A model to predict transformations and losses of nitrogen in UK pastures grazed by beef cattle

Abstract

The model simulates the cycling of N in grassland systems grazed by beef cattle and predicts the annual amount of N in liveweight gain, and the amounts lost through ammonia volatilization, denitrification and leaching, on the basis of fertilizer application and soil and site characteristics. It aims to provide a better understanding of the way in which these various factors interact in their influence on N transformations. The model has been programmed to run on IBM-compatible personal computers and responds rapidly to changes in input parameters.The model has been constructed from the average annual amounts of N passing through various components of the N cycle in ten field systems grazed by beef cattle. The amounts were either measured directly or were calculated from empirical sub-models, assuming a balance between inputs to, and outputs from the soil inorganic N pool. The model is given wide applicability through the inclusion of a mineralization sub-model which is sensitive to soil texture, sward age, previous cropping history, and climatic zone. Another important sub-model determines the partitioning of soil inorganic N to either plant uptake or the processes of loss: the proportion partitioned to plant uptake decreases as the total amount of soil inorganic N increases.Outputs from the model indicate that fertilizer N has a strong influence on ammonia volatilization, denitrification and leaching at a given site but that, over a range of sites with a given rate of fertilizer N, total loss and the proportions lost by the three processes are greatly influenced by the amount of N mineralized by the soil. The model indicates how fertilizer N should be matched with mineralization to limit gaseous and leaching losses and to achieve optimum efficiency of N use in grazing systems.

Forage intake and botanical composition of feed for cattle fed Brachiaria/legume mixtures

“…The relationship presented by Scholefield et al (1991) did not show a very close relationship to the results obtained in this study (Table 7). This is not surprising as the data used for Scholefield's model were obtained from ryegrass, clover and other temperate forages, considerably higher in N content than the B. dictyoneura hay (11.4 gN kg DM -1 ) and that the high content of polyphenols of the D. ovalifolium, unusual in temperate forages, which immobilise protein in the rumen such that it is excreted in the faeces rather than being assimilated.…”

“…If it is assumed that the total N in the animal carcass is 2.5% of the live weight (Scholefield et al, 1991) then the total N gained by the animals each day should be 8.8 g/head/day. So the total N excreted in urine should be the difference between the N consumed and the N excreted as faeces and that accumulated in the LWG (Berry et al, 2002).…”

“…Estimating urine production from grazing animals is extremely difficult, but if total forage consumption and the digestibility and concentration of N of this forage can be measured or estimated, then this ratio can used to calculate total N excreted in urine. Scholefield et al (1991) determined that the ratio (R u/f ) of N excreted in urine to that excreted could be described by the formula: R u/f = [1.2725 * (%N in diet)] -1.09.…”

“…This latter effect was apparent when the legume content in the diet increased from 25 to 50%, the concentration of N in the faeces increased from 14.8 to 17.0 gN kgDM -1 (Table 5). While the linear regression model of Scholefield et al (1991) indicated that dietary N concentration increasing from 14.4 to 18.2 g N kg DM -1 when the legume content increased from 50 to 100%, the ratio of urine N to faecal N (R u/f ), should increase from 0.74 to 1.22, but for this forage legume/Brachiaria mixture the R u/f increased from only 0.54 to 0.83 (Table 7).…”

Integrated regulation of nonpoint pollution: Combining managerial controls and economic instruments under multiple environmental targets

“…The IRRIGUIDE model (Bailey et al 1996) was 140 used to give crop-dependent weekly values of evapo-transpiration over winter; while 141 elution was modelled using the SLIMMER algorithm (Anthony et al 1996). Grass land 142 leaching was estimated using NCYCLE (Scholefield et al 1991;Lockyer et al 1995). Here, 143 leaching refers to the nitrogen not taken up by the plant which drains to the sub soil water.…”