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a Dép. de Phytologie, Faculté des Sciences de l'Agriculture et de l'alimentation, Université Laval, Québec, QC, Canada G1K 7P4
b Agric. and Agri-Food Canada, Soils and Crops Research and Development Centre, Québec, QC, Canada G1V 2J3
* Corresponding author (belangergf{at}agr.gc.ca)
Received for publication August 4, 2005.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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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 • PCA, principal component analysis • SEM, standard error of the mean
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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A ration with a low DCAD has been shown to reduce the incidence of hypocalcaemia in dry dairy cows (Horst et al., 1997). Many equations have been proposed to calculate the DCAD of a ration, but two are more commonly used. The first one, referred to as the short DCAD (DCADS), includes two cations and two anions as follows:
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Forages produced on intensive dairy farms often have a high DCAD because of the high K content of their soils (Kayser and Isselstein, 2005). When those forages are fed to dry dairy cows, anionic salts are usually added to reduce the DCADS of the ration to a recommended value of around –50 mmolc kg–1 DM according to Goff and Horst (2003). Anionic salts, however, reduce the palatability of the ration and represent a substantial cost for producers (Schauff et al., 2000). Indeed, if the ration DCAD is higher than 250 mmolc kg–1 DM, it is particularly difficult to add enough anionic salts to lower the DCAD to the recommended value without experiencing palatability problems (Horst et al., 1997). Forages fed to dry dairy cows should therefore have a maximum DCAD of 250 mmolc kg–1 DM but, preferably, the DCAD should be as low as possible to reduce the amount of anionic salts to be used.
Timothy (Phleum pratense L.) has been identified as a potentially good grass for dry dairy cows. It has a lower DCAD than other cool-season grass species because of its lower K concentration (Thomas et al., 1998b; Tremblay et al., 2006).
Potassium and Cl have the greatest effect on DCAD because of their high concentrations in plants and their coefficient of 1.0 in the DCAD equations. Other mineral elements, such as N and P, also affect DCAD. Nitrogen is not included in DCAD equations but N fertilization may impact forage DCAD because it affects the concentration of cations and anions that are used in its calculation. Nitrogen fertilization can either increase grass K concentration when K is highly available in soil, decrease it when soil K supplies are low (Grant and MacLean, 1966; Horst et al., 1997; Whitehead, 2000), or have no effect (Hopkins et al., 1994). The influence of N fertilization on plant Cl uptake depends on the form of N applied. Antagonism may occur between Cl and nitrate (NO3–), while the uptake of Cl may be increased when ammonium (NH4+) is present in large amounts (Glass and Siddiqi, 1985; Britto et al., 2004). Nitrogen fertilization also increases the concentrations of Mg, P, and Na and decreases the concentrations of Ca and S in forage grasses (Hopkins et al., 1994; Thomas et al., 1998a; Bélanger and Richards, 1999).
Phosphorus fertilization can directly influence DCADL because it affects timothy P concentration (Grant and MacLean, 1966). Since P fertilization is known to increase plant Na, Ca, and Mg concentrations, and to slightly reduce plant K concentrations (Fleming, 1973; Wilman et al., 1994; Bruulsema and Cherney, 1995), it can also have an indirect effect on DCADS and DCADL. Moreover, a P deficiency can limit plant response to N fertilization (Bélanger et al., 1989), which would alter the effect of N fertilization on the uptake of the elements used to calculate DCAD (Whitehead, 1995).
Development stage also affects the mineral concentration of forages which, in turn, is likely to affect their DCAD. Decreases in K, S, and P concentrations with advancing maturity have been reported for several forage species (Wilman et al., 1994; Bélanger and Richards, 1999; Whitehead, 2000). In contrast, Ca, Mg, and Cl concentrations have been reported to increase with development stages (Wilman et al., 1994), while results on Na concentration are inconsistent (Whitehead, 2000). The effect of development stage on the DCAD of forage grasses has not yet been established.
