Historia, actualidad e impacto
2.2. Desarrollo histórico de la energía termosolar
Farmers are strongly advised to rebuild nutrient availability through an expert nutrient cycling system in pasture paddocks. Paddocks which have been in pasture continuously for more than 1 5-20 years should be rotated with cultivation of a crop (Figure 4.8). The main components of pasture ecosystems are animals, plants (pasture) and soil with
Chapter 4: Pastoral dairy production for Tonga
nutrients (Kemp et al. , 2002). The nutrient pathways in grazing pasture are: from the soil or air, to the plant, returned to the soil in p lant residue or through the grazing animals (Crowder, 1 98 5 ) . Figure 2.9 in Chapter 2 illustrates the component and maj or pathways associated with nutrient cycles in grazing pasture. Kemp et al. (2002) stated that 30-95% of plant nutrients obtained from soil by grazing animals and 65-95% of nutrient uptake by plants, are returned to the soil in the form of litter, root residues or animal excreta (urine and faeces).
The fertility of the soil can be maintained by executing j udicious pasture management. Apart from urine and faeces, inclusion of legume species in pasture undoubtedly boosts pasture yield and replenishes soil minerals. Another alternative to improve pasture productivity is rotating permanent pasture with cultivat ing crops (Figure 4 . 8 ), which is more economical than regular application of expensive ferti liser.
Figure 4.8. a) Forage re-growth on a [allowing cultivated paddock; b) the same paddock re ploughed for cultivation on its third rotation (Fisi ' ihoi, 2006).
In Figure 4.8, the physical properties of the soil become normal and minerals have been replenished to make it more fertile than it was before. The benefits of using this technique in Tonga have been observed on both crop yields and pasture growth. Based on these observations, this method is advised due to the following reasons:
• the physical properties of the soil could be renewed and minerals replenished • more OM collected from crop residues and extra minerals added from fertiliser
applied to crops
• improvement of aeration, pH, soil temperatures and moisture content
• establishment of new pastures can be quicker.
Maintaining the soil nutrient cycle is very important for upholding p asture productivity in Tonga.
4.3.5. TOPPING, MOWING PASTURES OR SLAS HING
This style o f grazing management has been used profitably in developed countries such as New Zealand and Australia. This technique is suggested for Tonga because of its advantages to both pastures and dairy production. Holmes et al. ( 1 987) cited that mowing in early summer could stimulate re-growth of new vegetative tillers and increase the FV of re-growth. Irregular topping can be done i f rain occurs regularly and during the wet season. Theoretically, there should be advantages to topping grass inflorescences in their early development to avoid apical dominance and to stimulate the growth of existing and new vegetative tillers (McDonald, 1 986).
McDonald ( 1 986) showed that topping at various stages o f flowering during summer improved herbage quality when seed head density on pasture ranges from 1 ,200-2,000 m2,
reducing pasture dead matter levels and improving herbage N and digestibility, despite the fact that total herbage accumulation may temporally reduce after topping. Pasture should be topped or mown either before or after grazing to achieve high quality pasture and increase M S during summer, even though the total DM production will be reduced (Kolver et al. , 1 999). The MS production will be increased by 4.0% on topped pasture compared to non-topped pastures (Bryant, 1 982 cited by Holmes and Hoogendoom, 1 983). In addition to MS, LW also increases by 0. 76 kg/cow/day after either mowing before grazing or topping after grazing, as a response associated with an increase in the ME content of summer pastures, of 0.2 and 0.6 MJ ME/kg DM respectively. In conclusion, it is suggested that pasture should be mown after cows have grazed it, especially during late spring and early summer.
Slashing is one management technique used to reduce bulk and increase leaf percentage of sward (Chacon and Stobbs, 1 976; Minson, 1 976). Davidson and Cowan ( 1 98 1 ) experimented with this method in Australia using three treatments : control, slashing
( 1 0- 1 4 cm height above ground) and variable SR (three and five Friesian cows/ha). The trial was designed to determine the effect of different pasture management strategies on
Chapter 4: Pastoral dairy production for Tong a
individual cows' milk yield during the summer wet season using P. maximum, Gatton and B. decumpen (signal grass). The results indicated that milk yields/cow/day was higher (P < 0.05) for cows in the slash treatment in week 1 0, 1 1 , 1 2 and 1 3 and on cows in the variable treatment in week 1 1 (Figure 4 . 9). However, each time the pasture was slashed or an extra cow was added, the milk yield fel l and the extent of this fall depended on the severity of slashing or the number of extra cows added. Additionally, yield steadily increased as pastures re-grew or whenever the extra cows were removed.
•--• Gatton panic o----o Signal graee Slashed 2.0 �
�
"' 0.0 e?! §
-2.0 l2!
21 -4.0 -o Variable stocking � ,le 2.0 8.3 .E .!:�
0.0 "' .r:: u -2 0 0 3 5 7 9 1 1 1 3 1 5 1 7 WeekFigure 4.9. Milk yields of cows grazing Gatton panic and signal grass that were slashed or stocked at variable rates relative to the milk yields of cows grazing continuously. S denotes the time and height of slashing (cm above ground) and the numbers on the variable stocking rate (cows ha- 1 ) (Davidson et al., 1 9 8 1 ).