The influence of mineral nutrition of pasture grasses on the grazing preference by sheep

Abstract

This investigation on grazing preference of sheep used two experimental methods; rate of intake by penned sheep and field grazing observations.

Ewe hoggets were fed fresh cut leaf to examine the rate of intake of six pasture grasses of differing nutrient status. Sheep exhibited no specific preference in rate of intake trial one during autumn and it was concluded that sheep appetite may have influenced the result. Phalaris leaf in rate of intake trial two during autumn was consumed at the greatest rate (43.2g WM/min - 8.7g DM/min) with timothy leaf being least preferred (17.4g WM/min - 4.8g DM/min). Both on a wet and dry matter basis cocksfoot leaf in rate of intake trial three during spring increased when nitrogen levels increased from 2.83 to 5.22% N. In contrast Yatsyn ryegrass showed no relationship (2.16 to 4.48% N) on a wet matter basis, but on a dry matter basis rate of intake decreased with increasing nitrogen levels (11.02 - 6.36g DM/min). Ryegrass endophyte (Acremonium lollii) hyphae concentrations were doubled at high nitrogen levels and this may have caused the reduction in intake.

Field grazing trials measured the rate of decline in height of six pasture species of differing nutrient status. Phalaris was the preferred species in field trial one during autumn, being reduced in height at a fast rate and to a low residual height (1.45cm). With field trial two during autumn there appears to be more than one preferred species as tall fescue, timothy/Marsden ryegrass mixture and Marsden ryegrass over the first two days were reduced at similar rates to phalaris. In field trial three during spring there was no preferred species. After nitrogen fertilisation at equivalent rates to a urine patch, sheep in field trial one showed no strong preference for fertilised herbage and in field trial two there was an init preference as leaf heights were rapidly reduced in the first day (6-13cm), but there after, rate of decline was similar between treatments. Phalaris in field trial three after nitrogen fertilisation was clearly preferred and reduced in height at a fast rate with cocksfoot also being reduced severely over the first two days. Therefore it was concluded that high nutrient herbage levels will not always overcome differences in preferences between species. Also fertilisation can mask preference of less preferred species such as cocksfoot. It was also suggested that presence of legumes in plots may have reduced the ability of sheep to discriminate between species and treatments in field trials one and three.

Rate of intake trials did not in all cases correlate well with preference observed in the field such as in rate of intake trials two and three.