Nutritional value of mixed diets in cattle

Abstract

Field trials suggest that the nutritive value of high quality, high moisture feeds may be improved by the addition of a small quantity of roughage.

The nutritional value to steers of two high qualities, high moisture feeds, fresh cut pasture and pressed beet pulp silage (PBPS), with and without various levels of straw was investigated. The general effects of straw substitution on the two diets and possible associative effect on DMD and MRT of DM, PEG and Cr-mordanted straw in the reticulo-rumen were measured in steers.

Barley straw was fed with pasture at 0, 20, 50 and 100% of DMI in a 4x4 Latin Square design over an 80 day period, and with PBPS at 0, 50 and 100% of DMI in a 3x3 Latin Square design over 60 days to rumen fistulated steers. On two selected days, a single meal was offered to steers and from 0 to 12 hours after feeding total rumen contents were baled and weighed on four occasions. At 0 hr, PEG and Cr-mordanted fibre as liquid and solid phase markers were mixed with rumen contents. Rumen digesta was sub-sampled for OM, PEG and Cr analysis at each baling session. In vivo DM and OM digestibility’s were measured from day 10 to 20.

The general effect of adding 10% straw to pasture and PBPS was to reduce DMD by 2% units and 1.8% units respectively. MRT of DM, PEG and Cr in the rumen increased by 1.6, 0.5 and 1.2% for pasture, and by 1.0, -1.2 and 1.0% for PBPS respectively for every 10% increase in straw level in diet.

There was little associative effect on DMD of feeding barley straw with PBPS, but an increase of 1.8% units and 3.1% units over predicted values were measured with 20% and 50% straw DM fed with pasture.

MRT for DM was 1.0 and 10.0% shorter at 20% and 50% straw with pasture respectively, and 6.5% shorter at 50% straw level with PBPS than would have been predicted from results of the diets fed separately. Similarly, shorter retention times for PEG, and Cr were observed when straw was fed with pasture or PBPS. A possible explanation for the positive associative effect on DMD of feeding straw with pasture is that pasture and/or straw digestibility was improved. Because straw DMD was likely to be close to its potential, it was calculated that pasture had to improve by 2.3% units on 20% straw and by 6.2% units on 50% straw fed with pasture to give the DMD values attained on the mixed diets. Had the improvement in digestibility of the mixed diets been due to an improvement in DMD of straw only, the DMD of straw would have to improve 9% units with the 20%, and 6.2% units with the 50% straw supplemented diets.

Total OM mass of the rumen was higher at all times on the straw supplemented diets compared with the 100% pasture diet. This potential source of slow releasing energy in the rumen may allow better utilization of the high levels of ammonia in rumen fluid from pasture degradation. The trend towards shorter retention times of digesta in the rumen when straw is fed with pasture or PBPS, is likely to pass soluble carbohydrates and N into the small intestines, possibly improving ME utilization, and voluntary intake of the diets.