Archived at http://orgprints.org/7226 

Organic Forage Crop Production in Atlantic Zones: an Ecophysiological Approach
N. PEDROL and A. MARTÍNEZ
Servicio Regional de Investigación y Desarrollo Agroalimentario (SERIDA).
Estación Experimental ‘La Mata’. Apdo 13. 33820 Grado (Asturias), Spain.
ABSTRACT
Within a schedule of long-term organic forage crop rotations, two mixed stands of ryegrass-clover were evaluated as following crops after winter cereal-legume mixtures under different sowing and tillage methods in Asturias (N. Spain). Significant differences were found in productivity, sward components, chemical composition and in growth dynamics. The crop sequence perennial ryegrass-white clover established under a triticale-field bean canopy produced an additional 6 t DM/ha, compared with oat-vetch followed by Italian ryegrass-red clover under regular post-harvest sowing, thus being a promising method for improving organic forage productions with minimum tillage. Changes in productivity and feeding values observed for both these ryegrass-clover swards along different growth periods were studied  together with soil nutrient evolution and seasonal climatic fluctuations. Correlation analysis showed ecophysiological links among crop, soil and climate variables.

INTRODUCTION
With the disadvantages of small holding size, organic farmers in the Atlantic zones of Spain face a great challenge of self-sufficiency, in order to reduce as much as possible external fodder purchase. Thus, new strategies to maximize forage production are required that are not detrimental to sustainability. Research in organic farming (OF) is demanding more holistic approaches, by linking as many components of the agroecosystem as possible (Isart and Llerena, 1999). Useful agronomic results obtained from OF studies may be considered from a wider ecophysiological perspective, by taking into account the complex interactions among crop growth, soil and climate (Lambers et al., 1998). In this work, we tested minimum tillage methods to improve organic forage productions in the Atlantic zones and propose an ecophysiological approach to the growth evolution of ryegrass-clover swards.

MATHERIALS AND METHODS
Two forage crop sequences were started on a mixed meadow site at Llanera, Asturias, N. Spain (46º26/N, 5º50/W) on 30 October 2001. Based on previous soil analysis, tillage and amendment was in accordance with the organic management rules (commercial organic fertilizer, N:P:K 42:56:105; dolomite, Ca:Mg 883:494). Sequence 1 was started with a mixed crop of oats (O)  (cv. Prevision)/vetch (V) (cv. Acisreina) (OV 3:2, 265 seeds/m2) followed by a mixture of Italian ryegrass (Ir) (cv. Serenade)/red clover (Rc) (cv. Marino) (IrRc 5:4, 45 kg/ha) sown on 16 May 2002 after harvesting the cereal/legume crop. For sequence 2, an open mixed stand of triticale (T) (cv. Senatrit)/field bean (B) (cv. Rutabon) (TB 6:1, 195 seeds/m2) was sown simultaneously with a mixture of perennial ryegrass (Pr) (cv. Tove)/white clover (Wc) (cv. Huia) (PrWc 6:1, 40 kg/ha), so that the grass/clover sward was already established once the cereal/legume crops were harvested on 30 April 2002. Each rotation was replicated four times in plots of 300 m2.
Different cuts corresponding to subsequent growth periods (GP) were obtained from each grass/clover sward until late April 2003. For IrRc the cutting dates were 9 November 2002 (GP2, summer), 11 December 2002 (GP3, autumn), and 29 April 2003 (GP4, winter-spring). The PrWc was mown on the same dates plus an extra early cut on 8 July 2002 (GP1, late spring-early summer). Following soil nutrient-status analyses (3 May 2002 and 27 February 2003), organic fertilizer (N:P:K 25:33:63) was applied to both rotations on 4 March 2003, during GP4. Climatic variables (Tmin, Tmax, Tmed and rainfall) were measured daily along all growth periods. For each sward and cut, growth rate (GR), yield and its components (ryegrass, clover, other species, and dead matter) were obtained on a DM basis. Subsamples were analysed for chemical composition: crude protein (CP; TECATOR, 1995), water soluble carbohydrates (WSC; Hoffman, 1937), neutral-detergent fibre (NDF; Van Soest et al., 1991), and acid-detergent fibre (ADF; Goering and Van Soest, 1970). Evolution of parameters within mixtures along growth periods were statistically analysed by one-way ANOVA and LSD-test. Total yields were compared by t-test. Effects of mixtures, common growth periods (GP2 to GP4), and their interactions were tested by two-way ANOVA. Correlations among all measured plant, soil and climate variables were analysed by Pearson’s coefficient. Chemical composition and yields were also measured for initial cereal/legume mixed crops and compared by t-test. 

