Influence of 2-, 3-, 4- and 5-year stands
of alfalfa on winter wheat yield
E. Campiglia, F. Caporali, P. Bàrberi and R. Mancinelli Department of Crop Production, University of Tuscia S. Camillo de Lellis, 01100 Viterbo, Italy
Summary
Legumes such as alfalfa (Medicago sativa L.) are fundamental components of organic cropping systems. The effect of 5-year long cropping sequences [sunflower (Helianthus annuus L.)-winter wheat (Triticum
aestivum L.) 2-yr rotation; 3-yr Italian ryegrass (Lolium multiflorum Lam.) + 2-yr alfalfa; 2-yr Italian
ryegrass + 3-yr alfalfa; 1-yr Italian ryegrass + 4-yr alfalfa; 5-yr alfalfa] on winter wheat yield was studied in Central Italy in a long-term experiment which started in 1992. Sowing date (standard and late) and mineral nitrogen fertilisation rate (0, 60 and 120 kg N ha-1) applied to wheat were included as additional
experimental factors. Winter wheat grain yield in the unfertilised preceding crop sequences ranged from 1.4 t ha-1 in the sunflower-wheat sequence to 3.7 t ha-1 following 5-yr alfalfa. When N fertiliser was
applied, the wheat grain yield was increased irrespective of preceding crop sequences, the maximum yield being reached following alfalfa at 60 kg N ha-1. Total N content in wheat grain was positively
influenced by alfalfa stand age and N fertiliser rate. Results indicate that in a Mediterranean environment a satisfactory winter wheat yield can be achieved without any mineral N fertilisation provided that alfalfa is grown as a preceding crop for a minimum of 3 years. Inclusion of an alfalfa stand in organic cropping systems is therefore strongly recommended.
Introduction
Legumes such as alfalfa are fundamental components of organic cropping systems because they are particularly suited to make crop rotations less dependent from auxiliary energy inputs (Pierce & Rice, 1988) and they can positively influence the structure and functioning of the agroecosystem (Caporali & Onnis, 1992). Documented effects include reduction of disease, weed and insect infestations, increase in soil organic matter content, and enhancement of soil particle aggregation and water infiltration (Angers, 1992). Several studies have shown that improved crop yield and product quality usually emerge when alfalfa is grown as a preceding crop (Caporali & Onnis, 1992; Seliga & Shattuck, 1995; Anderson
et al., 1997; Holford & Crocker, 1997). One of the major effects of alfalfa is its well-known ability to fix symbiotically large amounts of atmospheric nitrogen (N2), most of which is carried over to succeeding crops (Pantanelli, 1952; Bruulsema & Christie, 1987; Harris & Hesterman, 1990). The amount of N2
fixed can range from 174 kg N ha-1 in the first growing year to 466 kg N ha-1 in the third year (Kelner et
al., 1997). An additional benefit of including alfalfa in a cropping sequence is its ability to extract and utilise NO3--N that has leached beyond the root zone of most annual crops (Schuman & Elliot, 1978).
Although large amounts of fixed N are removed with forage harvest, significant quantities are added to the soil system with the incorporation of herbage and roots (Sheaffer et al., 1991). In organic cropping systems, where one of the key points to achieve long-term sustainability is the replenishment of the N removed from the soil by agricultural products, the inclusion of alfalfa can be suitable to replace the majority of nitrogen taken up from the system. Wheat is the most widely grown winter cereal in central
and southern Italy. Because of its high yielding ability, wheat is believed to have a large requirement for nitrogen and therefore to rapidly deplete soil N content. Few studies have documented the effects of alfalfa stands of different age on the performance of the subsequent wheat. Our objective was to determine the effects of 2-, 3-, 4- and 5-year stands of alfalfa on grain yield, grain protein content, and N fertiliser requirements of a subsequent wheat crop, in order to evaluate the potential of alfalfa to substitute for mineral N fertilisation.
Materials and methods
A field trial was conducted at the experimental farm of the Tuscia University in Viterbo, Central Italy (12°40'E, 42°26'N). The main soil characteristics (0-30 cm depth) at the beginning of the trial (year 1992) were: pH 6.9, organic matter 1.6%, total N 0.101%, available P2O5 22 mg kg-1 and exchangeable
K2O 136 mg kg-1. Five different crop sequences were carried out for 5 years (from autumn 1992 to
autumn 1997) before winter wheat (Triticum aestivum L.) was grown to determine their residual effect. The cropping sequences (Table 1) selected were the following:
(1) sunflower (Helianthus annuus L. cv HS 90) - winter wheat (Triticum aestivum L. cv Pandas) 2-yr rotation, fertilised annually with 120 kg N ha-1 (applied at hoeing in sunflower and at tillering in wheat), 100 kg
P2O5 ha-1 and 100 kg K
2O ha-1;
(2) 3-yr Italian ryegrass (Lolium multiflorum Lam cv Asso), fertilised pre-sowing with 100 kg N ha-1, 80 kg
P2O5 ha-1 and 80 kg K2O ha-1 + 2-yr alfalfa (Medicago sativa L. cv Maremmana), fertilised pre-sowing with
200 kg P2O5 ha-1 and 100 kg K2O ha-1 and annually (on every autumn) with 50 kg P2O5 ha-1 and 30 kg
K2O ha-1;
(3) 2-yr Italian ryegrass + 3-yr alfalfa, fertilised as above; (4) 1-yr Italian ryegrass, + 4-yr alfalfa, fertilised as above; (5) 5-yr alfalfa, fertilised as above.
