Sodium fertilizer application to pasture. 8. Turnover and defoliation of leaf tissue

The effects of application of sodium fertilizer on the turnover and defolia tion of leaf tissue were investigated in a perennial ryegrass (Lolium peren ne) pasture grazed by dairy cows. Eight plots were allocated to treatments either with or without sodium fertilizer, with the former receiving 32 kg N a ha(-1) applied in five applications of NaNO3 over the grazing season. An equivalent amount of nitrogen was given to the controls as ammonium nitrate , the application of which was reduced in the sodium treatment to equate ni trogen fertilizer applications for the two treatments. In nine periods betw een April and September, marked tillers were recorded to measure leaf turno ver, leaf lamina growth and specific leaf weight and, when combined with ti ller density measurements, gave an estimate of herbage flux for the sward. The defoliation and net growth of the marked tillers were monitored at 3-da y intervals and the data were combined with tiller density and specific lea f weight data to determine the intake of the expanding, penultimate and old est live leaf laminae. Sodium fertilizer application did not affect the rat e at which leaves appeared, but it retarded their rate of disappearance. Th e extension rate and the specific weight of green laminae were both increas ed by sodium fertilizer application and therefore the net gravimetric growt h rate was increased. Tiller density was not affected by sodium fertilizer application and hence the estimated herbage growth and net herbage flux wer e increased by sodium fertilizer application. Application of sodium fertili zer did not affect lamina length, and in both treatments the penultimate la minae were approximately twice as long as expanding and oldest live laminae . Defoliation frequency decreased from the expanding to the oldest live lam inae in the control treatment without sodium. Sodium fertilizer application increased the frequency of defoliation of the oldest live leaf and also in creased the length of the expanding leaf that was defoliated. For penultima te leaf laminae sodium fertilizer application reduced the defoliation frequ ency and length of foliage grazed. The dry-matter (DM) intake of the oldest live laminae was increased by the application of sodium fertilizer. It is concluded that sodium fertilizer application increases net herbage growth b oth by increased extension rate of leaf laminae and specific leaf weight an d by delayed laminae senescence, and that it increases herbage DM intake by increasing the defoliation frequency of the oldest live leaf laminae.