Defoliation management and nitrogen fertiliser rate affect canopy structural traits of grazed guineagrass (Megathyrsus maximus) cv. Zuri under rotational stocking

ABSTRACT Context. Defoliation management by grazing or clipping, combined with nitrogen (N) fertilisation, can increase forage production in tropical grasses. Use of pasture canopy height as a practical tool for monitoring forage mass and accumulation has been proposed because of the relationship between height and canopy light interception. However, this relationship can vary due to variations in canopy structural changes and N supply. Aims. Our objectives were to study canopy structural responses and forage accumulation of grazed Zuri guineagrass (Megathyrsus maximus) under rotational stocking. Methods. Combinations of two pre-graze canopy heights (55 and 75 cm) and two N fertilisation rates (150 and 300 kg N ha−1 year−1) were used during two consecutive years. Cattle grazed the pastures to 50% of pre-graze height. Key results. Regardless of N rate, pastures managed at 75 cm always reached 95% light interception at pre-graze. The 55 cm/300 kg N pastures underwent structural changes such as greater leaf proportion, tiller population density and specific leaf area, and smaller mean foliage angle, which resulted in 95% light interception at pre-graze. Forage accumulation was 20% greater for 75 cm pastures (22 120 kg DM ha−1 year−1) than 55 cm pastures. Pastures receiving 300 kg N ha−1 year−1 had 38% greater forage accumulation than those receiving 150 kg N ha−1 year−1. Conclusion. Zuri guineagrass pastures should be defoliated when canopy height reaches 75 cm owing to greater forage accumulation, but can be grazed at 55 cm when N is applied at 300 kg ha−1 year−1. Implications. Zuri guineagrass can be managed with shorter pre-graze canopy height; when grazed at 55 cm and with 300 kg N ha−1 year−1, there is no negative effect on forage accumulation, allowing for shorter rest periods.