What effect does pasture height have during lambing?
Updated: Oct 11, 2019
There has been an increase in the number of triplet lambs born in many NZ sheep flocks due to increased selection for improved lambing percentages. This results in a greater feed demand in late pregnancy. A study published in the New Zealand Journal of Agricultural Research indicates that pasture grazing height is more important prior to lambing than after lambing.
The paper by authors Prof. Steve Morris and Prof. Paul Kenyon is summarised below.
During late-pregnancy, all ewes should be fed on pasture heights of at least 4cm. This helps to achieve optimum lamb birth weights.
There is no additional benefit from feeding on pasture height of over 4cm.
Twin and triplet bearing ewes do not need to be grazed as separate mobs from one another as their requirements for pasture height are the same.
The pasture height guidelines for single and twin bearing ewes have been previously recommended as 4cm, but should the recommended pasture height be any different for triplet-bearing ewes given they have less space in their rumen for pasture compared with single- and twin-bearing ewes. It is important to feed triplet bearing ewes adequately to ensure that the lambs have the best start at life and to increase their chances of survival.
This study compared ewe and lamb performance in twin- and triplet-bearing/rearing ewes offered varying pasture heights from mid-pregnancy through to weaning. There were four different grazing heights offered to different groups of sheep over mid-pregnancy and late-pregnancy (2, 4, 6, and 8 cm pasture height) and two post-lambing (4 and 8 cm). The ewes were grazed on ryegrass white clover swards in flat paddocks.
There was no effect of number of lambs born (2 or 3) on the live weight of the ewe during pregnancy and during lactation, therefore, only pasture height was important for ewe performance in this study.
The graph below (Figure 1) shows the live weight of the ewes from mid-pregnancy to lambing. Lambing occurred at day 152 of pregnancy. The ewes grazing the 2 cm sward height during pregnancy were significantly lighter than ewes grazing 4, 6 or 8 cm pasture up to lambing.
The ewes grazing the 2cm pasture height also had significantly lower dry matter intakes and body condition scores. The lambs born to the ewes grazing the 2 cm pastures during pregnancy were lightest at birth, but these lambs must have had compensatory growth up to weaning, as there were no differences in lamb weaning weight.
What was interesting was that there was no significant difference in dry matter intake between the twin- and triplet-bearing ewes throughout pregnancy and lactation as shown in Table 1. This means that the farmer would not need to graze twin and triplet bearing ewes as separate mobs from one another.
Also there were no differences in lamb survival from birth to weaning among the pasture height groups. This means that pasture height does not affect lamb survival, however, there were differences between twin and triplet born lambs. Triplet born lambs were 0.9 kg lighter than twins at birth, and 4.7 kg lighter than twins at weaning. Lamb losses were 14% in twin-born lambs and 32% in triplet-born lambs, therefore, factors other than pasture height are influencing lamb survival which were not covered in this study.
There is no need to feed above 4 cm in pregnancy or lactation to optimise lamb weaning weight, lamb survival or ewe weight and condition score gain. At pasture height levels above 4cm, the ewe possibly cannot utilise the extra feed due to rumen restriction.
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