The objective of this study was to determine the effects of development stage and N and P fertilization on the DCAD and mineral concentrations of timothy. In particular, we wanted to determine what combination of development stage and N and P treatments resulted in a DCAD of <250 mmolc kg–1 DM for timothy grown on a soil with a high K content, typical of most dairy farms in northeastern North America.
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MATERIALS AND METHODS |
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In the spring of 2001 and 2002, timothy was harvested at four developmental stages: stem elongation (6 June in both years), early heading (13 June in 2001 and 12 June in 2002), late heading (20 June in 2001 and 19 June in 2002), and early flowering (26 June in both years). Dry matter yield was determined by harvesting forage at a 5-cm height from a 2.1 x 0.9-m area in each plot using a self-propelled flail forage harvester (Carter MGF Co., Brookston, IN). A fresh sample of approximately 500 g was taken from each plot, weighed, and then dried at 55°C in a forced-draft oven for 3 d for determination of percentage DM. Samples were then ground using a Wiley mill (Standard model 3, Arthur H. Thomas Co., Philadelphia, PA) fitted with a 1-mm screen.
Chemical Analyses
Plant N, P, K, Ca, and Mg were extracted using a method adapted from Isaac and Johnson (1976). Dried and ground forage samples of 100 mg were digested during 45 min at 380°C with a 1.5-mL mixture of sulfuric and selenious acid plus 2 mL of H2O2 30%. After cooling, the mixture was diluted to 75 mL with deionized water. Chloride was extracted using a method adapted from Liu (1998); 250-mg samples of dried and ground forages were mixed with 20 mL of H2SO4 7 mMol L–1 for 60 min, centrifuged at 32 570 x g for 30 min, and the Cl concentration was determined in the supernatant. Extraction of S followed a method adapted from Mills and Jones (1996). Three milliliters of nitric acid (HNO3) were added to dried and ground samples of 350 mg in digestion tubes. These tubes were covered with perforated aluminum foil, placed on a digester block, heated at 120°C for 30 min, and cooled for 2 min before adding 3 mL of H2O2 30%. Tube contents were digested again for 15 min using the same procedure. The previous steps were repeated five times until the solution became colorless. Tubes were cooled and the solution was diluted to 40 mL with deionized water. Finally, Na was extracted by dry ashing (Miller, 1998).
A QuikChem 8000 Lachat autoanalyzer was used to measure N with the method 13-107-06-2-E, S with the method 10-116-10-1-G, and P with the method 15-501-3 (Zellweger Analytics, Lachat Instruments, Milwaukee, WI). A PerkinElmer 3300 atomic absorption spectrometer (PerkinElmer, Überlingen, Germany) was used to determine K by flame emission, and Ca, Na, and Mg by atomic absorption. Chloride was measured with a Dionex DX 500 chromatograph equipped with a ASII HC column (Dionex Corp., Sunnyvale, CA). 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
The experimental design was a split-split plot with four replicates. Stages of development were assigned to main plots, P fertilizer treatments to subplots, and N fertilizer treatments to sub-subplots. Harvest years and replicates were considered to be random effects and consequently, data were analyzed using the Mixed procedure (Littell et al., 1996) of SAS (SAS Institute, 1999). Sources of variation are presented in Table 1. The effects of N treatments and stages of development were partitioned into linear and quadratic contrasts; the stages of development were quantified as 1 to 4 as the harvest dates were 6 or 7 d apart. Statistical significance was postulated at P 
0.05. Least square means and standard error of the means (SEM) were calculated. Principal component analysis (PCA) was used to understand relationships among variates related to DCAD and it was performed on the least square means using the correlation matrix method (SAS Institute, 1999) to give equal weight to all variates. The contribution of each variate to a PC axis can be seen from its loadings (Fig. 1
). Pearson correlation coefficients among variates were also calculated.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The DCADL also decreased with stages of development for all N fertilizer treatments, but this decrease was quadratic for treatments of 60, 120, and 180 kg N ha–1; for these N treatments, the decrease of DCADL with stages of development was greater between stem elongation and early heading than between later stages of development (Table 2). From stem elongation to early heading, the rate of decrease in DCADL was greatest with 60 kg N ha–1 (13 mmolc kg–1 DM d–1) and least with no N fertilization (3 mmolc kg–1 DM d–1). The rate of decrease of DCADL was least between the two last stages of development for all N treatments (1–4 mmolc kg–1 DM d–1) except for no N fertilization (7 mmolc kg–1 DM d–1).
