growth chamber were higher at the first harvest than at the second harvest made 5 wk later due to depletion by the initial growth. Differences in mineral composition between spring growth and summer regrowth may affect the DCAD.
Very few direct comparisons of mineral concentrations and DCAD among C3 forage species, cultivars, and harvests have been made in which the plants were grown under similar conditions. Our objective was to evaluate the DCAD and mineral composition of a few cultivars of five cool-season grass species grown in eastern Canada and harvested twice or three times a year.
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MATERIALS AND METHODS |
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Four soil samples (one per replication) were collected in May 2003 at each location and for each species (n = 60). At each replication of a given species, four cores from the 0- to 20-cm soil layer were pooled to obtain representative samples that were used to characterize replicate variability. Soil samples were air dried and sieved to 2 mm before analysis.
Smooth bromegrass and timothy were harvested twice a year, whereas orchardgrass, tall fescue, and meadow bromegrass were harvested three times a year at all locations. All grasses were first harvested when they reached the early to mid-heading stage of development. For timothy, the second harvest was also taken at the early heading stage, 6 to 8 wk after the first harvest. Meadow bromegrass, smooth bromegrass, orchardgrass, and tall fescue do not produce reproductive stems in their regrowth following the first harvest; consequently, an interval was used for the timing of the second harvest of those four species. For smooth bromegrass, the period of growth after the first harvest varied among locations: 8 wk at Normandin, nine at St. Augustin, and five at Ste. Anne-de-Bellevue. For all other species, the second and third harvests were taken 5 to 6 wk after the previous harvest. These intervals were chosen based on recommended and common practices. Dry matter yield was measured in each plot by harvesting, at a 5-cm height, a strip 5 or 6 m long and 0.6 m wide with a REM flail forage harvester (Swift Machine and Welding Ltd., Swift Current, SK) at Ste. Anne-de-Bellevue and St. Augustin, and by harvesting a strip 5 m long and 0.9 m wide using a self-propelled flail forage harvester (Carter Mfg. Co., Brookston, IN) at Normandin. A forage sample of 
500 g of fresh matter was taken from each plot, dried at 55°C in a forced-draft oven for approximately 3 d, and ground using a Wiley mill (Standard Model 3, Arthur H. Thomas. Co., Philadelphia, PA) fitted with a 1-mm screen.
Chemical Analyses
Soil pH was measured in a 1:1 soil/water solution according to McKeague (1978). The K, P, Ca, Mg, and Na were extracted by the Mehlich-III solution at pH 2.3 ± 0.15 (Tran and Simard, 1993). The cation exchange capacity was estimated from the sum of K, Ca, Mg, Na, and Al extracted by the 0.1 M BaCl2 solution, and expressed on a molar basis (Sumner and Miller 1996). The K concentration was determined by flame emission and the Ca, Mg, Na, and Al concentrations were determined by atomic absorption spectrometry (PerkinElmer 3300, Überlingen, Germany). The P concentration was analyzed on a Lachat QuikChem 8000 flow injection analysis system (Zellweger Analytics, Lachat Instruments Div., Milwaukee, WI) using an adaptation of Method 12-115-01-1-A (Lachat Instruments, 2005). Soil Cl and S were extracted in a 1:1 soil/water solution (Adriano and Doner, 1982; Johnson and Fixen, 1990) and analyzed by conductivity on a Dionex DX500 equipped with an AS11HC column (Dionex Corp., Sunnyvale, CA). Soil organic matter content was estimated from organic C concentration [organic matter (g kg–1 DM) = 3.5 + (1.80 x organic C [g kg–1 DM])] determined using a dry combustion procedure (Nelson and Sommers, 1982) in a combustion furnace (Leco CNS-1000, Leco Corp., St. Joseph, MI). Soil characteristics at each location are reported in Table 1.
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Subsamples of 0.5 g of dried and ground forages were mineralized at 500°C for 4 h, the ashes were dissolved with 1.0 M HCl (Miller, 1998), and the Na concentration was determined by atomic absorption spectrometry (PerkinElmer 3300). The Ca concentration in samples harvested in 2003 was determined by atomic absorption spectrometry (PerkinElmer 3300) on samples mineralized by dry ashing.
