Completely hide asset related logging in Rails 3.2

development.rb

config.assets.logger = false

suppress_asset_logging.rb

if (Rails.application.config.assets.logger != nil) && !Rails.application.config.assets.logger
  Rails::Rack::Logger.class_eval do 
    def call_with_quiet_assets(env)
      previous_level = Rails.logger.level
      Rails.logger.level = Logger::ERROR if env['PATH_INFO'].index("/assets/") == 0 
      call_without_quiet_assets(env).tap do
        Rails.logger.level = previous_level
      end 
    end 
    alias_method_chain :call, :quiet_assets 
  end
end

Add the config line to config/environments/development.rb and add "supress_asset_logging.rb" to config/initializers.

With thanks and credit to https://github.com/guilleiguaran in this comment: rails/rails#4569 (comment)