These effects of development stage on DCAD were related to variations in the concentrations of the elements included in the DCAD equations. The most influential elements were K and Cl; timothy K concentration decreased by 17% between stem elongation and early flowering, while Cl concentration increased by 11% between stem elongation and early heading and then remained stable for the last two stages of development (Table 2). Therefore, the combined effect of timothy K and Cl concentrations explains most of the decrease in both DCADS and DCADL with stages of development. The decrease in timothy Ca concentration with development stage was highly significant (Table 1), especially between stem elongation and early heading when it decreased by 42%. This contributed to a greater decrease of DCADL (61 mmolc kg–1 DM) between the first two stages of development compared to the decrease of DCADS (49 mmolc kg–1 DM). Timothy Mg concentration decreased by 12% between stem elongation and late heading, whereas timothy S concentration decreased by 12%, and P concentration by 28% between stem elongation and early flowering. This decrease in S and P concentrations should have resulted in an increase in DCADL. However, it was not enough to counteract the decrease in DCADL resulting from the decrease in plant K concentration and the increase in plant Cl concentration. Magnesium, S, and P were in lower concentrations than K, Ca, and Cl. Moreover, they are affected by a coefficient that reduces their weight in the calculation of DCADL whereas K and Cl account for 100% of their concentration. The overall variations in timothy Mg, S, and P concentrations only tempered the decrease of DCADL with development stage caused by the decrease in K concentration and the increase in Cl concentration.
Nitrogen fertilization affected timothy DCADS but did not significantly influence DCADL (Table 1). However, the interaction between development stage and N treatment was significant for both DCADS and DCADL. At stem elongation, there was a significant increase in both DCADS and DCADL when N fertilization went from 0 to 60 kg ha–1 with no further increase at higher N applications. This result is attributable to a more than twofold increase in timothy K concentration (5.8 g kg–1 DM) as compared with Cl concentration (2.3 g kg–1 DM) when N fertilization went from 0 to 60 kg ha–1. At the three later development stages, there was no significant effect of N treatment on either DCAD (Table 2). The timothy K concentration also increased (2.7–4.4 g kg–1 DM) when N fertilization went from 0 to 60 kg ha–1 at the three later stages of development, but this increase did not significantly affect DCADS or DCADL; the increase in Cl concentration with 60 kg N ha–1 (1.9–3.2 g kg–1 DM) was sufficient to compensate for the increased timothy K concentration.
Under our experimental conditions where soil P did not limit DM yield, P fertilization did not affect DCADS or DCADL (Table 1) even though timothy S concentration increased by 3% and P concentration by 13% when P fertilization went from 0 to 45 kg ha–1 (data not shown).
Dry Matter Yield and Nitrogen Concentration
Dry matter yield was affected by the interaction between development stage, N treatment, and P treatment but it was mostly influenced by N treatment and development stage; P fertilization alone had no significant effect on DM yield (Table 1). Dry matter yield increased with development stage and N treatments (Table 2). With no N fertilization, DM yield was between 32 and 41% of the maximum yield observed with 180 kg N ha–1; this indicates that plant growth was severely limited when no N was applied. The increase in DM yield with N fertilization was greatest when fertilization went from 0 to 60 kg N ha–1 at all stages of development. As expected, timothy N concentration increased with increasing N fertilization and decreased with stages of development (Table 2). The increase in timothy N concentration with 180 kg N ha–1, as compared with the no N treatment, was greater at stem elongation (75%) and early heading (59%) than at late heading (40%) and early flowering (49%).
