Investigations into the use of the faecal nitrogen method to estimate the energetic feeding value of pasture and to prove the selective feed intake of grazing animals

Improved equations for estimating the digestibility and the energetic feedi ng value by means of the composition of the animal's faeces were developed. The nitrogen content of the organic matter of faeces from sheep and cattle was used as an internal marker ("faecal nitrogen method") and a hyperbolic function was applied as a mathematical model to describe the relationship between faecal nitrogen and organic matter digestibility. Evidence was prov ided that the feeding value of the forage which is really ingested by selec tively grazing animals can be reliably estimated in this way. In order to derive the equations, 33 samples from pasture taken during the whole grazing season and tested in vivo by sheep were used. It was found th at the accuracy of estimation is influenced by the season. This effect can be taken into account by using the number of days after April 30(th) as an additional term. By integration of this correction term into the equation, a substantial improvement in precision was achieved. Equations for estimati ng the digestibility without (1) or with (3) this correction term were deri ved, wherein DOMest [%] means the estimated percentage of organic matter di gestibility and figure WT the number of days after April 30(th). The energetic feeding value also depends on the ash content of the feed whi ch is reflected in the ash content in the faeces (XA(faeces) [g/kg DM]). It proved to be possible to estimate the parameters ME or NEL via DOM, estima ted by the faecal nitrogen method, and the crude ash content of faeces usin g the equations (5) and (6). A comparison of the results obtained by application of these equations with the in vivo-parameters and with the results of an in vitro-method, based o n testing the forage with cellulase according to De Boever et al. (1993) an d a mathematical model published by Weissbach et al. (1999), showed good ag reement. This enables to prove the effect of the selective feed intake by g razing animals. The cellulase method can be taken for estimating the feedin g value of the offered forage and the faecal nitrogen method of the ingeste d forage. The difference between these both issues indicates the intensity of selection by the animals. An independent validation of these equations f or estimating DOM, ME and NEL by means of 36 other in vivo tested grass sam ples resulted in only slightly higher standard errors of prediction (3.7,4. 5 and 5.3 % respectively). Further evaluations clearly indicated that the e quations derived from the results of trials with sheep are valid also for c attle. The effect of selective feed intake during grazing on the feeding value of ingested pasture was further studied by grazing experiments over 5 years wi th growing cattle on extensively managed grassland. On average of the whole season and all experimental years a 5 units higher DOM and a 0.8 MJ/kg DM higher ME for the ingested compared to the offered pasture was recorded. Th is animal behaviour enabled a better body weight gain as could be expected from the pasture quality offered.