Table 1 Crop preceding sequences used for winter wheat.
Crop Crop sequence description 1993 1994 1995 1996 1997
3-year sunflower + 2-year wheat 1 S W S W S
3-year ryegrass + 2-year alfalfa 2 R R R A A 2-year ryegrass + 3-year alfalfa 3 R R A A A 1-year ryegrass + 4-year alfalfa 4 R A A A A
5-year alfalfa 5 A A A A A
A= alfalfa; R= ryegrass; S= sunflower; W= wheat.
Mineral N fertiliser was used instead of organic fertiliser in order to better assess the capacity of alfalfa to provide the cropping system with nitrogen.
In all cropping sequences, alfalfa was mowed three times per year and Italian ryegrass twice per year, from the spring following crop establishment onwards, the biomass being removed in any cases. Sowing and mowing dates varied according to the seasonal conditions.
In autumn 1997, a split-split-plot experimental design, arranged in a randomised complete block layout with three replications, was superimposed on the plots where the preceding crop sequences were grown. Preceding crop sequences were allocated in the main plots, winter wheat sowing dates (standard and late) in the sub-plots and winter wheat N fertiliser rates (0, 60 and 120 kg N ha-1) in the sub-sub-
plots (6 by 3 m).
Mouldboard ploughing at 30-35 cm depth was performed in September 1997, and followed by seedbed preparation by disking (at 15 cm depth) and rotary harrowing at 7-10 cm depth in mid-October. Winter wheat (cv. Pandas) was sown at a rate of 220 kg ha-1 seeds in rows spaced 15 cm apart on two different
dates (standard sowing on 10 November, and late sowing on 10 December). Phosphorus (100 kg P2O5
ha-1) and potassium (100 kg K
2O ha-1) fertilisers where applied to all plots immediately before the last
harrowing pass. The three mineral N fertiliser rates were applied (as ammonium nitrate) as top dressing at the wheat tillering stage (27 February). Weeds were controlled by hand immediately after N fertilisation. In each sub-sub-plot, four 4 m-long central rows of wheat were harvested in mid-July to determine the straw and grain yields, which were expressed on a dry-matter basis following oven-drying at 70°C until constant weight. A 500 g grain sample per sub-sub-plot was ground on a 0.5 mm diameter sieve, and a sub-sample was used for determination of grain total N concentration by the Kjeldahl method.
Data were subjected to split-split-plot analysis of variance using the ANOVA procedure of the SAS program (SAS Institute, 1985). Means were separated by the least significant difference (LSD) test at P
≤ 0.05. Regression analyses of wheat grain yield on alfalfa stand duration and N fertiliser rates were performed using the GLM procedure of SAS.
Results and discussion
Winter wheat grain yield was influenced by all the experimental factors – of which the preceding crop sequence was the most important – and a significant “preceding crop sequence × N fertilisation rate” interaction occurred. Compared with the sunflower-winter wheat 2-yr rotation, inclusion of alfalfa in the preceding crop sequence – regardless of the duration of the alfalfa stand – resulted in a significantly higher wheat grain yield across all N fertiliser rates (Table 2). In the plots which did not receive N fertiliser, the largest (164%) and smallest (93%) yield increase occurred after 5- and 2-yr alfalfa respectively, with the actual size of the effect decreasing progressively with the duration of the alfalfa stand, to such an extent that no statistically significantly differences (P ≤ 0.05) were found among 3-, 4- and 5-yr alfalfa.
Table 2 Winter wheat grain yield (t ha-1) as affected by preceding crop sequence and N fertilisation
level.
