A. Farrugia and A. Pflimlin
Institut de l'Elevage, 149 Rue de Bercy, 75595 Paris Cedex 12, France
Three trials have been set up in Brittany (Finistère) by the Institut de l'Élevage and the local advisery services, comparing nitrate leaching risks under grazing conditions with either pure grass swards receiving over 300 mineral nitrogen or grass-clover sward without (or 30) mineral nitrogen.
At TREVAREZ, nitrogen studies are made on:
1 - a grass-clover sward continuously grazed by milk heifers. One paddock of 6 ha has been divided in three plots: one grass plot receiving 300 kg nitrogen fertilizer ha -1, one grass-clover plot with 150 kg nitrogen fertilizer ha -1 and one grass-clover plot without N fertilizer.
2 - two swards rotationally grazed by milk cows with either grass-clover without nitrogen fertilizer or a pure grass-plot receiving 300 kg fertilizer N ha-1.
3 - At KERLAVIC, the interest of grass-white clover rotationaly grazed by sheep is studing on two plots: one plot of grass receiving 330 kg N fertilizer and one plot of grass-clover with 50 kg fertilizer N.ha-1).
The quantities of mineral nitrogen (N03 - N and NH4+ -N) are assessed by soil-cores sampled during the winter at three horizons: 0-30 cm, 30-60 cm and 60-90 cm. In 1991-92, the samples have been made only at two depths 0-30 cm and 30-60 cm at TREVAREZ.
The soil of this two sites are differents: 20-25% of clay, 80 cm depth on a schist subsoil at TREVAREZ and 14-16% of clay, less deep on a granite at KERLAVIC.
Rainfall from October to february have been various during the 3 winters:
- 740, 507 and 691 mm in 90-91, 91-92, 92-93 at TREVAREZ and
- 575 mm in 92-93 at KERLAVIC
The quantities of nitrogen in soils, will be examined on these 3 trials but the comments will focus on nitrate mainly.
Continuously grazed sward by milk heifers: low level of nitrate under the grass-clover sward without nitrogen fertilizer
The paddocks were establisched in spring 1989 and followed a rotation of maize-ray-grass and cereal peas fertilized with pig slurry (25-30 m3 ha-1).
Heifers grazed from the end of march to the end of October. Nitrogen fertilizers are applied to grassland from march to July (6 supplies for the grass 300 N ha-1 and 3 for the grass-clover 150 N ha-l).
The content of clover in autumn for the three years was 46%, 44%, 33% clover for the fertilizer plot (150 N ha -1) and 44%, 46%, 49% for the unfertilizer plot.
The number of heifers on the sward is adapted to the production of grass. The infertilized grass-clover yield has been less than the too others plots (less 15%) (in UFU*).
The quantities of nitrate of grass-clover ON plot are significantly lower than these of the grass plot receiving 300 N and grass-white clover 150 N during the 3 winters. The level of nitrogen under the grass-clover plot with 150 N is similar to the grass receiving 300 kg fertilizer N ha-1
This confirm that it is unuseful to fertilizer the grass-clover: animals performances will decrease as fertilizer costs and nitrate leaching risks will increase.
Rotationally grazed sward by milk cows: various results according to the years
Grass-clover paddock has been reseeded in spring 1989 and followed a rotation of pasture and two years of maize-ray grass fertilized with pig slurry (25-30 m3 ha -1). The grass plot has been seeded in 1985. Cows grazed from the end of march to november even in december-january in 1992-93. Animals stocking rate is 5 cow/ha in spring and 4 - 3, 3 cow/ha in summer. Last supply of nitrogen on the grass "300 N" is applied in July according to the rainfall. Clover content is substantial: 50 a 70% in autumn over the three years. Yield are similar for the two plots (11 a 12 T DM ha -1).
During the first winter 1990-91, the quantities of nitrate in the profil of the grass-clover plot were not significantly different from the grass-only plot and were relatively high 70-115 kg -N03-N ha -1 in 0-90 cm depth (fig. 4 a and 5 a). After the level of nitrate of the grass-clover plot was modarate in 1991-92 and 1992-93 and slightly less high than the pure grass-plot.
How to explain theses differences compared to the grass-clover sward grazed by heifers: clover content, length of the pasture period, plot history, soil sample or interaction between all theses factors?
Rotationally grazed sward by sheeps: similar results to the first experiment
The paddock have been reseeded in autumn 1991 and followed.
Sheeps grazed from the 25 of march to the end of October. The animals stocking rate is 24-25 ewes ha-1 Nitrogen supplies 50 kg ha -1 are applied after each passage for the pure grass and only once in february for the grass-clover (50 kg N ha -1).
Clover content is not as high as for the two others trials: 25-35% at the end of summer.
Yields of grass-clover plot have been less than those of the pure grass-plot (less 20-25%) over the two years.
Nitrate quantities results are similar to the continiously grazed plot by heifers. Nitrate level of the grass-clover (+ 50 N) are less than the pure grass plot (330 N).