The Cl present in plants is mainly in the ionic form; therefore, the Cl in forage samples can be quantitatively extracted with water, diluted acid, or diluted salts (Liu, 1998). Subsamples of 0.25 g of dried and ground forages were mixed with 20 mL of 0.007 M H2SO4 for 60 min, centrifuged at 32 570 x g for 30 min, and the Cl concentration of the supernatant determined by conductivity on a Dionex DX500 equipped with a AS11HC column.
Subsamples of 0.35 g of dried and ground forages were digested in concentrated HNO3 (Mills and Jones, 1996); organic and inorganic forms of S were converted to the SO4 form that was precipitated with acidified BaCl2, kept 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.
The DCADS and DCADL were then calculated with Eq. [1] and [2], with each element expressed in mmolc kg–1 DM (g kg–1 DM x 1000 x valence/atomic weight).
Statistical Analyses
Data were analyzed using the mixed-model procedure of SAS (Littell et al., 1996), according to a completely randomized design where harvest years (2002 and 2003), the interaction location x species, and replicates were considered random effects. The harvests were considered to be repeated measurements on each experimental plot using the repeated statement with the un(1) unstructured covariance option of the mixed-model procedure. For each species at each location, cultivars were replicated four times. Sources of variation were partitioned as locations, species, cultivars within species, harvests, species x harvests, and cultivars within species x harvests (Table 2). Data normality was verified using the Shapiro-Wilk statistic and the variance homogeneity was verified visually with graphics of residuals (SAS Institute, 1999). Raw data were transformed (logarithmic or square root transformation) when it was deemed appropriate. Least square means or detransformed least square means are reported (Tables 2 and 3). A protected LSD multiple test was used to compare species. A set of simple contrasts (Table 3) was defined a priori for testing differences between the two cultivars of tall fescue (‘Courtenay’ vs. ‘Kokanee’), meadow bromegrass (‘Fleet’ vs. ‘Paddock’), and smooth bromegrass (‘Bravo’ vs. ‘Radisson’). Within these three species, the cultivars did not differ in maturity. For the three cultivars of orchardgrass and the four cultivars of timothy, simple contrasts were selected to compare early- to intermediate- and late-maturing cultivars. ‘Benchmark’ orchardgrass is an early-maturing cultivar, whereas ‘Kay’ and ‘Okay’ are late-maturing cultivars. ‘Tiller’ and ‘Toro’ timothy are early-maturing cultivars, ‘Champ’ is intermediate, and ‘Climax’ is late maturing (Centre de Référence en Agriculture et Agroalimentaire du Québec, 2004). Statistical testing was done at the 0.05 significance level unless noted otherwise. The main statistical analysis used the five grass species harvested twice at each of the three locations (Table 2), but a secondary statistical analysis used the same model but with three species harvested three times a year at the three locations (data not shown).
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RESULTS |
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Dietary Cation Anion Difference
Orchardgrass had the highest DCADS and DCADL and timothy had the smallest in both spring growth and summer regrowth (Table 2). Meadow bromegrass, tall fescue, and smooth bromegrass had intermediate and similar DCADS and DCADL in spring growth. In summer regrowth, meadow bromegrass had a higher DCADS and DCADL than smooth bromegrass, and tall fescue was intermediate between those two species.
On average for all species, the DCADS decreased by 4% between spring growth and summer regrowth but this variation between harvests was different from one species to another, as indicated by the significant species x harvest interaction (Table 2). Between spring growth and summer regrowth, the DCADS decreased by 3 to 14% for orchardgrass, tall fescue, smooth bromegrass, and timothy but it increased by 5% for meadow bromegrass. The DCADL for all species was, on average, similar in spring growth (484 mmolc kg–1 DM) and summer regrowth (485 mmolc kg–1 DM); between the two harvests, however, it was unchanged for tall fescue, decreased by <10% for orchardgrass, smooth bromegrass, and timothy, and increased by 13% for meadow bromegrass. From spring growth to summer regrowth, timothy had the largest decrease in DCADS and DCADL, whereas meadow bromegrass was the only species with an increase in both DCAD values.
