EFFECTS OF AN INCREASE IN NITROGEN-FERTIL IZER OR IN SUPPLEMENTARY CONCENTRATE ON PERFORMANCES AND ON PLASMA UREA WITH LACTATING DAIRY-COWSGRAZED EITHER CONTINUOUSLY OR IN A ROTATIONAL SYSTEM

A grating trial was conducted with spring calving dairy cows grazed ei ther continuously or with the rotational system in order to improve yi eld per ha with a simultaneous increase in stocking rate and either in concentrate or in nitrogen fertilizer The trial was repeated over 2 c onsecutive years. There were 34 cows divided into six treatment groups : groups 1, 2 and 3 were grazed with the rotational system and groups 4, 5 and 6 were grazed continuously. Three methods of grazing were use d in each system. In the first method of grazing the stocking rate was four cows/ha (groups 1 and 4). In the two other methods of gracing, t he stocking rate was five cows/ha: the increase in stocking rate was a llowed by an increase in the amount of concentrate (groups 2 and 5) or by an increase in nitrogen fertilizer (groups 3 and 6). The supplemen tary concentrate offered in groups 2 and 5 as compared with groups 1 a nd 4 was calculated to cover the requirements of the supplementary cow with the average milk yield of the group. In groups 3 and 6, the amou nt of nitrogen fertilizer was increased in order to compensate for the increase of stocking rate, which reduced the grazing area per cow, by an improvement in grass production induced by supplementary nitrogen fertilizer, 1 kg nitrogen fertilizer allowing 1.3 days extra grazing. Concentrate was offered to all the cows when they were milked. The cow s received 1.41 and 1.28 kg concentrate daily in groups 1 and 4, respe ctively In groups 2 and 5, the cows were offered a large amount of con centrate: 4.18 kg/day in group 2 and 3.89 kg/day in group 5, The cows in group 3 ate 1.21 kg of concentrate and those in group 6, 1.07 kg. N itrogen fertilizer was applied at the rate of 113 kg/ha in the pasture grazed by the cows in groups 1, 2, 4 and 5 and at the rate of 225 kg N/ha in the pasture grazed by the cows in groups 3 and 6. Each year, f ive cows were utilized in groups 1 and 4 and six cows in the other gro ups. Grass production was improved by an extra 1 435 kg dry matter in groups 3 and 6 as compared with groups 1 and 4 and the proportion of c lover was reduced from 12 to 8%. The contents in dry matter, crude fib er and crude protein of grass were not affected by the treatments, the corresponding figures being 16.3%, 20.1% and 23.8%. The milk yields i n groups 2 and 5 were higher than in groups 1 and 4 (15.61 vs 14.56 kg /day; P > 0.10) while in groups 3 and 6 they were lower than in groups 1 and 4 (13.12 vs 14.56 kg/day; P > 0.10). There were no interactions between grazing systems and methods of grazing. Grass yields, express ed in UFL/ha, were higher in groups 3 and 5 than in groups 1 and 4 (8 168 vs 7 159 UFL/ha) and in the rotational system than in the continuo us system (7 650 vs 7 309 UFL/ha). Urea concentrations in blood plasma were reduced in groups 2 and 5 as compared with groups 1 and 4 (218 v s 231 mg N/l; P > 0.10); in groups 3 and 6, the concentrations were th e highest (269 vs 231 mg N/l; P < 0.001). The rotational system induce d lower concentrations than the continuous system (209 vs 270 mg N/l; P < 0.001). There was an interaction between the grazing systems and t he methods of grating.