The growth curves of closed grassland swards obtained during the growing seasons of six successive years were compared with each other and with theoretical growth rates calculated by DE Wit's method and based on the mean light energy input data for the same years. The form of the growth curve consisted of a period with a constant rate, lasting for 6-7 weeks in May-June, then rapidly shortening to 3 weeks in August-September, after which there was a rather abrupt change to zero values. A comparison with theoretical values showed that a period of constant growth rate was possible, because the promoting influence of increasing LAI counteracted the effect of increasing respiration. Before the longest day this period was lengthened by an increasing daylength; thereafter the reverse was found. There is no ready explanation for the sudden decline to zero values. This is thought to be associated with changes in the rate of photosynthesis.

In 3 trials with perennial ryegrass given N in the range 0-1000 kg/ha and from 3 to 28 times/yr, DM production, N uptake and NO3 content in relation to N application rates and cutting frequencies of herbage were compared. Higher N application rates increased the herbage yield more at longer growing periods than at higher cutting frequencies. N uptake was positively affected by the application rate and not or only slightly by the cutting frequency. Consistent with this, appreciably high N conc. (max. 5.4%) were found on av. in the herbage at high fertilizer N rates and frequent cutting than at lower rates and less frequent cutting (1.6%). NO3 content increased during the growing season, especially as the N application rate was higher and the growing periods longer. (Abstract retrieved from CAB Abstracts by CABI’s permission)

Stigter, C. J., Goudriaan, J., Bottemanne, F. A., Birnie, J., Lengkeek, J. G. and Sibma, L., 1977. Experimental evaluation of a crop climate simulation model for Indian corn (Zea mays L.). Agric. Meteorol., 18:163-186.For the evaluation of a micrometeorological model, extensive measurements were taken of the microclimate in a maize cwp. Both the model and the measurements are discussed in this paper. In a comparison of the measured and simulated characteristics of the mic1·oclimate it is concluded that the absorption of radiative energy and the leaf and boundary layer resistances are adequately simulated, so that this micrometeorological model presents a sound basis for simulation models of crop production. Profiles of aerial temperature and humidity are also reasonably simulated. They can still be improved upon by a better incorporation of spatial and temporal variations of turbulence and wind as a function of the canopy characteristics, and probably by a better simulation of exchange at the soil surface. ·

Statistical treatment of yield data from potatoes, sugar-beet, peas, wheat, barley, flax and maize grown in short- and long-term experiments where water and mineral supplies were optimum showed that variations in yields, especially those of potatoes, were positively correlated with the total global radiation received during the growing period. Partial correlation coefficients showed that the length of the growing period and the annual yield increase might explain much of the yield variation.-G.M.H. (Abstract retrieved from CAB Abstracts by CABI’s permission)

The yields in kg/ha were calculated for potatoes, sugar beet and winter wheat for conditions under which water and mineral supply were not limiting growth rates. The date at which a closed crop canopy can be obtained in spring was an important criterion for the yield. Growth analyses of silage maize and sugar beet confirm that relatively high yields can be further improved by accelerating leaf development in spring. (Abstract retrieved from CAB Abstracts by CABI’s permission)

Highest growth rates of 10 crops with a closed leaf canopy and non-limiting supplies of water and minerals ranged from 162 (grass) to 228 (potatoes) kg DM/ha per day. When root DM was taken into account, the growth rate of most crops was similar, about 250 kg DM/ha per day, the determining factor being the weather. Differences in DM yields between crops were determined mainly by the duration of the growing period.-R.B. (Abstract retrieved from CAB Abstracts by CABI’s permission)

Aboveground net photosynthesis and respiration of maize cv. LG11 was determined in the field between mid-June and end-Oct. at regular intervals (1-2 weeks) at 12.5 and 22.5 degrees C by measuring the CO2 uptake or release in mobile crop enclosures. The actual growth rate of the crop was determined from fortnightly harvests. Temp. dependence of photosynthesis was highest in the early (until mid-July) and very late (early Oct.) stages of crop growth, showing a reduction of about 50% at the higher irradiances (>400 W/msuperscript 2). In the period of major DM production (mid-July to Sep.) the reduction was only 12-15%. Assuming maintenance respiration to become constant for cobs and grain exceeding a yield of 1000 kg/ha and for stems exceeding 2500 kg/ha, the measured and calculated dark respiration at 22.5 degrees matched fairly well. At 12.5 degrees the calculation, using the same assumptions, significantly overestimated dark respiration during the first part of the growing period. The carbon balance sheet showed that from the total amount of CO2 absorbed by the crop (equivalent to 30.7 t DM/ha), 30% was lost by aboveground respiration and 50% was invested in aboveground harvestable material. The remaining 20% was assumed to be transported to plant parts below ground. Substantial losses of DM at the end of the growing season did not occur. (Abstract retrieved from CAB Abstracts by CABI’s permission)

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