Michigan State University animal scientists studied four hay feeder design types: cone, ring, trailer or cradle.  All feeder types allowed approximately 14.5 inches for each animal.  Dry matter hay waste was 3.5%, 6.1%, 11.4% and 14.6% for the cone, ring, trailer and cradle feeders, respectively.  There were differences in the behavior of cows at the feeders that may be involved with the differences in wastage.  Cows eating from the cradle feeder had about 3 times as much butting and displacement behavior as other feeder types and four times as many entrances compared to cows feeding at the other type of feeders.  The researchers determined that slanted bar designs encourage animals to keep their head in the feeder for longer periods while eating. Source: Buskirk, et al. 2003. 
http://www.msu.edu/~buskirk/Publications/JAS%2081-109.pdf 

        The effect of hay feeding method on cow performance and economics was also evaluated using mature beef cows at the Dickinson North Dakota Research Extension Center. Methods evaluated included 1) rolling round bales out on the ground, 2) a PTO driven round bale processor that shreds round bales into windrows, and 3) a tapered-cone round bale feeder engineered with a center tapered cone creating a manger around the inner circumference of the feeder. Pregnant cows were fed for a period of 58 days to document feed waste, and cow performance (weight gain, ultrasound fat depth change, body condition score change and hay intake).  Labor inputs, and feeding time, were also subsequently used to develop an economic analysis. 
The cows eating at the tapered cone feeder had increased ending weight, rib and rump fat depth change, increased ending body condition score, and reduced hay usage. In the economic analysis model, which was developed for 100 head cow herd sizes, feeding with a tapered-cone round bale feeder offered substantial cost savings per cow arising from lower hay usage and reduced equipment operating time. Feeding costs per cow in the 100 head herd for rolling out bales, shredding bales with a processor and feeding bales in a tapered-cone feeder were $97.99, $107.44, and $85.36, respectively. Using a PTO driven bale processor to shred bales into windrows before feeding was the most expensive due to higher equipment ownership cost and higher hay disappearance per cow compared to the tapered-cone bale feeder. Rolling bales out on the ground or shredding into windrows with a bale processor increased hay usage and winter feeding cost without enhancing cow performance.  Source: Landblom and co-workers. Dickinson North Dakota Research Extension Center 2003 Research Report.  http://www.ag.ndsu.nodak.edu/dickinso/research/2002/beef02p.htm