For orchardgrass, tall fescue, and meadow bromegrass, the second summer regrowth of both production years was also harvested (a third harvest, data not shown). The DCADS was significantly higher in orchardgrass (632 mmolc kg–1 DM) and meadow bromegrass (609 mmolc kg–1 DM) than in tall fescue (463 mmolc kg–1 DM). Orchardgrass (603 mmolc kg–1 DM) and meadow bromegrass (619 mmolc kg–1 DM) also had a significantly higher DCADL than tall fescue (463 mmolc kg–1 DM). In spring growth and the first summer regrowth, there was no significant difference between meadow bromegrass and tall fescue for DCADS and DCADL (Table 2), but in the second summer regrowth, both DCAD values were higher in meadow bromegrass than tall fescue.
Cation Concentrations
Potassium greatly influenced the DCAD because it was present in the highest concentration and also because the variation in K concentration among the five grass species was important and highly significant (Table 2). The K concentration was highest in orchardgrass, lowest in timothy, and intermediate in meadow bromegrass, tall fescue, and smooth bromegrass; the ranking of the five grass species was exactly identical for K, DCADS, and DCADL in spring growth (Table 2). The species ranking was different in summer regrowth than in spring growth because changes in K concentration between the two harvests were different among species. The K concentration was 4, 5, and 6% lower in summer regrowth than in spring growth for orchardgrass, smooth bromegrass, and timothy, respectively, whereas it was 5 and 2% higher in summer regrowth than in spring growth for meadow bromegrass and tall fescue.
Among the DCADL cations, Ca was the second and Mg the third in importance in terms of concentration in the five grass species. Variations in their concentrations need to be large to significantly influence the DCADL because their concentrations are affected by a low value coefficient of 0.15 in the calculation of DCADL (National Research Council, 2001). In spring growth, Ca concentration was the lowest in orchardgrass and smooth bromegrass, intermediate in meadow bromegrass and timothy, and highest in tall fescue. In summer regrowth, the ranking among species was different—orchardgrass had the lowest, timothy the highest, and tall fescue, meadow bromegrass, and smooth bromegrass had intermediate Ca concentrations. The Ca concentration increased by an average of 29% from spring growth to summer regrowth but this increase varied among species; it increased by 24, 33, 9, 37, and 47% in orchardgrass, meadow bromegrass, tall fescue, smooth bromegrass, and timothy, respectively. These different increases explained the different species ranking for Ca concentration in spring growth and summer regrowth. In summer regrowth, timothy was the grass species with the lowest DCAD and K concentration, and the highest Ca concentration.
Tall fescue had the highest Mg concentration and meadow bromegrass, timothy, and smooth bromegrass had the lowest Mg concentration in both spring growth and summer regrowth; orchardgrass had an intermediate Mg concentration. Even though the species x harvest interaction was significant, the ranking among grass species was quite similar in both spring growth and summer regrowth. The Mg concentration was 40, 42, 30, 31, and 55% greater in summer regrowth than in spring growth for orchardgrass, meadow bromegrass, tall fescue, smooth bromegrass, and timothy, respectively.
The Na concentration varied between 0.015 g kg–1 DM for timothy to 0.051 g kg–1 DM for orchardgrass in spring growth but there was no significant difference in Na concentration among grass species in summer regrowth (average of 0.026 g kg–1 DM, Table 2). The significant variation among species observed in spring growth had practically no influence on the DCAD because the Na concentration was much less than that of the other elements.
Anion Concentrations
Chloride was the anion present in the highest concentration in the five grass species, followed by P and S (Table 2). As stated above, the interaction between species and harvests was not significant (P = 0.082, Table 2) for forage Cl concentration; in both spring growth and summer regrowth, Cl concentration was significantly lower in timothy than in all other grass species. Timothy was the species with the lowest Cl concentration, but because it also had the lowest K concentration, its DCAD values were the lowest of all grass species.
The ranking among species based on the S concentration was not exactly the same in spring growth than in summer regrowth but it was quite similar. Tall fescue had the highest and timothy the lowest S concentration, whereas orchardgrass, meadow bromegrass and smooth bromegrass had intermediate S concentrations (Table 2). In spring growth, orchardgrass and meadow bromegrass had higher P concentrations than tall fescue, smooth bromegrass, and timothy. In summer regrowth, orchardgrass still had the highest P concentration.
Cultivar within Species
The effect of cultivars within species was highly significant only for Ca and Mg concentrations and tended to be significant (0.05 < P < 0.10) for DCADL and concentrations of Na and P (Table 2). For these five variates, means and statistical significance of all a priori cultivar contrasts are presented in Table 3.