RESULTS AND DISCUSSION
For cereal/legume mixed crops, TB stands were significantly more productive than OV (9600 vs. 8320 kg DM/ha; P<0.05). The success of TB mixtures in previous work has been based on the early growth and high competitive ability of field beans compared with other forage legumes, resulting in TB organic productions that compete in yield and quality with conventional ones (Pedrol and Martínez, 2003). Both mixed crops did not show significant differences in chemical composition (data not shown).
From the sum of yields taken at subsequent cuts from ryegrass/clover swards, total yields for PrWc were 1.7 times higher than for IrRc, (11113 vs. 6502 kg DM/ha; P<0.001). This was attributed to the contribution of a productive cut of PrWc in early summer (GP1, Table 1) obtained after an excellent establishment of the mixture under the open canopy of TB, which protects seedlings from chilling, imposing minimum shading. In this first growth period PrWc had the most intense growth rate, the highest ryegrass and fibre contents, and a good WSC balance (Table 1). Undersowing the organic sward with a winter cereal/legume crop is shown to be a good strategy to encourage grass growth in the forthcoming spring-summer.
The two-way ANOVA obtained for the subsequent common cuts (Table 1) illustrates the differences in yield parameters and chemical composition between PrWc and IrRc mixtures as a whole. Growth period (cGP) had highly significant effects on independent variables, but seasonal differences in T and rainfall did not covariate with all of them (Table 2). For example, the relative contribution to yield of ryegrass and dead matter was not correlated with the climatic conditions of each GP, but covariated significantly in different directions with changes in soil fertility. In both of the grass/clover swards, ryegrass percentage decreased with litter increase (Table 1) probably due to progressive depletion of N and K, whereas this tendency changed notably after fertilization (GP4). CP contents were, surprisingly, negatively correlated with soil N and K, and were not correlated with clover contents (Table 2). Regarding the highly significant positive correlation with litter percentage, the maintenance or even the rise in CP contents was probably due to resorption from litter. The nitrogen mainly invested in the photosynthetic apparatus is exported via phloem from senescent tissues (as amino acids or active protein pools) to growing tissues. Nutrient resorption is particularly efficient in grasses such as ryegrass, which have active growth of new leaves (Aerts, 1996), especially under limited nitrogen supply, e.g. some organic swards (Culleton et al., 2002). Of course, soil N availability in each GP was also positively correlated with yield and GR (Table 2).
Table 1. Above: Evolution of yield, components, and chemical composition in subsequent growth periods (GP) of two organic swards: perennial ryegrass/white clover (PrWc) and Italian ryegrass/red clover (IrRc). For each mixture asterisks denote significant differences among growth periods (one-way ANOVA) and mean values (n=4) labelled with distinct letters are significantly different (P<0.05; LSD-Test). Below: Two-way ANOVA of the effects of mixtures and their common growth periods (cGP). ***P<0.001, **P<0.01, *P<0.05, NS (not significant) P>0.05. [All values refer to DM, except DM % (over FM)].

		DM	Yield	rye	clo	others	litter	GR	CP	WSC	NDF	ADF
sward	GP	%	kg/ha	%	%	%	%	kg/ha·day	%	%	%	%
PrWc	1	20.2a	4352a	67.0a	3.8a	26.3a	3.3a	63.1a	11.0a	12.7a	58.6a	31.8a
	2	11.4b	1151b	37.8b	13.5b	34.3a	14.5b	17.7b	17.5b	6.3b	53.1b	31.7a
	3	16.0c	1629b	24.5b	1.8a	49.5a	23.5c	17.9b	17.7b	14.0a	46.3c	27.0b
	4	17.9c	3981a	36.5b	15.0b	39.5a	8.8ab	29.7c	14.5c	12.2a	49.8bc	26.9b
		***	***	**	*	NS	***	***	***	**	***	**
IrRc	2	10.9a	1000a	39.8a	9.0a	38.8a	12.5a	8.5a	16.7a	6.5a	51.4a	31.1a
	3	15.5b	1361a	17.0b	9.3a	40.5a	32.8b	15.0b	17.8a	11.7b	44.7b	24.7b
	4	17.7c	4142b	29.3ab	36.8b	25.3a	8.3a	30.9c	17.3a	11.5b	47.0b	24.1b
		***	***	*	***	NS	***	***	NS	***	**	***
	M	NS	NS	NS	*	NS	NS	*	NS	NS	*	*
	cGP	***	***	**	***	NS	***	***	*	***	***	***
	McGP	NS	NS	NS	*	NS	*	*	*	NS	NS	NS
Legend: rye, ryegrass; clo, clover; others, other species; litter, dead matter; GR, growth rate; CP, crude protein; WSC, water-soluble carbohydrate; NDF, neutral-detergent fibre; ADF, acid-detergent fibre.

