Influence of the endophyte fungus in ryegrass on grazing preference by sheep

Abstract

Animal productivity may be enhanced through greater intake associated with the use of preferred pasture species provided they are of high nutritional value. Grazing preference by ewe hoggets was investigated in three grazing trials and an indoor feeding trial.

The first grazing trial (April 3-12, 1990) examined the preference of ewe hoggets for three grass treatments, each containing white clover. The grass treatments were endophyte (Acremonium lolii) infected (+E) perennial ryegrass, endophyte free (-E) perennial ryegrass and tall fescue. Two 0.2 ha blocks, each containing three replicates of the three grass treatments were sown in spring 1989. Endophyte incidence (% infected tillers) was 40% in +E ryegrass and 0% in -E ryegrass. Tillers were vegetative. Ewe hoggets (15) were introduced into the plots and their preferences recorded by visual observations from a raised viewing tower, and agronomic measurements on the plots. On day 1 of grazing a high proportion (76%) of ewe hoggets grazed tall fescue while a low proportion (12%) grazed both +E and –E ryegrass. From day 2 to 10 of grazing a similar number of sheep grazed each grass treatment. Grass leaf height declined less in +E ryegrass (48%), than -E ryegrass (64%) and tall fescue (71%) during grazing. Pseudostem height of grasses was reduced 4.2mm in tall fescue and -E ryegrass but only 1.5mm in +E ryegrass by grazing. Pasture mass declined more in -E ryegrass (1230 kg DM/ha) than +E ryegrass (1100 kg DM/ha) or tall fescue (1020 kg DM/ha). The percentage green grass cover (point analysis first hits) decreased with grazing in all grass treatments but less in +E ryegrass (21%) than -E ryegrass (34%) and tall fescue (28%). White clover percentage was low in all grass treatments (4-7%) prior to grazing and declined with grazing. Both +E and –E ryegrass contained more dead material than tall fescue.

In a second grazing trial (May 14-21) the same plots were either mown to 30mm or left unmown two weeks prior to grazing and also subdivided into urine and non-urine patches. No endophyte was detected in either +E or -E ryegrass. Ewe hoggets (20) were introduced and agronomic measurements made during grazing. The height of grass leaves in all treatments was always greater in urine than non-urine patches but grass leaf height declined more rapidly in urine patches. The grass leaf height of +E and –E ryegrass was twice that of tall fescue in urine and non-urine patches throughout grazing. Pasture mass, prior to grazing, over all treatments was higher (1990 vs 1578 kg DM/ha) but declined more during grazing (928 v 751 kg DM/ha) in urine than non-urine patches. Mowing created a constant pseudostem height (30-32 mm) but did not influence pasture mass or grass leaf height changes.

In the third grazing trial (25 September - 5 October) measurements of grass leaf height were made throughout grazing on +E (60% infection) ryegrass and -E ryegrass (0% infection) plots that were part of a grazing preference study consisting of four replicates of eight grasses or herbs. Plots had been ungrazed since May 23, 1990. Reproductive tiller development had begun. The height of +E and -E ryegrass were similar prior to grazing (180mm) but declined more in -E ryegrass (135mm) than +E ryegrass (80mm). Endophyte incidence in laxly grazed areas of +E ryegrass plots was 70% but only 30% in intensively grazed areas.

The rate of eating of different feeds (white clover, tall fescue, +E ryegrass (70% infection) and -E ryegrass (0% infection)) was measured by offering 100g fresh samples of each alone to eight hungry sheep and recording time taken to consume each sample. Eating rates were highest for white clover (5.52 g DM/minl and lowest for +E ryegrass (2.89 g DM/min) with tall fescue (3.86 g DM/min) and -E ryegrass (3.94 g DM/min) intermediate. The three grass feeds were then separated into pseudostem and leaf, and the rate of eating measured. Leaf material was eaten at similar rate for all grasses (3.86-4.21 g DM/min). The +E ryegrass pseudostems were eaten more slowly (1.85 g DM/min) than -E ryegrass (2.65 g DM/min) or tall fescue (3.47 g DM/min).

The slower rate of decline of grass leaf height, the smaller amount of pseudostem, pasture mass and green leaf removed, and the lower rate of eating of endophyte infected +E ryegrass when compared to -E ryegrass is interpreted as discrimination by sheep against endophyte infected +E ryegrass. It is proposed that low animal production of sheep grazing +E ryegrass can be attributed to the reduced intake of +E ryegrass because of avoidance behaviour. It is also suggested that the avoidance of sheep by fungus from eating the grass, is an adaptive mechanism of the fungus to ensure continued survival via seed spread.