Managing Lolium perenne L. (perennial ryegrass) in a sub-tropical environment in KwaZulu-Natal, South Africa.

Abstract

Lolium perenne L. (perennial ryegrass) generally fails to persist

under the sub-tropical cpnditions of South Africa. Furthermore,

little research data are available on how to manage this species

locally. This study was designed to identify the management

options, particularly with r espect to grazing defoliation, which

would help enhance the longevity of perennial ryegrass pastures.

This was addressed by:

1) reviewing on-farm management practices of perennial

ryegrass in KwaZulu-Natal;

2) conducting a detailed two-year field study of the effects

of grazing frequency (HF, MF and LF = high, medium and low

frequency, respectively) and intensity (HI, MI and LI =

high, medium and low intensity, respectively), rotationally

applied with the addition of a continuous grazing treatment

(CG), on parameters linked to persistency.

tiller population dynamics, dry matter

These included:

(DM) yield and

quality, perennial ryegrass vigour, weed invasion and root

development; and

3) examining effects of different levels of applied nitrogen

(N) during the establishment year on various parameters

linked to persistency. These included: tiller population

densities, DM yield and quality, perennial ryegrass vigour,

weed invasion and root development.

The review of on-farm management practices of perennial ryegrass

growers in KwaZulu-Natal revealed that reasonably high rates of

N application (e.g. 350 and 250 kg N ha¯¹ a¯¹ to perennial ryegrass

as pure and clover-based stands, respectively) are important for

pasture survival. However, a consistent distribution of the

applied N is even more important (i. e. at least seven split

applications of N onto pure stands of perennial ryegrass and five

onto perennial ryegrass-clover). In terms of grazing management,

the period of absence of animals from the pasture during summer

was identified as the most important grazing variable affecting

pasture survival (i.e. ≥ 21 days). Also, the length of the

period of occupation by animals should be as short as possible,

particularly during summer (i. e. ≤ 3 days). Paying careful attention to summer irrigation is also an important variable

contributing to pasture survival. Grazing intensity was not

highlighted as an important contributor to pasture survival.

In terms of tillering potential, DM yield and quality (cellulose

dry matter disappearance and herbage N) and perennial ryegrass

vigour, perennial ryegrass followed definite seasonal trends.

These were highest during autumn and spring and were lowest

during the mid to late summer period. Perennial ryegrass was

most susceptible to general sward degradation through poor

management during the mid to late summer period when the danger

from weed invasion is greatest and its growth potential, vigour

and tillering abilities are lowest within these seasonal

periods, grazing defoliation produced marked effects. In terms

of tiller survival, DM yield, plant vigour, reduced weed invasion

and root production, treatments incorporating low frequency

grazing (e.g. LFLI and LFHI) generally out-performed (P≤0.05)

those incorporating high frequency grazing, irrespective of the

intensity (e.g. HFHI, HFLI, and continuous grazing (CG)). The

defoliation treatment incorporating medium frequency and

intensity (MFMI) (currently the recommended defoliation strategy

for perennial ryegrass) was also out-performed in many instances

(P~0.05) by the low frequency treatments (e.g. LFHI and LFLI) .

During the establishment year, increasing levels of applied N

increased (P≤O. 05) perennial ryegrass DM yields and herbage

quality. Models predicting the response of DM yield and quality

to applied N suggest linear responses up to 720 kg N ha¯¹ a¯¹.

Further refinement of such models and the inclusion of animal

production parameters is recommended. Maximum (P≤0.05) tiller

population densities occurred at applied N levels of 480 kg ha¯¹

a¯¹. Perennial ryegrass vigour increased (P≤O. 05) with increasing

levels of applied N up to 480 kg h¯¹ a¯¹, but individual tiller

vigour decreased. Increasing levels of applied N up to 360 kg

ha¯¹ a¯¹ suppressed (P≤O. 05) weed tiller densities. Increasing

levels of applied N (up to 600 kg h¯¹) increased (P≤O. 05) the

root organic matter (OM) per unit volume of soil in the top 5 cm

of the soil and decreased root OM per unit volume in the 10 - 20

cm soil depth category.