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Last update: May 2021

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Substitution rate between grazed pasture and a conserved forage in grazing dairy cows

Thesis : substitution rate between grazed pasture and a conserved forage in grazing dairy cows
The main objective of the thesis is to quantify the influence of grazing conditions and of the level of forage supplementation on pasture intake and milk production in dairy cows.

Study the pasture intake regulation in dairy cow

In the future, the average dairy herd size will continue to increase, while the area available for grazing will probably decrease on a per cow basis. In this context, more dairy herds will be fed during longer periods (several months) on mixed diets comprising grazed pasture and conserved forages. If concentrate supplementation has been studied for a long time, enabling to predict accurately the substitution rate between pasture and concentrate, knowledge on pasture intake regulation when conserved forages are fed is less, making difficult to predict accurately cow performance in such feeding systems. Particularly, it seems difficult to predict the range of pasture/forage substitution rate, as well as to quantify the effects of main sources of variation.

The main hypothesis of the thesis is that grazing conditions, and mainly pasture allowance, along with the level of forage supplementation, are the main determinants of substitution rate. The INRA model, developed at the unit Pegase (GrazeIn) are not able to predict very low substitution rate (close to 0) reported in the literature under very severe grazing conditions.


An experimental and meta-analysis approach to determine the factors of variation

The thesis in mainly based on an experimental approach to improve partial knowledge available in the literature. Three grazing experiments are planned, with the aim to define the interaction between levels of maize silage supplementation and pasture allowance on the pasture/forage substitution rate. A quantitative meta-analysis of the literature will be done to complete the experimental approach and to determine the main factors of variation of pasture/forage substitution rate in grazing dairy cows.

In the grazing experiments, individual pasture intake will be estimated from the n-alkanes method, clearly identified as the best method for high levels of supplementation. Grazing behaviour will also be recorded to better understand behavioural adaptation of the cows while increasing forage supplementation level.

The thesis of Marcolino F. Miguel is a sandwich PhD carried out partly in Brazil (direction H.M.N. Ribeiro Filho, Santa Catarina State University, Lages), with the completion of two experiments in the University of Lages. The thesis is planned from March 2012 to February 2016, with 12 months spent at UMR Pegase (Syslait team), from november 2013 to october 2014, with a direction by Rémy Delagarde. The third experiment and the meta-analysis are planned during this stay.