Relationships among Variates
The relationship among variates (Fig. 1A) and the interaction between development stage and N treatment (Fig. 1B) are represented by the PCA diagrams. The first component of the PCA explains 55% of the total covariation. A contrast is defined between DM yield, on the left side, and DCADS, DCADL, and N, P, K, Ca, and S concentrations, on the right (Fig. 1A). This contrast is primarily driven by development stages as their scores are well separated along the axis of the first component (Fig. 1B). The two DCAD values and concentrations of N, P, K, Ca, and S decreased, while forage DM yield increased with development stage (Table 2). In the first component, the three highest N treatments at the stem elongation stage (S1N2, S1N3, and S1N4 in Fig. 1B) were opposed to all N treatments at the early flowering stage (S4N1–S4N4 in Fig. 1B); all other treatments were intermediate (Fig. 1B).
The first component of the PCA reflects the dilution that occurred with increasing DM yield (Fig. 1A). This dilution is caused by a greater increase in the concentration of the C-rich structural component compared to the N-rich metabolic component of the plant (Bélanger and Gastal, 2000). The dilution of N and P with increasing DM yield in timothy is well documented (Bélanger and Richards, 1997, 1999). The decline of S concentration with stages of development has been reported for other forage species (Whitehead, 2000) but, to our knowledge, it is the first time that a dilution of K, Ca, and S with increasing DM yield is reported in timothy. For each N treatment, the decrease of timothy K concentration with increasing DM yield associated to stages of development was similar to that of N and P concentrations and was consistent throughout the growing cycle (Fig. 2 ). For S concentration, the dilution was primarily observed from stem elongation to late heading, while for Ca concentration there was a marked decrease between stem elongation and early heading with limited changes at later stages (Fig. 2).
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The second component of the PCA explains 24% of the total covariation and is driven by N treatment. Indeed, as evident on the second component axis, the scores for the treatment with no N applied at all development stages were opposed to N applications of 120 and 180 kg N ha–1; N at 60 kg N ha–1 was intermediate (Fig. 1B). The second component indicates that timothy N, K, and Cl concentrations, and DM yield, were positively correlated to each other but negatively associated with Na and Ca concentrations (Fig. 1A). It was expected that timothy N concentration would be associated with DM yield, as they both increase with N fertilization. However, these two parameters were not significantly correlated (Table 3); this lack of correlation can be explained by the positive relationship between N concentration and DM yield with increasing N fertilization and the negative relationship between N concentration and DM yield with advancing maturity. The latter two phenomena occurred simultaneously. Timothy N and K concentrations are known to follow a similar pattern in plant tissue and they were significantly correlated (r = 0.85) in this study (Table 3). Finally, the presence of timothy Cl concentration in the same group as timothy K concentration on the second component axis is probably linked to the small KCl spring application. The contrast between timothy K concentration and timothy Na and Ca concentrations on the second component axis may be explained by an antagonistic effect among cations.
A significant and negative correlation (r = –0.61) was observed between DM yield and DCADL (Table 3). This is of particular interest economically, as it means that greater DM yields at later development stages will lead to a lower DCADL. A similar trend was also observed for DCADS (r = –0.47, P = 0.06).
Our results indicate that harvesting timothy at a later development stage would be preferred if the main objective is to produce forages with a DCAD of <250 mmolc kg–1 DM. In this study on a soil with a high K content, DCADL and DCADS were reduced to <250 mmolc kg–1 DM when timothy was harvested at early heading and late heading, respectively. The concomitant reduction in N concentration and in other nutritive traits would be of less importance for dry cows that have relatively lower requirements for forage digestibility and protein. Our results also indicate that an appropriate N fertilization should be used; it increases DM yield and N concentration while having no effect on the DCAD of timothy when it is harvested at early heading or later.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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