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. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1994.

Perennial ryegrass is an intensive, temperate pasture grass that responds well to applied fertiliser. The purpose of this project was to study the effects of fertiliser on the productivity and quality of perennial ryegrass in KwaZulu-Natal and how this impacts on animal intake. It was hypothesised that over-application of fertiliser to a perennial ryegrass pasture would negatively affect the quality of the herbage for grazing by dairy cattle and that intake would be affected. Thus the project aimed to assess the effects of applied fertiliser on yield, quality and intake of an established perennial ryegrass pasture. The trial consisted of a set of six separate experiments. Each experiment focused on the interaction between two of the major nutrient elements nitrogen (N), phosphorus (P), potassium (K) and sulphur (S). The experiments (NxP, NxK, NxS, PxK, PxS and KxS) were managed separately to avoid possible transfer of nutrients during grazing, which would result in the contamination of treatments. Each factor had three levels (low, medium and high), giving a total of nine treatments per experiment. Each of the experiments was replicated three times in a randomised block design. Increased fertiliser N application rates increased perennial ryegrass yield with a pattern of diminishing return, where split applications above 40 kg N ha-1 produced smaller increases in yield when compared with the response at lower applications of N. Applied P, K and S did not affect yield, suggesting that even the lowest application levels were sufficient to not limit production. Nitrogen application affected apparent intake, but it is suggested that this is due to the yield effect rather than a direct effect of N on apparent intake. The application of P, K and S did not affect apparent intake. Results from this study showed that the quality of perennial ryegrass herbage, especially in terms of feed value to dairy cows, can be significantly affected by applied fertiliser. The extent of the response was affected by sampling date (i.e. time of year) and this must be taken into account when planning a fertiliser management strategy. This is particularly so with respect to N fertiliser recommendations. Crude protein (CP) content of herbage increased with increasing levels of applied N and the extent of the response was influenced by season. P, K and S did not affect CP concentration in herbage, except in the PxK experiment where increased levels of K lowered herbage CP. Applied N considerably increased the concentration of non-protein nitrogen (NPN) in perennial ryegrass herbage. P and S did not affect NPN levels, whereas applied K decreased NPN content in the iv NxK and PxK experiments. Non-structural carbohydrate (NSC) content of herbage was decreased by applied N but was unaffected by applications of P, K and S. Neither neutral detergent fibre (NDF) nor acid detergent fibre (ADF) was affected by applied fertiliser. In this study herbage P declined and herbage Ca increased with increasing levels of applied N. The addition of fertiliser K resulted in lower herbage Ca values. There was no herbage S response to applied fertiliser in this study. Classification and regression tree (CART) analysis identified the primary determinant of apparent intake in experiments containing N as a factor as the amount of material available to be grazed and that NSC, NPN and ADF are also determinants of apparent intake. Cows do not regulate diet choice within the short-term time frame of a meal. Thus intake is determined by short-term needs rather than by meeting long-term nutrient requirements. Fibre creates physical fill within the rumen, thus restricting intake. High NPN content is associated with high nitrate values. The reduction in intake of herbage with high nitrate content may be due to reduced palatability or to a negative feedback system limiting the intake of nitrate and ammonium. Increased NSC content is associated with increased intake, possibly through the effect of sugar on herbage palatability.