Grazing time: the missing link : a study of the plant-animal interface by integration of experimental and modelling approaches

A series of grazing (chapters 2, 3, 5 and 6) in-vitro (chapter 4) and modelling trials (chapters 1 and 7) were combined with the following objectives: a) to gain insight in the main mechanisms controlling dry matter intake (DMI), intake rate (IR) and grazing time (GT), during the first grazing session after a.m. milking, b) to judge the relative importance of rumen fill and the concentration of fermentation products in the rumen liquor as candidates to signal the end of the grazing sessions and c) to develop new and modify and evaluate existing simulation model, to operate under non-steady state conditions with the aim to predict DMI, rumen fermentation and supply of nutrients.Increasing the length of the allowed grazing time significantly increased DMI (P<0.01), the proportion of time spent actively eating (P<0.01) and DM rumen pool size after grazing (P<0.05). The allowed grazing time did not have any significant effect on total and liquid rumen pool sizes after grazing but did have (P<0.05) on DM and OM (slope, 0.5 kg h-1) rumen pool sizes. DMI as well as GT were greater after a starvation period of 16.5 h and were reduced by the presence in the rumen of indigestible material (P<0.01).The interaction between starvation time and rumen fill before grazing on GT, although not significant (P<0.06), supports the idea of a combination of signals controlling meal size under grazing conditions. Grazing time did not follow a significant trend with period of regrowth. In this trial, rumen fill (as represented by total, DM or NDF rumen pools size), volatile fatty acids (VFA), ammonia, pH and osmotic pressure as individual variables were not correlated with GT or DMI. Rumen pools can be accurately predicted under discontinuous feeding regimes, although the representation of rumen ammonia pools requires further development. This general finding is highly relevant since the distance between the "sward-driven" and "metabolic driven" models can be shortened and the whole and unique process of "ingestion-digestion" of nutrients under grazing tackled. Grazing time control remains a difficult obstacle to understand the whole process.This research offered valuable information about the relative importance of several factors in the control of GT. Clearly it is necessary to understand the way in which the different signals produced at different places are integrated for the animal to modulate eating and other behaviour. In this sense the combination of analytical and synthetic research was proven to be an effective strategy.PhD Thesis, Animal Nutrition Group, Wageningen Agricultural University, Marijkeweg 40, 6709 PG, Wageningen, The Netherlands.