As feed fermentation in the rumen generates heat, to maintain constant body temperature during hot weather, one of the strategies employed by the cow is to reduce feed intake. However, other mechanisms (i.e. panting) that operate to dissipate heat actually increases the maintenance energy requirements, making it necessary for the diet to have a higher energy density.
In general, nutritionists accomplish this by increasing concentrate and reducing forage in the ration. This is a sound approach, however sufficient effective fiber should be maintained in the diet to stimulate cud chewing and rumination, thereby, maintaining adequate rumen pH.
The National Research Council (NRC 2001) suggests the concentration of forage neutral detergent fiber (NDF) in the TMR should be between 15 and 19%, depending on the amount and type of non-fibrous carbohydrates (NFC) present in the diet.
This suggestion should be taken as an absolute minimum, as it was developed for TMR diets, based on alfalfa of adequate particle size. The reason why part of the diet NDF needs to come from forages is to ensure an adequate amount of effective fiber.
Forage fiber stimulates rumination, and thus cud chewing, which increases saliva production that neutralizes rumen acidity. The total NDF concentration in the diet has usually been associated with the incidence of acidosis, although this correlation is not very strong. However, there is a stronger correlation of acidosis with forage NDF in the diet. In a meta-analysis of 106 diets, ruminal pH was positively correlated with the percent of forage NDF (P<0.0001; r2=0.63), but not total NDF (Allen, 1997).
Research has confirmed the importance of forage inclusion in diets of cows under heat stress. In a University of Georgia experiment, heat-stressed cows were fed four experimental diets having a 40:60 forage:concentrate ratio (West et al. 1999. Table 1). Dietary fiber concentration was achieved by partial substitution of corn silage with Bermuda grass hay.
Forage NDF increased gradually from 17 %, without hay, up to 24.7% (22.8% hay) in the ration. Milk yield was highest, 58.1 and 56.8 lbs. /d for cows fed diets with an intermediate concentration of forage NDF (23.5 and 19.2%, respectively), while milk fat percentages increased linearly with forage NDF inclusion rate. However, it was necessary to include a minimum of 23.5% forage NDF in the diet to maintain milk fat at 3.5% or greater.
These results do not agree with those of Halachmi et al. (2004) in heat-stressed cows fed 12 and 18% forage NDF diets . The concentration of forage NDF did not affect intake, but affected milk yield. Cows fed low forage fiber diets produced 6% more milk at 84.7 lbs. /d. Differences in energy density (0.79 vs. 0.75 Mcal/lb., for 12 and 18% forage NDF, respectively) likely affected production as intakes were similar between diets. However, contrary to the University of Georgia experiment, in this study, no differences were observed in milk fat with both diets yielding 3.4%.