Editor's note: The following answer is excerpted from a paper that Rick Grant, president of the Miner Institute, presented in October at the Cornell Nutrition Conference.
Q: Can productivity be maintained when feeding higher-forage diets versus diets that are closer to a 50:50 forage-to-concentrate ratio?
A: There is refreshed interest in feeding higher-forage diets to dairy cows. We are now in what seems to be an era of high-priced corn and other feed commodities, which encourages feeding of lower-starch diets. We can achieve lower dietary starch content by either replacing starch with fermentable carbohydrates from non-forage fiber sources or by feeding higher forage-to-concentrate ratios. With higher-forage diets, there are the long-recognized environmental benefits and the beneficial health consequences of feeding higher forage-NDF diets to cows.
Now, more than ever, we require nutrition models that do a good job of predicting forage digestion and passage as we formulate diets containing higher proportions of forage. We need to evaluate multiple pools of fiber for both digestion and passage. Ultimately, our goal will be improved prediction of NDF digestibility and animal response.
A fundamental question when feeding higher-forage diets is whether productivity can be maintained in comparison to a diet with closer to a 50:50 forage-to-concentrate ratio. Also, based on the preceding discussion, what differences exist in chewing behavior and ruminal function between diets containing substantial forage NDF and those containing considerable non-forage NDF?
Recently, we conducted a study at Miner Institute that evaluated how a higher-forage diet based on corn silage compared with a standard diet containing 50 percent forage and a diet containing non-forage sources of fiber in place of corn meal. The high-forage and the non-forage fiber diets both contained similar, and lower, starch content than the standard diet. The standard diet contained 20 percent conventional corn silage, 20 percent BMR corn silage, and 10 percent hay crop silage (mixed, mostly grass) for a total of 50 percent forage in the ration DM. This diet also contained 15 percent corn meal, 8.3 percent soybean meal, 5 percent beet pulp, 5 percent wheat midds, and 16.7 percent of a supplement. In contrast, the higher-forage diet contained 53.3 percent BMR corn silage and 10 percent hay crop silage for a total of 63.3 percent forage in the ration DM. We used BMR corn silage as the primary forage in an effort to enhance the digestibility of NDF from forage. The remainder of the diet was comprised of 6.7 percent soybean meal, 5 percent each of beet pulp and wheat midds, and 16.7 percent of a supplement. The diet based on non-forage sources of fiber contained the same forage sources and amounts as the standard diet (50 percent total forage), but corn meal was reduced to only 3.8 percent of ration DM and replaced with beet pulp (10.8 percent), wheat midds (10.8 percent) and distillers grains with solubles (4.2 percent).
All three diets contained similar crude protein (16.5 percent) and protein fractions, fat (3.9 percent) and sugar (6.8 percent). Ration NDF was lower for the standard diet (34.7 percent) than either the higher- forage or non-forage fiber source diets which were similar (38.1 percent). The peNDF content of the standard diet was also lower (18.5 percent) than the higher-forage diet (25.9 percent) simply reflecting the differences in forage content, and the non-forage fiber source diet was intermediate at 22 percent. Starch content was greatest for the standard diet (26.0 percent) and lower for the higher-forage diet (21.4 percent) and non-forage fiber source diet (21.3 percent).
Dry matter intake was greatest for cows fed the standard diet, intermediate for the non-forage fiber source diet, and lowest for the higher-forage diet. The lower intake for the higher- forage diet likely reflected the higher forage NDF and peNDF content of this diet. The intake of NDF as a percentage of body weight was greater for the higher-forage diet and the non-forage fiber source diet relative to the standard diet. In fact, 1.35 percent of body weight intake of NDF is a very high level of NDF consumption, and consequently the high forage NDF intake may have limited dry matter intake for cows fed the higher-forage diet despite the fact that it was comprised primarily of BMR corn silage. Although milk yield was reduced for cows fed the higher-forage diet, milk fat percentage was elevated, and so solids-corrected milk yield was similar for all diets as was efficiency of solids-corrected milk production.
Total chewing time was greatest for cows fed the higher-forage diet, intermediate for the non-forage fiber diet, and least for the standard diet. Ruminal pH was least for cows fed the standard diet, highest for cows fed the higher-forage diet, and intermediate for the non-forage fiber diet. Finally, microbial protein production was similar for all three diets.
The results of this study show us that we can successfully feed higher-forage diets if the forage contains highly digestible NDF (as in the case of BMR corn silage).
The question remains: how can we best model forage particle dynamics in the rumen to consistently formulate successful high-forage diets? (Note: Elsewhere in the paper, Grant describes efforts of the “Fiber Working Group,” a multi-year collaborative research effort to improve the current nutrition models, with a particular focus on the Cornell Net Carbohydrate and Protein System.)