In pen-fed groups, minimizing sorting is just as important as providing adequate effective fiber.
A client recently commented that about 20% of the animals in a mixed cow-heifer pen had loose manure. The ration had more than adequate fiber (on paper), but recent forage dry matter changes had altered the dry matter of the Total Mixed Ration (TMR).
My first thought was that all cows were not eating the same diet, in other words; they were sorting the TMR. Indeed, the dry matter content of the TMR had changed enough that the animals were sorting the TMR, evidenced by the fact that the cleanup feed contained more straw than normal.
A recent paper in the Journal of Dairy Science addressed the question of feed sorting and its effects on chewing behavior, production and rumen fermentation. In a well-designed study, the authors fed the same TMR once per day to eight cows in individual stalls in a 4x4 Latin square designed trial. The only difference in the TMRs was the length of the grass hay. As the length of the hay increased, more sorting would have been expected.
Cows tend to sort against longer particles and for finer particles. The cleanup feed (refusals) in this study showed that cows did select the finer particles first. The feed that the cows actually consumed on the shorter grass-hay TMR was closer to the TMR that was fed than that of the longer-grass TMR. However, at the end of the day, each cow had consumed approximately the same amount of NDF and starch. Rumen fermentation, milk production and milk components weren’t different between cows. The authors stated that the cows may have sorted to a desired mean particle length.
The results of this study imply that cows will eventually consume enough long fiber to compensate for a preference for grain and finer particles over a day’s time. This is good news for tie-stall fed cows, since the cows in this study were individually fed. However, in pen-fed cows, the same cows don’t always eat the leftovers. It’s like a buffet. The first people at the buffet might get all the jumbo shrimp and “death by chocolate” cake. The last to the buffet might only get a choice of three bean salad and alfalfa sprouts.
I contend that in pen-fed groups, minimizing sorting is just as important as providing adequate effective fiber. So, as this study indicates, it’s better to feed a TMR that can’t be sorted in pen-fed animals, even if the average particle size of the TMR is reduced. That’s why the fiber in the middle box in the Penn State Forage Particle Separator is so important.
Reference: Maulfair, D.D., G.I. Zanton, M.Fustini and A.J.Heinrichs. 2010. Effect of feed sorting on chewing behavior, production, and rumen fermentation in lactating dairy cows. J. Dairy Sci. 93:4791-4803.