Meanwhile, the demand of the ewe flock increases, reaching a peak prior to weaning and declines thereafter as the requirements of dry ewes decline and lambs are being drafted for sale.
Pasture management for high lamb performance
The main objective of grassland management is to have a plentiful supply of highly digestible grass available to the animals for the duration of the grazing season. However, as the grazing season progresses grass matures and goes from vegetative to reproductive state. This change increases the proportion of stem and reduces digestibility and intake potential.
Therefore, to achieve optimum levels of lamb performance from grazed grass, pasture must be managed to maximise the proportion of leaf in the sward canopy, thus maintaining herbage digestibility and intake potential. This is achieved by grazing swards to predetermined residual heights during the grazing season. Sward height measurement is the easiest and most effective way of managing pasture. For ewes and their lambs target post-grazing sward heights, which differ for rotational and set stocked grazing systems, based on studies undertaken at Athenry are in table 2 (right).
Similar levels of lamb performance are achievable from well-managed set-stocked and rotational grazed systems. The main advantage of the rotational grazing system is that it simplifies the removal of excess herbage (paddocks) from the system during periods of rapid grass growth, such as early May, for forage conservation. It also enables the inclusion of extra herbage, such as aftergrass, when grass growth slows down in mid-summer.
Also, the rotational grazing system facilitates higher grass utilisation which reduces the cost of production.
Drafting for Athenry flock
The flock at Athenry, which is used for grassland and nutrition research consists of 350 crossbred ewes. Each year, for experimental reasons, approximately 100 ewe hoggets are included in the flock. Depending on experimental treatment, ewes receive an average of 20kg concentrate in late pregnancy. Post-lambing ewes rearing singles and twins and their lambs receive no concentrate supplementation while at pasture. Ewes rearing triplets receive 0.5kg concentrate per ewe daily for five weeks post lambing, while lambs reared as triplets receive up to 300g concentrate daily until weaning. Post weaning all lambs are grazed as one flock and receive no concentrate supplementation.
The drafting information for all the lambs from the Athenry flock for 2008, 2009 and last year are presented in figure 1 (right). During these years, the number of lambs reared per ewe was 1.7 each year. However, average carcass weight increased from 19kg to 20.6kg between 2008 and last year.
During three consecutive years with dramatically different weather patterns and grass production, the data shows that lambs from a prolific flock can be consistently finished from grazed grass offered as the sole diet. At Athenry from April 1 to June 30, total rainfall was 225mm, 350mm and 156mm in 2008, 2009 and last year respectively.
In 2008, grass supply was scarce due to low temperatures in mid-April, with the post grazing sward height being as low as 2.6cm at times. However, no concentrate was offered to either the ewes or their lambs. For April, May and June 2008 mean post-grazing sward heights were 3.5, 4.8 and 5.5cm and mean pre-grazing sward heights were 6.4, 8.6 and 7.9cm respectively.
Managing the sward as described above resulted in mean daily liveweight gains from birth to weaning of 336, 292 and 296g daily for singles, twins and triplets respectively.
April and May in 2009 were characterised as extremely wet with total monthly rainfall of 150mm and 131mm, respectively. For April, May and June, mean post-grazing sward heights were 3.8, 3.8 and 4cm and pre-grazing sward heights were 5.8, 7.0 and 6.6cm respectively. Pre-weaning lamb daily liveweight gains were 338, 279 and 284g for singles, twins and triplets, respectively.
Last year, March, April and May were characterised as a period of low temperatures, so consequently grass supply was scarce. Ewes were supplemented with concentrate for three weeks post lambing. Furthermore, June was extremely dry with only 13ml of rainfall during the first 26 days of the month. Consequently, in June, due to drought conditions, grass supply was scarce and its feed value was low due to seed head elongation.
For April, May and June mean post-grazing sward heights were 3, 4.6 and 4.9cm and pre-grazing sward heights were 4.4, 7.2 and 7.7cm, respectively. Pre-weaning lamb daily liveweight gains were 310, 265 and 257g for singles, twins and triplets respectively.
The data presented shows that even when there was a grass shortage, such as in April and May 2008 with the cold or in 2009 which was extremely wet, high levels of lamb performance can be achieved consistently when grazed grass is the sole diet offered to ewes and their lambs.
The data presented in this article, based on many years of research at Athenry, clearly illustrates that high levels of lamb performance is achievable from grazed grass offered as the sole diet. The key to achieving high levels of lamb performance from pasture is the provision of adequate quantities of high digestibility herbage. The easiest way to manage grassland for the flock is to use sward height when deciding on moving the flock to new pasture or the removal of paddocks for forage conservation.
A farmer produces heavy lamb carcasses from grass without any concentrate supplementation offered to ewes rearing singles or twins or their lambs post lambing. Ewes rearing triplets receive 0.5kg concentrate daily for five weeks post lambing, while lambs reared as triplets are offered up to 300g supplement until weaning. The mean lambing date varied from March 15 to 20 for the years 2008, 2009 and 2010. Weaning rate for this flock, for the years 2008-2010, varied from 1.7 to 1.8 lambs weaned per ewe to the ram.
Post weaning, all lambs are grazed as one flock without any concentrate supplementation. The mean carcass weight for the lambs in 2008, 2009 and 2010 was 21.8, 21.1 and 21.7kg respectively. The drafting patterns for the flock, for the past three years, are presented in figure 2 (right).
The data in figure 2 clearly illustrates that heavy carcasses can be consistently produced from mid-season prime lamb systems where grass is offered as the sole diet to lambs reared as single and twins pre-weaning and to all lambs post weaning.
In addition, with the adoption of good grassland management techniques such as grazing to the recommended post grazing sward heights and tight grazing in April and May, the drafting pattern improved between 2008 and last year.
•Heavy lamb carcasses can be produced from grazed grass offered as the sole diet.
•To achieve high lamb performances match grass supply and feed value with animal requirements.
•Graze pastures to pre-determined sward heights.
•Graze swards tight, post-grazing sward heights of 3.5 to 4cm, during April.
•Increase post-grazing sward height as the season progresses.
Dr Tim Keady works at the Animal and Grassland Research and Innovation Centre, Teagasc, Athenry, Co Galway