I have a 50 acre grass pasture that is being taken over with multiflora...
Chemical control is probably the way to go. Mowing only works if you do it several times preseason for 2-4 years to really wear it down.
Crossbow is usually accessible to most producers and would be a good product to start with. The active ingredient is 'triclopyr', which is is recommended for good control. The active ingredient 'metsulfuron' is also recommended and can be found in a product called Cimarron.
Crossbow is a broad leaf killer so it will take out any broadleaf plant such as clovers and other legumes. Here is a brief article that has some useful comments on woody brush control in pastures: http://u.osu.edu/beef/2016/06/08/problem-pasture-weeds/.