Grasses Gaining Ground For Lactating Dairy Cows

By Jane Fyksen Crops Editor
Thursday, October 23, 2008 7:19 AM CDT

Alfalfa has long been the queen of forages for lactating dairy cows. However, there’s renewed interest in utilizing grass forages in milking-string diets, in large part because of farm nutrient management issues. Longer stand life, their need for nitrogen and more tolerance to manure spreading are elevating grasses to princess status. Whether they take over alfalfa’s throne or not, remains to be seen.

Jim Paulson, University of Minnesota regional Extension dairy educator at Hutchinson, Minn., recently spotlighted this emerging alfalfa versus grass power struggle for acreage within the royal family of dairy forages. While yield and perceived quality is generally lower for grass species compared to legumes, other agronomic factors that allow for greater and more frequent manure applications than on alfalfa, may make grasses more desirable, reported Paulson at this year’s 69th Minnesota Nutrition Conference.

“Grasses may also complement diets with high levels of co-products from the ethanol and food industries better than legumes because grasses are generally moderate to low in crude protein compared to alfalfa and most co-products contain a significant amount of CP,” he states.

With the ethanol competing with cows for corn, co-product feeding will likely become more prominent on dairy farms in the future, likewise more grasses on farmland.

Paulson reviews some results and limitations of previous lactation studies comparing the feeding value of grass and legume forages, and provides a look at nutritional and cell wall differences between grasses and legumes in order to better understand the utilization of these two types of forage by dairy cows.

“Given the complexity of factors that determine the composition and digestibility of forages, blanket statements claiming superiority of alfalfa or any other forage for milk production are impossible,” he states. “High levels of performance have been observed for cows fed either legume or grass forages when included as part of a typical total mixed diet with multiple ingredients.”

He says the standard advice to feed high quality (i.e. low fiber, high digestibility) forages to lactating dairy cows as part of appropriately balanced mixed diets still stands. But that standard operating procedure may be tougher to attain with grasses at present.

“The criteria or parameters on which to formulate diets to optimize milk production from grasses are less well understood than it is for alfalfa or corn silages. Current forage analysis methods are incomplete in identifying the chemical or nutritional components in grasses which alter their digestion and support high milk production,” says Paulson. “Better methods of forage evaluation are needed to discriminate among and between forage species as to their nutritional superiority and quality.” While neutral detergent fiber (NDF) digestibility analysis can offer some information about forage quality, he says this test alone “cannot account for all the relevant sources of variation that determine forage quality.”

So how do cows perform when fed grass versus legume diets?

He reviews six studies comparing alfalfa to one or more of the following: brome grass, orchard grass, tall fescue and perennial ryegrass. He says comparing legumes to grasses in lactation studies is “confounded” by NDF differences between the two species. Grasses generally contain more NDF than legumes. Therefore, when diets are formulated to contain an equal amount of forage dry matter (DM), total dietary NDF concentration will be higher for those containing grasses compared to legumes. Increasing dietary NDF concentration most often has a negative impact on the amount of DM consumed by lactating dairy cows which generally translates into reduced milk production.

The studies Paulson examined all had the grass forage higher in NDF compared to the alfalfa fed within the study, but DM intake of cows and milk production wasn’t always compromised by a higher NDF content in the forage. One study reported the impact of NDF concentration on rumen fill isn’t always consistent as it’s also influenced by the chemical composition and digestibility of the NDF fraction and particle size.

One research project supports this as first cutting of both orchardgrass and fescue had a significantly higher (20 to 25 percent units) NDF digestibility than the first cutting alfalfa (48 percent NDF digestibility), but milk production and DM intake weren’t different between diets containing orchardgrass, fescue or alfalfa. Second cuttings of orchardgrass and fescue were similar in NDF digestibility to alfalfa, and cows fed diets containing these forages were lower in DM intake and milk production than cows fed the alfalfa diet. Similarly, another earlier study reported no difference in milk production or DM intake when comparing orchardgrass with 52 percent NDF to alfalfa with 40 percent NDF, but NDF digestibility of the orchardgrass NDF was 75 percent compared to 49 percent for the alfalfa. (Unfortunately, none of the studies he looked at reported forage particle size.)