Among the eight predefined contrasts, only the contrast comparing the tall fescue cultivars was significant for DCADL (Table 3); on average for both harvests of the two production years, ‘Courtenay’ had a higher DCADL (539 mmolc kg–1 DM) than ‘Kokanee’ (482 mmolc kg–1 DM). This was because ‘Courtenay’ had higher Na, Ca, and Mg concentrations than ‘Kokanee’ and because there was no significant difference between the two cultivars for any of the anion concentrations (Tables 2 and 3).
The early-maturing ‘Benchmark’ orchardgrass had higher Ca and Mg concentrations and a lower P concentration than the late-maturing ‘Kay’ and ‘Okay’ (Table 3). These variations, however, were not large enough to significantly affect the DCADL of the three cultivars, probably because the three elements are affected by a coefficient in the calculation of the DCADL.
For Mg concentration, there were slight but significant variations between the two cultivars of meadow bromegrass and between the two cultivars of smooth bromegrass. The early-maturing ‘Tiller’ and ‘Toro’ timothy also had higher Mg concentrations than the intermediate (‘Champ’) and the late-maturing (‘Climax’) timothy. These variations in Mg concentration, however, were not large enough to create significant variations among smooth bromegrass, meadow bromegrass, and timothy cultivars for their DCAD.
Locations
Locations did not significantly differ in DCADS and DCADL, Na and S concentrations, and DM yield but they differed in forage K, Ca, Mg, Cl, and P concentrations (Table 2). The K concentration was significantly higher in forages grown at Normandin and Ste. Anne-de-Bellevue than at St. Augustin, and it was not necessarily related to soil fertility because the soil K concentration was higher at Normandin but similar at Ste. Anne-de-Bellevue and St. Augustin (Table 1). Forages grown at Normandin had a higher Cl concentration than those grown at Ste. Anne-de-Bellevue, which, in turn, had a higher Cl concentration than those grown at St. Augustin. Forage Cl was related to soil Cl concentration; soil Cl concentration was much higher at Normandin than at Ste. Anne-de-Bellevue and St. Augustin (Table 1). Even though the forage K concentration significantly varied among sites, the DCAD did not; this is probably because both K and Cl concentrations in forages were higher at Normandin and Ste. Anne-de-Bellevue than at St. Augustin (Table 2).
Concentrations of Mg and P in forages were significantly lower at St. Augustin than at Normandin and Ste. Anne-de-Bellevue, and this is probably because soil Mg and P concentrations were lower at St. Augustin than at the other two locations (Table 1). Forage Ca concentration was higher at St. Augustin than at the other locations; soil Ca concentration was also the highest at this location (Table 1).
Relations between Dietary Cation Anion Difference, Mineral Composition, and Dry Matter Yield
In spring growth, the five species differed significantly for DCAD, mineral composition, and forage DM yield (Table 2). On average for spring growth of both production years, 74% of the total covariation among variates was explained by the first axis of the PC (principal component) analysis (
1 = 74%, Fig. 1b). The contribution of a variate to a PC axis can be seen from its loadings (Fig. 1a) or its correlation coefficient (Table 4); both were used. The PC scores for the first axis defined a contrast: DM yield vs. DCADS, DCADL, Na, K, Cl, S, and P concentrations (Fig. 1a). They were negatively associated with DM yield (r = –0.99) and positively associated with DCADS (r = 0.94), DCADL (r = 0.99), and concentrations of Na (r = 0.96), K (r = 0.99), Cl (r = 0.88), S (r = 0.86), and P (r = 0.83) (upper part of Table 4). Variates within the same group were positively correlated and variates in opposing groups were negatively correlated; as DM yield increased, both DCAD values and mineral concentrations except Ca and Mg decreased. A single descriptor for this axis could be called "a dilution effect." Forage DM yield was negatively correlated with forage DCADS (r = –0.97), DCADL (r = –0.99), and concentrations of Na (r = –0.96), K (r = –0.99), Cl (r = –0.81), and P (r = –0.90) (Table 4). Both DCAD values were positively correlated with Na, K, and P concentrations (Table 4). The DCADL was positively correlated with Cl concentration (r = 0.83); the low Cl concentration of timothy should have been reflected in a higher DCADL, but it was more than compensated by a lower K concentration (Table 2). On the first axis, orchardgrass had a higher score than timothy (Fig. 1b); orchardgrass also had significantly lower DM yield and higher DCADS, DCADL, Na, K, Cl, S, and P concentrations than timothy (Table 2). The dilution effect occurred because DM yield varied between 2.56 Mg ha–1 for orchardgrass and 4.35 Mg ha–1 for timothy during spring growth.