Clover contribution to total yield was significantly superior in IrRc, achieving values of optimal establishment of red clover in GP4, after fertilization (Table 1, above). This is a clear indicator of a well-developed tap root system during the preceding growth periods, which allows a fortified regrowth after winter (Culleton et al., 2002). The significant loss of white clover in GP3 could be due to the reduction of soil K levels (Baars and Younie, 1998), but soil K contents and depletion (from 72 to 43 mg/l) were almost identical for both swards. The % of “others” was negatively correlated with ryegrass and clover contents  (Table 2). Although total amount of weeds did not change significantly with soil fertility and seasonal climatic fluctuations (Tables 1 and 2), the relative contribution of different species differed between swards and changed along subsequent growth periods. White clover growth in GP3 (autumn) was restricted by the increased presence of the highly competitive Agrostis and Yorkshire fog (components of “others”), whereas this fraction in IrRc was mainly composed of  the non-aggressive plantain. 

Table 2. Correlation analysis of yield-, chemical composition-, climate- and soil fertility related variables measured in each subsequent growth periods of two organic ryegrass/clover swards. Asterisks denote the significance level of positive (+) or negative () bivariate correlations tested by Pearson’s coefficient (n=28): *** P<0.001, ** P<0.01, * P<0.05, NS (not significant) at P>0.05.
Variable	DM	Yield	rye	clo	others	litter	GR	CP	WSC	NDF	ADF
Yield	***+	
rye	NS	*+
clo	NS	NS	NS	
others	NS	*	***	*
litter	NS	***	***	NS	NS
GR	***+	***+	***+	NS	*	**
CP	***	***	***	NS	NS	***+	***	
WSC	***+	**+	NS	NS	NS	NS	*+	NS
NDF	NS	NS	***+	NS	*	*	***+	***	NS
ADF	NS	NS	***+	*	NS	NS	NS	*	*	***+
T	***	***	NS	*	NS	NS	NS	NS	***	*+	***+
Rainfall	NS	*+	NS	*+	NS	NS	NS	NS	NS	NS	*
N	NS	*+	*+	NS	NS	***	*+	***	NS	***+	***+
P	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS	NS
K	NS	NS	***+	NS	NS	***	NS	**	NS	***+	***+
T and Rainfall, mean temperature (ºC) and accumulated rainfall (mm) of each growth period. Others as listed in Table 1.

Cuts taken after summer growth (GP2, Table 1) were the least productive, having the lowest DM and WSC contents but high fibre proportions; all these parameters showed significant correlations with T (Table 2). As a response to high temperatures in summer, stomatal closure is induced to avoid excessive water loss by transpiration, thus leaf expansion and plant growth are reduced. Due to the compromise between carbon gain and water loss, diminished net photosynthetic rates lead to a decrease in WSC concentrations, whereas reduced growth leads to C storage and deposition in cell walls as reflected in higher fibre contents (Table 1).
From our results, we conclude that the establishment of the perennial ryegrass-white clover sward together with the starting crop triticale-field bean is a promising, low-cost method to improve organic forage production. In this trial we obtained an additional 6 t DM/ha, compared with oat-vetch followed by Italian ryegrass-red clover under regular post-harvest sowing. Besides being more productive, the system has faster work rates, requires minimum tillage and reduces crop establishment costs. While these are early results from a long-term trial, some of the ecophysiological processes that underlie our field agronomic observations can be explained. From consideration of correlations among some plant, soil and climate parameters, it is possible to achieve a better understanding of the crop performance in a given agroecosystem.

ACKNOWLEDGEMENTS
Reseach work was supported by the INIA project RTA01-144-C5-2. The authors wish to thank the team from "La Mata" for their essential field work assistance. We are also very grateful to Alan Hopkins for his careful revision of the manuscript.

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