2 July 2013

6 minute read

By introducing such feeds, intakes increased and feed efficiency and income over feed costs were lower than on a high forage/higher starch TMR. Dietary phosphorus contents were also greater than with the high forage TMR because of the byproducts used. There was a suggestion that feeding the highest amount of wheat straw may have increased mobilization of body tissues, so body condition should be watched carefully with these types of rations.

The drought has made for a challenging year, write Mr Chase and Mrs Hall. Feed prices are high and forage may be limited in both quality and quantity. So, what are our options for feeding cows? We know that dairy cows need fermentable and physically effective feeds to provide nutrients and maintain good rumen function.

Common forage substitutessuch as sugar beet pulp or straw provide either very fermentable or very physically effective fiber sources, respectively. In dealing with high feed prices, replacement of purchased corn grain or soybean meal with less expensive by-product feeds could reduce cost of the
ration.

However, a challenge is that formulating rations based largely on forage substitutes and byproducts has not been well explored. Do they need more effective fiber to keep byproduct fiber in the rumen to be digested? We ran an experiment to test some of the possibilities.

The objective of the feeding trial was to evaluate performance of lactating dairy cows offered different combinations of forage substitutes (wheat straw = more effective fiber, and sugar beet pulp = more fermentable) in dietsthat were relatively low in forage and supplemented solely with byproducts (no corn, no soy). Forty-eight late lactation cows, including 8 that were ruminally cannulated were used; cows averaged 1.3 lactations, 71 lb milk, 1442 lb body weight, and 280 days in milk at the start of the trial.

Cows were offered a high forage TMR (covariate diet) in the first 2 weeks of the study, and then switched to 1 of 4 experimental diets for 4 straight weeks of feeding (Table 1). The experimental diets contained only 40 per cent true forage (corn & alfalfa silages), and differed in the amount of chopped wheat straw or sugar beet pulp pellets they contained, ranging from 0 per cent straw + 12 per cent beet pulp to 9 per cent straw + 3 per cent beet pulp. All other ingredients were kept in the same proportions in all diets.

Molasses was included to bind the rations together and reduce sorting. Diets contained monensin. When analyzing the results, the cow responses from the initial 2 week feeding period on a common TMR was used to adjust the responses on the experimental diets so that results took into account the relative change in performance of individual cows. One of the cannulated cows was omitted from the performance evaluations because of low milk production (12 lb / day) and she was used only for rumen and fecal pH measures. 

Results

Compared to the “normal” higher forage/higher starch TMR, the experimental rations contained a lower proportion of NDF from forage (NDF from corn & alfalfa silages, and chopped wheat straw). Ranges of 0.8 to 0.95 of bodyweight as forage NDF have been recommended; the experimental diets were below and above these values. Starch content of the experimental diets was less than half as much as is commonly included in lactating dairy cow diets in the Midwest and Northeast and two-thirds higher in phosphorus (Table 1). 

3.5 per cent FPCM = 3.5 per cent fat- and protein- corrected milk, DMI = dry matter intake, N = nitrogen, SED =
standard error of the difference. Increase and then decrease with increasing wheat straw. Body weight increased on all diets, but we don’t know to what extent this was due to increased gut fill; no major, obvious condition score changes were noted, but body condition was not monitored in this study.

Cows were fed the covariate and forage substitute/high byproduct diets a month apart, so the change in DIM had to be taken into account, but, even so, 3.5 per cent fat- and protein-corrected milk did not differ between them (75.3 vs 76.2 lb, least squares means, P=0.48). Because of the large difference in dry matter intake, the efficiency of production for 3.5 per cent fat- and protein-corrected milk (1.50 vs. 1.28, P<0.01) and milk N/feedN (0.27 vs 0.23, least squares means, P=0.02) were greater for cows on the covariate diet.

Speaking to the influence of effective fiber sources, the amount of time cows spent ruminating
and eating increased as the amount of straw increased (linear increase in both, P<0.01; Figure 1).

Increased rumination/ chewing would deliver more saliva and buffer to the rumen. That affected
ruminal pH (Figure 2), where the pH of the 0 per cent straw diet was lower or tended to be lower than for the other diets (diet effect P<0.01; 0 per cent straw different from other diets, P<0.02). What was strange was that rumen pH did not change between right before and 4 hours after feeding on the experimental diets (P=0.73).

This is different than what we see with our common diets that are higher in starch; rumen pH declines after feeding and usually hits its low point within 6 hours(covariate diets had ruminal pH
averaging 6.3 and 6.0 just before and 4 hours after feeding, respectively).

The low pH noted on the 0 per cent  straw diet would likely slow down but not stop rate of fiber digestion, which would be important for digestion of fibrous byproducts. Fecal pH did not differ by diet (P>0.24) suggesting that ruminal escape of carbohydrates that would ferment appreciably in the hindgut were not an issue or differentamong diets (6.39, 6.52, 6.43, and 6.48 for 0, 3, 6, and 9 per cent straw, respectively; 6.51 for covariate diet).

Income and Feed Costs 

Income over feed costs increased as straw inclusion increased as cows largely maintained production and intakes declined; also, wheat straw is less expensive than sugar beet pulp. Although the 9 per cent straw diet gave a $0.49 advantage over the 6 per cent straw diet, caution is urged; the increase in butterfat per cent and MUN as intake declined on the 9 per cent diet may suggest body condition mobilization greater than with the other diets. That could be a problem if done over an extended period of time.

Verification of what was actually happening with the 9 per cent straw diets is needed to understand whether the diet formulation needs to change to support late lactation production, reproduction, and maintenance or increase in body stores. Although the high forage covariate diet had the greatest ration cost per hundredweight, the milk production similar to cows on the forage substitute/high byproduct diets but at lower intakes combined to give the highest income over feed costs.

Summary
Late lactation cows maintained performance on the low forage, low starch diets based on byproduct feeds. Using up to 6 per cent wheat straw gave good performance without noticeable body weight loss/condition change. If such diets are tried, cows should be carefully observed to assure that body weight is maintained.

The high phosphorus content of the byproducts elevated phosphorus content in the diet; not desirable long term for nutrient management issues and impact on the environment. On the 60 per cent forage “more standard” lactating cow diet, cows gave similar milk production performance, had lower intakes, more efficient production, and better income over feed costs than cows on the experimental diets.

However, the forage substitute/high byproduct diets are viable substitutes to feed lactating dairy cows when other more traditional feeds are in short supply.