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2 = 20%, Fig. 1b), with the score largely determined by Ca concentration (r = 0.95; Table 4). On this axis, tall fescue had a high positive PC score, whereas orchardgrass had a large negative score (Fig. 1b). On average for both production years, spring growth of tall fescue had significantly higher Ca and Mg concentrations and lower P concentration and DCADS than orchardgrass (Table 2). For the first summer regrowth, timothy had a lower DM yield than smooth bromegrass; other forage species had intermediate DM yields (Table 2); however, the DM yield was not included in any contrast defined by the PC scores estimated using variate averages of the two production years (Table 4, Fig. 1c). The range of variation among species for DM yield was less in summer regrowth (1.80 Mg ha–1 for timothy to 3.23 Mg ha–1 for smooth bromegrass) than spring growth (2.56 Mg ha–1 for orchardgrass to 4.35 Mg ha–1 for timothy). The first PC axis explained 56% of the total covariation (Fig. 1d), with the scores being largely determined by DCADS (r = 0.94), DCADL (r = 0.95), and K concentrations (r = 0.97), and negatively associated with Ca concentration (r = – 0.81) (lower part of Table 4 and Fig. 1c). On this axis, timothy had a lower score than the other extreme species, orchardgrass (Fig. 1d); timothy had significantly lower values than orchardgrass for DCADS, DCADL, and K concentrations, whereas it had a significantly higher Ca concentration than orchardgrass (Table 2).
On the second axis (2 = 23%, Fig. 1d), there were some unique species with extreme scores largely determined by Cl concentration (r = 0.81; Table 4); tall fescue had a high positive score, while orchardgrass had a high negative score (Fig. 1d). Tall fescue had a significantly higher Cl concentration and a lower P concentration than orchardgrass; it also had a numerically higher S concentration and a lower Na concentration than orchardgrass (Table 2).
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Along with differences in K concentration, grass species also differed in all other element concentrations. Timothy had lower Na and Mg concentrations than most other grass species but tended to have a higher Ca concentration. Pritchard et al. (1964), in eastern Canada, also noted that timothy had a low Na concentration. Several studies cited by Whitehead (2000) reported lower Na and Mg concentrations in timothy than orchardgrass; however, Sleper et al. (1989) reported a lower Mg concentration in orchardgrass than in timothy. The high Ca concentration of timothy, especially in summer regrowth, adds to the benefits of its low DCAD for the prevention of hypocalcaemia or milk fever in dairy cows.
Timothy had a lower Cl concentration and also tended to have a lower concentration of S. There are few reports of the concentrations of S and Cl in individual grass species. In a review of S and Cl in forage plants, Whitehead (2000) concluded that differences in S and Cl concentration among species appear to be small and often inconsistent. Even though our observed differences were not always significant, our results confirm the reported higher S concentration of tall fescue than orchardgrass or smooth bromegrass (Whitehead, 2000). Roche et al. (2002) indicated that precalving dietary S and Cl concentrations play an important role in Ca homeostasis, in addition to their role in acid–base balance. They reported that dietary S concentration was more important in the control of hypocalcaemia than either dietary K or Cl concentrations; however, Goff et al. (2004) stated that the addition of Cl to prepartum rations would prove more effective than SO4 because SO4 has about 60% of the blood-acidifying activity of Cl.
Differences in DCAD between harvests were relatively small. We had expected a larger decrease in DCAD between spring growth and summer regrowth because of the potentially reduced availability of soil K under summer growing conditions. In fact, the average K concentration for the five species did not differ between spring growth (31.5 g kg–1 DM) and summer regrowth (31.1 g kg–1 DM). The proportion of leaves was also probably higher in summer regrowth than in spring growth, even in timothy, the only recurrent-flowering species (Bélanger and McQueen, 1998). The concentration of certain elements (K, Ca, and Mg) is usually higher in leaves than in stems (Pritchard et al., 1964). This, combined with a greater proportion of leaves in the summer growth, may have counterbalanced the reduced nutrient availability.
