Productivity of grasslands under continuous and rotational grazing

Abstract

In the Netherlands, rotational grazing, with grazing periods of 2 to 5 days, is the most common grazing system at present. In contrast with other countries of North-western Europe, the continuous grazing system is used here only to a limited extent. However, the results of numerous comparative trials at high nitrogen fertilization levels and high stocking rates, carried out in the 1970's, suggest that there is no significant difference in animal production between the two grazing systems.Experiments were carried out to determine the physiological and environmental limits to herbage production under continuous and rotational grazing. This was done by measuring the seasonal patterns and seasonal totals of sward CO 2 assimilation and animal production. The experiments were carried out on heavy clay soils at two nitrogen fertilization levels (125 and about 450 kg N ha -1yr -1). With 450 kg N ha -1yr -1cumulative gross assimilation over the grazing season was 9% higher with rotational than with continuous grazing, but there was no difference in animal production. The higher efficiency of utilization of gross assimilation products under continuous grazing was due to lower topping losses and lower costs of above-ground maintenance respiration. Under both grazing systems, gross CO 2 assimilation per unit leaf area was not depressed at all at 125 kg N ha -1yr -1. but there was a marked reduction of the rate of leaf area development in the second half of the grazing season. The absence of any effect in the first half of the grazing season was due probably to a residual effect of previously applied nitrogen. This effect can be considerable on heavy clay soils.The herbage intake under rotational grazing is often estimated using Linehan's formula, which takes into account the herbage production during grazing. This formula was evaluated by means of dynamic simulation, with measured assimilation-light response curves as the main input. It emerged that in some practical situations of rotational grazing, the herbage production during the grazing period is significantly underestimated using Linehan's formula. This is mainly because Linehan's formula assumes exponential growth of the sward at all stages of growth. Since this is not correct for a sward in the absence of grazing, a new comprehensive formula is derived, using the assumption that the sward is in the linear growth phase at the start of grazing. Comparisons with the simulation output show that this new formula for estimating herbage intake is valid for all situations of rotational grazing.To manage the continuous grazing system successfully, information is needed about the seasonal changes in production capacity with this grazing system. An experiment with dairy cows revealed that throughout the grazing season a constant proportion (equal to 0.25 at the high nitrogen level) of the carbohydrate pool, derived from gross assimilation minus above-ground maintenance respiration, was ingested by the grazing cattle. This observation formed the basis for a dynamic model to compute the net herbage production under continuous grazing throughout the grazing season, using data on radiation and temperature. It appears that, although there is a gradual decline from spring to autumn, the highest production rates occur in June. The average seasonal pattern of net herbage production, predicted by the model using average weather data, can be used for management purposes of the continuous grazing system. It is then possible to adjust stocking density during the grazing season, to achieve the maximum pasture output without adversely affecting the botanical composition and tiller density of the sward. Although the limited time available for this study made it impossible to examine effects of rotational and continuous grazing on the long term, there are good indications that continuous grazing is preferable in this respect. Moreover, the continuous grazing system seems better able to withstand periods with low rainfall, owing to the higher tiller density. During prolonged periods of water stress, however, the mean sward height must be lowered to about 6 cm, instead of the normal optimum level of 7 to 8 cm. It was observed on the clay soils that there is then actually no need for irrigation to achieve high pasture outputs. This is due partly to the unchanged carbon allocation pattern during periods of water stress.The simulated production rates under continuous grazing were compared with measured production rates of grass swards with an initial herbage mass close to the recommended grazing stage for rotational grazing (1700 kg DM haha -1above 4 cm), in order to develop a simple method for estimating the undisturbed herbage production rate at the onset of a rotational grazing period. The herbage production of a sward in the grazing stage for rotational grazing was, on average, 2 or 2.5 times the production level under continuous grazing. The higher value was found for the period early May-early June and was a result of stem elongation. This observation may serve as a simple method for estimation of the undisturbed production rate.A new procedure was developed to deal with grazing losses. Here, the utilization efficiency is calculated by comparing the total amount of harvested dry matter (herbage intake plus silage grass) with the total production under a certain cutting regime. This approach is applicable in any grassland management situation.The CO 2 assimilation of a leaf canopy is strongly dependent on the ratio between diffuse and direct radiation. This produces much of the scattering often observed in field measurements on the assimilation-light response of crop surfaces. It is shown that the crop production models PHOTON and BACROS, developed at the Department of Theoretical Production Ecology of the Wageningen Agricultural University, treat the distribution of diffuse and direct radiation over the leaves of a canopy correctly, but that the proportion of the diffuse component is significantly underestimated over a wide range of radiation levels. Radiation measurements were used to improve the section in these models describing the separation between diffuse and direct radiation. Literature survey showed that the leaf assimilation-light response in field-grown grass swards can be best calculated using a Blackman curve, rather than the frequently used asymptotic exponential curve. With these improvements, the relation between daily total radiation and daily total gross assimilation (and dry matter production) is shown to approximate to a Blackman curve, with the intersection at 60% of the maximum radiation total on that day, i.e. that under a perfectly clear sky