Crop N fertilisation level (kg ha-1) Sequence 0 60 120 1 1.4 2.3 3.2 2 2.7 4.3 4.0 3 3.4 4.1 4.3 4 3.5 4.6 4.4 5 3.7 4.3 4.5 LSD 0.95 0.6
Winter wheat grain yield response to N fertilisation was higher for the sunflower-winter wheat preceding crop sequence than for any crop sequences including alfalfa. Consequently, wheat following alfalfa showed low requirement for additional N fertiliser to achieve maximum grain yields. Regardless of the duration of the legume stand, the unfertilised wheat which followed alfalfa yielded as much as wheat in rotation with sunflower when fertilised at the highest N rate. Therefore, a 120 kg N ha-1
fertilisation rate could be considered equivalent to the residual effect of a 3- to 5-yr alfalfa stand in the first year after incorporation of the legume biomass, the value for 2-yr alfalfa being some 90-100 kg N ha-1. Despite the evident beneficial effect of alfalfa on soil N fertility, maximum wheat grain yield
(about 4.5 t ha-1) was reached after any alfalfa stands at 60 kg N ha-1. This suggests that alfalfa had a
beneficial effect on soil N fertility, which was complementary to that of mineral N fertiliser; this was probably associated with an enhancement of soil fertility sensu latu, as it is well known by farmers. The largest relative responses to the rate of 60 kg N ha-1 occurred following 2-yr alfalfa, in which grain yield
increased from 2.7 t ha-1 (without N fertiliser) to 4.3 t ha-1. The high N contribution of alfalfa to this
cropping system signifies that, without the inclusion of alfalfa as a preceding crop, far more than 120 kg N ha-1 are presumably necessary in a cash crop sequence such as sunflower-wheat to reach
maximum wheat grain yields. The standard sowing date generally decreased wheat grain yield in all cropping sequences (data not shown): this effect may have occurred because of the excessive soil moisture conditions encountered immediately before and after sowing consequent to a heavy rainfall period (Figure 1). Regression analyses of wheat grain yield at each N fertiliser rate on alfalfa stand duration (averaged over sowing dates) are shown in Figure 2. It is evident that, without N fertilisation, wheat grain yield increased by 0.2 t ha-1 for each additional year of alfalfa in the crop sequence,
although the increase was less pronounced for the older alfalfa stands. This effect disappeared at the 60 and 120 kg N ha-1 fertiliser rates, where relatively high wheat grain yields were obtained in all crop
sequences which included alfalfa.
Figure 1 Precipitation (bars), and maximum (---) and minimum (——) temperatures (recorded at 10-day intervals) at Viterbo, during the 1997-98 wheat growing season.
-5 5 15 25 35 45 55 S O N D J F M A M J J Temperature [°C] -10 0 10 20 30 40 50 60 70 80 90 100 110 Precipitation [mm] 1997 1998
The effect of preceding crop sequence on wheat straw yield was similar to that observed on wheat grain yields with the largest unfertilised yields following 5-yr alfalfa (Table 3). The effect of alfalfa stand duration on wheat straw yield was actually smaller than that on grain yield because of the statistically significant (P ≤ 0.05) increase exerted on wheat harvest index (data not shown).
Both alfalfa stand duration and mineral N fertilisation had a large positive effect on wheat grain N concentration, but the interaction between the two factors was not significant (Figure 3). The highest N concentration occurred after 5-yr alfalfa (1.83%), but differences with the values observed for 3- and 4- yr alfalfa were not significant at P ≤ 0.05. Grain N concentration was lowest in wheat following the sunflower-winter wheat 2-yr rotation (1.59%).
Figure 2 Relationship between wheat grain yield and alfalfa stand duration for different N fertiliser levels. * and nsdenote significant and not significant effects at P ≤ 0.05, respectively.
Table 3 Winter wheat straw yield (t ha-1) as affected by preceding crop sequence and N fertilisation
level.
Crop N fertilisation level (kg ha-1)
sequence 0 60 120 1 2.3 3.4 4.7 2 3.9 6.4 5.8 3 5.5 5.9 6.5 4 5.0 6.6 6.5 5 5.7 6.2 7.0 LSD (P≤ 0.05) 0.7 0 1 2 3 4 5 2 3 4 5
Alfalfa stand duration (years)
Grain yield (t ha
-1 )
0 kg N ha-1 y = -0.048 x2 + 0.673 x + 1.507 R2 = 0.997 * 60 kg N ha-1 y = 0.081 x + 4.041 R2 = 0.189 ns
In conclusion, in the absence of mineral N fertilisation applied to wheat, the preceding crop sequences that included alfalfa were clearly the most effective in increasing grain yield and grain N concentration of the cereal. Surprisingly, the 3-, 4- and 5-yr alfalfa stands resulted in similar residual effects; it remains to be seen whether this substantial equivalence would persist over time. The results obtained in this research confirm that satisfactory winter wheat yield can be achieved without any direct N fertilisation in a Mediterranean environment, provided that alfalfa is grown as a preceding crop (Caporali & Onnis, 1992). Alfalfa is thus particularly suited to be included in organic cropping systems where maximum exploitation of natural resources is sought for.
Figure 3 Nitrogen concentration (%) in wheat grain after different preceding crop sequences
and fertilisation levels. Bars labelled with the same letter are not significantly different at P ≤ 0.05.
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