Uno Tamm
Estonian Research Institute of Agriculture, Saku, Teaduse 1, EE3400, ESTONIA.
Introduction
Materials and methods
Results
Conclusions
References
The duration of the grazing period of dairy cattle in Estonia is 145-150 days. During this period 1200-1600 kg of the pasture DM is consumed by each cow. The amount of consumed DM depends on the duration of the grazing period, botanical composition and quality of the herbage. In May when the grazing period starts, the growth and the development of the plants depend primarily on the weather conditions. While the grass is ageing its nutritive value and content of digestible nutrients are diminishing.
Several comparisons of the most widespread swards, namely grass/white clover and grass swards have been made. Besides agronomic evaluation the productivity of the dairy cows has also been determined (Parol, 1986; Older et al, 1991), the results of which indicate the advantages of the grass/white clover swards.
The aim of the experiment described below was to investigate the impact of the time of the first grazing on the DM yield and the changes in compositional quality and nutritive value of the grass/white clover and grass swards. Simultaneously, the possibilities of prolonging the grazing period by using these different swards were investigated.
Swards used in the experiment were sown at the Juuliku Experimental Cattle Farm, Saku in 1987. The botanical compositions of the sown swards were as follows.
1. Grass/white clover swards: timothy, meadow fescue, smooth-stalked meadow grass, perennial ryegrass and white clover.2. Grass swards: cocksfoot, timothy, smooth-stalked meadow grass and perennial ryegrass.
The field experiment was carried out in 1989-1992. Early grazing began at the phenological stage of tillering (yield of DM 1.2-2.1 t ha-1). The second and third grazings began 7 and 14 days later, respectively from the date of the beginning of early grazing (yield of DM, 2.0-2.8 t ha-1 and 3.2-3.7 t ha-1, correspondingly). The grass swards were fertilized with 180 and 300 kg N ha-1 annually.
The complete proximate analysis of DM was performed according to the generally known methods. Digestibility of DM in rumen was measured in sacco. The amount of metabolizable energy was calculated from the results of the DM analysis. Statistical data processing was performed using regression analysis.
The time of beginning the grazing influenced the yield of the pastures and also their chemical composition and nutritive value. The development of white clover was slower initially than that of grasses. Regression analysis reflected the dependence of the yield of DM of grass/white clover swards on the sum of efficient (T > 5°C) temperatures (R2 = 0.64, P < 0.01). The temperature factor had not so strong an influence on the grass swards (R2 =0.19, P > 0.05) and the yield of DM could be estimated by the numbers of days needed for its accretion (R2 = 0.51, P < 0.05). Grass swards reached the required yield for grazing a week earlier than the grass/white clover pasture. Due to that the annual yields of DM were higher (Table 1).
Due to the white clover, the mixed sward was rich in protein, highly digestible and with high nutritive value during all the grazing cycles. In the last year of the experiment, delaying the beginning of grazing reduced white clover in the sward markedly due to shading by the taller grasses.
With regard to the annual yield of DM of the grass swards, yield was higher but with lower compositional quality when the first grazing was delayed. The quality was high only when the grazing started earlier although with a lower protein content and higher crude fibre content than grass/white clover pastures. With the delay in first grazing grass pasture aged rapidly and the digestibility of DM decreased. This was also indicated by the correlation between DM digestibility and content of crude fibre (R2 = 0.6, P < 0.01). The DM digestibility of grass swards was 7-12% lower that from grass/white clover swards. The amount of metabolizable energy in the DM decreased to under 10 MJ kg-1 with delayed grazing on grass pastures. Such nutritive value does not meet the needs of highly productive dairy cattle.
The accretion and chemical composition of the grass pasture were improved by the N fertilization. The protein content of DM was markedly dependant on the level of N (R2 = 0.82, P < 0.01). The effect of different amounts of N on the digestibility of the DM remained unclear.
Table 1. Yield, chemical composition and nutritive value of pasture grass at different times of first grazing.
|
Yield/quality indices |
Time of first grazing |
||||||||
|
Grass/white clover sward |
Grass sward |
Grass sward |
|||||||
|
I |
II |
III |
I |
II |
III |
I |
II |
III |
|
|
DM yield (t ha-1) |
8.1 |
7.6 |
7.0 |
8.3 |
8.9 |
9.8 |
9.9 |
11.6 |
12.1 |
|
Crude protein (%) |
16.9 |
17.6 |
17.1 |
14.0 |
13.8 |
13.2 |
15.7 |
14.9 |
14.5 |
|
Crude fibre (%) |
20.8 |
21.6 |
21.8 |
25.8 |
26.0 |
27.9 |
24.5 |
25.8 |
27.0 |
|
DM digestibility (%) |
73.8 |
73.6 |
72.6 |
67.1 |
63.1 |
60.9 |
66.6 |
64.8 |
63.5 |
|
Metabolizable energy MJ (kg DM)-1 |
11.5 |
11.4 |
11.2 |
10.4 |
9.8 |
9.6 |
10.5 |
10.1 |
9.9 |
In conclusion it is possible to say that the utilization of the pastures may be more efficient when there are different types of swards. Grass swards enable grazing to start one week earlier and therefore the season's grazing period is longer. The quality of the DM of the grass/white clover pastures remains high for a longer period than the grass pastures and this assists the grazing process during the season.
Older, H., Sarand, R.-J., Tamm, U., Kaldaru, R., Muld, U. (1991) Production and utilisation of high-quality grass feeds on the Juuliku Experimental Cattle Farm. Estonian Agricultural Centre of Information. Tallinn, 76 pp.
Parol, A. (1986) The productivity of cultivated pastures evaluated by dairy cows depending on the botanical composition and fertilisation. In: Bioproduction of Grasslands. Valgus, Tallinn, pp. 141-148.