The principal component analysis clearly establishes four groups of grasses when the relationship between DM yield, DCAD, and mineral composition is considered. Timothy is characterized by a high yield in the spring growth and a high Ca concentration in the summer regrowth but a low DCAD and low Na, K, Mg, Cl, and P concentrations in both spring growth and summer regrowth. Orchardgrass, on the other hand, is characterized by high Na, K, and P concentrations and a high DCAD in spring growth and summer regrowth. Tall fescue is primarily characterized by a high Ca concentration in spring growth and high Mg, Cl, and S concentrations in spring growth and summer regrowth. Smooth bromegrass and meadow bromegrass tended to be intermediate between timothy, orchardgrass, and tall fescue. These results clearly indicate major differences among forage grass species in their mineral composition.
We only observed a cultivar difference in DCADL with tall fescue; no cultivar differences in DCADS or DCADL were observed in the four other species. In tall fescue, the cultivar difference in DCADL was attributed to differences in Na, Ca, and Mg concentrations. Significant variations were reported for Ca and Mg concentrations in tall fescue cultivars grown in Québec (Drapeau et al., 2005) and for Mg concentration in tall fescue genotypes (Brown and Sleper, 1980). We also observed cultivar differences in Mg concentration in orchardgrass, meadow bromegrass, smooth bromegrass, and timothy, and in Ca and P concentrations in orchardgrass. Sleper et al. (1989) reported genetic variation for K, Ca, and Mg concentrations within C3 grass species. Our results suggest that there are limited cultivar differences in DCAD of forage grass species and that the choice of cultivars will have minimal or no effect on the forage DCAD; however, only a few cultivars for each species were tested in our study.
Variations in forage mineral concentration associated with soil types and prior soil fertility are well established (Soder and Stout, 2003). Although our locations differed in soil fertility and forage K, Ca, Mg, Cl, and P concentrations, there was no difference in DCAD among locations. The high forage K concentration at two of the locations was compensated by a high forage Cl concentration. At the three locations, soil available K was considered good to rich (Centre de Référence en Agriculture et Agroalimentaire du Québec, 2003) and this is probably the main reason why no relationship was seen between soil available K and the DCAD.
The target DCAD of the ration for milk fever prevention is around –50 mmolc kg–1 DM when using the DCADS equation or around +150 mmolc kg–1 DM when using the DCADL equation (Goff and Horst, 2003). In our study, the forage DCADS and DCADL were always higher than the target values of the ration. The soil K availability was considered good at two locations and rich at the other location (Table 1). It is then apparent that soils with lower soil K availability will be required to produce forages with DCAD near the target values for dry cows. Other options such as Cl fertilization could also be considered to reduce the DCAD. Choosing a grass species with a low DCAD, such as timothy, and growing it on a soil with low K availability and possibly with Cl fertilization, would contribute to reaching the target DCAD of the dry cow ration; this would then reduce the incidence of milk fever and increase milk production, resulting in lower production costs.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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V. S. Heron, G. F. Tremblay, and M. Oba Timothy hays differing in dietary cation-anion difference affect the capability of dairy cows to maintain their calcium homeostasis J Dairy Sci, January 1, 2009; 92(1): 238 - 246. [Abstract] [Full Text] [PDF]
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G. B. Penner, G. F. Tremblay, T. Dow, and M. Oba Timothy Hay with a Low Dietary Cation-Anion Difference Improves Calcium Homeostasis in Periparturient Holstein Cows J Dairy Sci, May 1, 2008; 91(5): 1959 - 1968. [Abstract] [Full Text] [PDF]
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E. Charbonneau, P. Y. Chouinard, G. F. Tremblay, G. Allard, and D. Pellerin Hay to Reduce Dietary Cation-Anion Difference for Dry Dairy Cows J Dairy Sci, April 1, 2008; 91(4): 1585 - 1596. [Abstract] [Full Text] [PDF]
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 S. Pelletier, G. Belanger, G. F. Tremblay, M. H. Chantigny, and G. Allard Dietary Cation Anion Difference and Tetany Index of Timothy Forage Fertilized with Liquid Swine Manure Agron. J., January 11, 2008; 100(1): 213 - 220. [Abstract] [Full Text] [PDF]
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