“When diets are formulated to contain similar forage NDF concentrations, less forage and more concentrate is required in grass as compared to legume diets. Unless concentrate inclusion reaches levels that depress rumen function, a lower forage-to-concentrate ratio typically will increase milk production. Therefore, milk production on grass diets should generally equal or exceed production on legume diets because of the lowered forage-to-concentrate ratio and generally higher energy (concentrate) intake of the grass-containing diet compared to legume diets when formulated for equal NDF,” Paulson points out.

One 2002 study clearly illustrates how formulating for an equal dietary NDF concentration increases the concentrate amount in grass diets compared to legume diets, but refutes the premise that milk production and DM intake increase with increasing concentrate feeding. In another in 2004, lactation diets containing alfalfa, orchardgrass or tall fescue silage were balanced to provide a similar amount of forage NDF as a percent of body weight (0.95 percent of BW). This resulted in forage inclusion levels of 62, 54, 51, 59 and 48 percent for alfalfa, orchardgrass first and second cutting and tall fescue first and second cutting, respectively. No difference in DM intake or milk yield for cows fed alfalfa, first-cutting orchardgrass or first-cutting tall fescue silage was seen, but cows fed second-cutting grass silages had lower production and DM intake. The second cutting grass diets had the lowest amount of forage in the diet DM and yet cows produced the lowest amount of milk, Paulson mentions.

Two earlier lactation studies directly compared grass and legume forage diets formulated on a forage-to-concentrate ratio basis, ranging from 40:60 to 60:40, and found no significant differences in DM intake or production due to forage type or amount of concentrate in the diet.

Based on all of these studies, Paulson admits “how to formulate grass based diets to optimize lactation performance isn’t well understood.” What is it about grass forages that in some cases result in similar or superior lactation performance compared to legume forages, while at other times performance is considerably depressed when grasses are fed?

He says such deviations in performance are most likely the result of “variation in nutrient content and digestibility as affected by forage maturity, leaf-to-stem ratio, and cell wall structure.” He looks at those differences.

The nutrient composition of grasses and legumes varies depending on many factors such as species, maturity, fertilization and soil fertility, growing environment, and harvesting conditions. The nutrient profile of the legume and grass hay and haylages shown in Table 2 is from analyses conducted by the Dairyland Labs in Arcadia on grass and legume samples submitted in 2006 and 2007.

Paulson briefly describes some of the similarities and differences in fiber, protein, and mineral concentration that generally exist between grass and legume forages at similar stages of maturity.

“Grasses contain higher concentrations of NDF and acid detergent fiber than do legumes. The higher fiber concentrations are found in both the leaf and stem fractions of grasses compared to legumes,” he says, citing research that compared forage species at a similar maturity and reported that leaves of alfalfa and red clover plants (mid-flower) were about 25 percent NDF and stems were 40 to 55 percent NDF. In contrast, leaves and stems of tall fescue, smooth bromegrass, and orchardgrass were approximately 50 and 70 percent NDF, respectively. However, the digestibility of NDF at 48 hours as reported by Dairyland Labs is very similar between legume and grass forages. Because of the higher fiber content of grasses compared to legumes at similar stages of maturity, forage quality indexes (RFV and RFQ) will always be lower for grasses than legumes.

CP concentration of legumes is higher than grasses. The majority of CP in fresh legumes or grasses is true protein with approximately 10 to 15 percent as non-protein nitrogen (NPN). The amount of NPN increases, as a percent of CP, when grasses are heavily fertilized with nitrogen or when either legumes or grasses are fermented (30 to 65 percent of CP). In both hay and haylage, the solubility of protein tends to be higher in legumes than grasses, Paulson reports.

Legumes tend to accumulate more total macro and micro-minerals and ash than grasses. Of the major minerals in forages, legumes contain two to three times the calcium found in grasses, while potassium and phosphorus concentration in legumes is only slightly higher or similar in legumes compared to grasses. Across all forage species, major factors impacting mineral composition include fertilizer application, stage of growth, and environmental conditions.

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