Pasture quality and quantity largely determine live weight gain, milk production, reproductive performance and health.
High quality pasture is especially important for ewes and lambs during lactation and for weaned lambs.
In any pasture there are a number of components – clovers (and herbs), grass leaf, stem and dead matter. The proportion of each determines the digestibility of the sward.
Digestibility refers to the proportion of the feed absorbed by an animal. It also gives an indication of energy availability measured in megajoules of metabolisable energy (ME) per kilogram of dry matter.
The high quality parts are clovers and green leaf from temperate grasses. These have a digestibility of close to 80 per cent or an ME of 12.
The digestibility of the leaf material of semi-tropical grasses like paspalum and kikuyu ranges from 70–75 per cent or an ME of 10.
Dead plant material has a digestibility lower than 50 per cent and sheep are reluctant to eat it. Dried (as in hay) or frosted pasture is much higher quality than the dead material from aged leaves and stems.
Dead material builds up in spring and can reach quite high levels during the late spring and summer. Most of this is stem material from grasses going to seed.
Once grasses have gone to seed there is a major and rapid decline in feed quality. This is clearly shown in the diagram.
Pasture which had gone to seed was once known as 'standing hay', but in reality its nutritive value is very much lower than real hay. Also, the longer it is left ungrazed, the lower its quality becomes.
Pastures (even if leafy) that have been spelled for several weeks in late spring-early summer, are also of low nutritive value (digestibility below 75 per cent and ME below 10) especially if stem and dead material has accumulated. Lambs will not gain any weight on such pasture - it is maintenance feed only.
As temperatures increase, lower quality summer and semi-tropical grasses start to grow, contributing to the decline in pasture quality over summer. This may be offset to some extent by an increase in the growth of white and red clovers.
The elongation of the reproductive tiller in ryegrass starts in mid- to late-October and continues on into November. The central tiller starts to lengthen and the immature seedhead forms inside the leaf sheath.
After ryegrasses go to seed they enter a dormant phase. So if seedhead production can be reduced or prevented, there is a double benefit - continued growth and the maintenance of high feed quality. In addition, some plants die after they have gone to seed.
In early- and mid-lactation the sheep are usually on top of the feed. However in October pasture growth rates usually outstrip the requirements of the flock. Feed surpluses start to occur and if left, grasses will start going to seed and accumulated feed will start to age.
Management at this time can have a huge bearing on feed quality in late-lactation and after weaning. It should aim to control pasture growth, prevent grasses going to seed and remove older growth that would otherwise accumulate and dies.
Once the reproductive tiller is removed, more young vegetative tillers emerge from the base of the plant.
Surpluses are more obvious under rotational grazing, but also occur under set-stocking. The trick is to anticipate the surplus and to retire a proportion of the farm from grazing by the ewes and lambs – taking care not to tighten them up so much that their production levels suffer. This allows the mob to keep on top of the rest of the feed.
Ewes should be spread out to maximise their intakes at this time. The aim is to keep pastures in the 3–4 cm range (1200–1500 kg DM/ha). Once pasture starts to go rank (approaching 10 cm) current and future sheep production will decline.
Closing up areas for hay or silage kills two birds with one stone. It controls feed quality and conserves feed for periods of shortage.
Silage has a number of advantages over hay (see In The Paddock 2). It is cheaper, made with earlier better quality pasture and some recovery of silage aftermath can be expected before drought. This will be high quality new grass growth, usually clover-dominant.
Topping prevents grasses going to seed and prevents the accumulation of seed stem, but will not remove older material further down in the sward unless the cutting height is quite low.
Some herbicides applied at low rates also prevent grasses going to seed and encourage clover growth.
On farms without tractor country, grazing animals are the only way to control the pasture. Cattle are ideal for preparing quality pasture for lambs and can do this sometime before the lambs enter the paddock, or can be rotated through the lamb mobs.
Strategic electric fencing can be used to concentrate them on really rough areas. Cattle will not suffer the same production penalty as sheep when they are grazed on older stemmy pasture.
After weaning, the ewe mob can also be used to clean up rank paddocks.
On some hill country farms, some paddocks may have to be left ungrazed in order to get enough stock power to keep feed on the rest of the farm under control.
In this situation, the ungrazed paddocks should be smaller paddocks on the easier more fertile areas of the farm if available. Research has shown these recover better and are easier to clean up later than dry infertile areas. Another option is to conserve surplus feed as silage or hay from these paddocks.
Your local farm consultant or veterinarian will also be able to help. Alternatively, contact your local WoolPro extension specialist:
Northern North Island: Sally Hobson tel 07-823 3321 or 025-924 751 Hawkes Bay/East Coast: Lew Willougby tel 06-835 1888 or 025-434 417 Southern North Island: Richard Gavigan tel 06-376 0006 or 025-499 851 Nelson/Marlborough/Canterbury: Alan Marshall tel 03-343 7913 025 329 399 South Canterbury: Julia Mackenzie, tel 03-680 6782 or 025-782 353 Otago: Robert Pattison, tel 03-489 9021 or 025-323 094 Southland: Aaron Meikle, tel 03-203 9071 or 025-846 377