Formulating diets that maximize income over feed costs often depends on the combination of forages and concentrates that best suit the herd’s nutritional needs, based on available forages. Ration formulation is greatly dependent on forage quality. The goal is to find a balance between allowing for adequate energy intake to stimulate maximum milk production while reducing feed costs. When there is an adequate supply of high-quality forages the need can often be reduced for supplemented concentrates.
Concentrates are used in dairy-cow diets because they provide more digestible energy per kilogram than forages. Concentrates are less gut-filling due to smaller particle size and reduced concentrations of neutral detergent fiber. Diets with greater proportions of concentrates provide a more consistent source of digestible carbohydrate compared to forages. A major disadvantage to those diets is they tend to be more expensive than those with greater proportions of home-raised alfalfa, grass or corn silage, and have less fiber than alfalfa or grass.
Similarly challenging is the wide variance in neutral-detergent-fiber content and neutral-detergent-fiber digestibility of forages such as alfalfa and grass. Those levels can vary widely depending on maturity, weather and time of year. Harvesting practices and storage methods – whether storing as hay or silage – also play a role.
Diets that contain greater proportions of concentrates contain smaller levels of neutral detergent fiber and therefore are less likely to limit feed intake caused by rumen fill. Dietary neutral detergent fiber and nonfiber carbohydrates are fermented by microorganisms that produce volatile fatty acids that reduce ruminal pH. Nonfiber carbohydrates are more rapidly and completely digested in the rumen compared to fiber, and therefore reduce rumen pH to a greater extent than neutral detergent fiber.
Another drawback of high-concentrate diets is that an excess of digestible carbohydrate can cause rumen acidosis or subacute rumen acidosis. The microbial digestion of nonfiber carbohydrates leads to the production of volatile fatty acids, an excess of which will cause a decrease in rumen pH. Rumen acidosis has been linked to reduced dry-matter intake and reduced milk-fat production.
Work at the University of Wisconsin-Madison has shown high-quality forages can replace costly supplemental concentrates without hindering milk production. Entitled “Evaluation of high-quality alfalfa silage on lactation performance and assessment of forage fiber digestibility in mid-lactation dairy cow diets,” that work found high-quality forages improve nutrient digestibility, reduce gut fill, and allow greater intake of protein and forage neutral detergent fiber in diets.
An experiment was conducted to determine how much high-quality alfalfa silage could replace concentrate feedstuffs in high-producing dairy-cow diets without reducing milk production. Forty-eight lactating Holstein cows – 24 multiparous and 24 primiparous at 141 plus or minus 22 days in milk – were randomly assigned to four treatments in a randomized complete block design. The experiment had a two-week covariate period followed by an eight-week treatment period. Diets consisted of 40 percent brown-midrib-corn silage, 10 percent conventional alfalfa silage, and either 0 percent, 6 percent, 12 percent or 18 percent high-quality alfalfa silage on a dry-matter basis. Diets were formulated to contain about 30 percent neutral detergent fiber, 26.4 percent starch and 17.4 percent crude protein on a dry-matter basis.
The diet with no high-quality alfalfa silage had a forage-to-concentrate ratio of 50:50. That ratio increased by 6 percent in diets as high-quality alfalfa silage replaced soy hulls as a percent of diet on a dry-matter basis. The high-quality alfalfa silage contained 33 percent amylase-neutral detergent fiber, 26.1 percent crude protein and 10.6 percent ash.
Increasing high-quality alfalfa silage in the diets linearly decreased dry-matter intake – P<0.05 – between the treatment diets. See Figure A. Dry-matter intake for cows on the diet of 18 percent alfalfa silage was about 1.5 kilograms per day less during treatment weeks one and two as cows were acclimating to the high-forage concentration of the diet. From treatment weeks three to eight, cows on the diet of 18 percent alfalfa silage increased their dry-matter intake.
It’s expected that the high portion of forage in the diet initially restricted gut-fill as cows were previously fed reduced-forage diets prior to the start of the study. As high-quality alfalfa silage replaced soy hulls, neutral detergent fiber intake decreased linearly – P<0.001 – across the diets. Increased particle size has been linked to a decrease in intake of neutral detergent fiber as well as dry-matter intake. The better quality of the high-quality alfalfa silage made for lesser gut-fill limitations compared to conventional alfalfa silages due to its reduced neutral-detergent-fiber composition.
Even with a decrease in dry-matter intake across treatments, milk production by diet was unaffected – P>0.10. See Table A. It’s expected the high quality and digestibility of the high-quality alfalfa silage allowed for greater nutrient absorption and utilization to be partitioned toward milk production. Because milk was unaffected, feed efficiency – calculated as energy-corrected milk divided by dry-matter intake – increased linearly from 1.63 to 1.83 when high-quality alfalfa silage increased incrementally in the diets. Milk-fat percentage and yield increased linearly as high-quality alfalfa silage replaced concentrate feedstuffs. See Figure B for the depiction of the increased milk- fat percentage.
Percentage and yield of both milk protein and lactose did not differ among the treatments. Milk-fat percentage and yield, lactose percentage and feed efficiency were greater for primiparous cows than multiparous cows. Digestibility of neutral detergent fiber didn’t change with the inclusion of high-quality alfalfa silage. Feed efficiency increased, suggesting underrepresented body-tissue mobilization by cows on higher-forage diets – or calculation errors associated with feed efficiency or diet digestibility. Substitution of protein and non-forage-fiber feedstuffs to as much as 18 percent of the diet, on a dry-matter basis, with high-quality alfalfa silage did not reduce milk production. It increased milk-fat yield, the milk-fat percentage and feed efficiency.
The research suggests an opportunity to improve income over feed costs by incorporating high-quality forages into high-producing dairy-cow diets while reducing the need for costly supplemental concentrates.
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