University of Minnesota, Department of Animal Science,
1364 Eckles Avenue, St. Paul, Minnesota 55108-6120 U.S.A.
Take Home Message
Forage quality can impact dairy producers through their effects on milk production, feed costs, and cow health. High quality legume/grass forages of 125 relative feed value (RFV) or higher with over 20% of their weight in particle lengths of 3.8 cm or greater are needed in lactating cow diets. Forages of lower quality are consumed slower and in less quantity, and thus provide less nutrients to the animal. Diets containing 21% NDF from high quality forages will return more milk production and reduce off-farm feed costs.
Forages are a major source of nutrients needed for milk production. In addition to protein and energy, forages provide the fiber needed in rations for cud chewing, rumination, and rumen health. In the formulation of diets for dairy cattle, the quality and the amount of forage needed to meet nutrient and fiber requirements must be considered first. Grains, proteins, co-product feeds, and other supplements are added to complement the nutrient contributions from forage. The importance of forages as the foundation of dairy diets was recently illustrated by Lundquist (9) in a feeding pyramid for dairy cattle (Figure 1).
Forages affect dairy rations in two ways: 1) by their contribution of nutrients to the diet, and 2) by their impact on ration costs. The quality of a forage affects its ability to contribute nutrients to the diet. High quality forages are able to supply the most nutrients and will have a larger inclusion rate in diets than lower quality forages. For most dairy producers, the least-cost source of nutrients is high quality forages whether they raise their own or purchase them. Additional feeds may be needed in diets utilizing low quality forages thus raising the cost of the ration.
Figure 1. The feeding pyramid (9).
Legumes, grasses, or mixtures of the two are commonly fed forages. Alfalfa, the most commonly fed legume, is an excellent source of protein, energy, and fiber for dairy cows when the quality is high. Grasses tend to be lower in protein and energy than legumes, but can be a valuable source of nutrients. The focus of this paper is to examine the impact of legume/grass forages on milk production and evaluate the cost of feeding as affected by forage quality.
Forage Quality Legume And Grasses
The quality of legumes and grasses can be assessed by the type and quantity of fibrous material in the plant. Neutral detergent fiber (NDF) is the cell wall material of the plant and is comprised of the hemicellulose, cellulose, and lignin. Increasing levels of NDF in plants and/or diets have been found to limit dry matter intake (DMI). Acid detergent fiber (ADF) is comprised of cellulose and lignin and is closely associated with digestibility. An increase in the indigestible lignin complex in the ADF fraction reduces digestibility of the plant. Therefore, ADF and digestibility are negatively correlated. Protein content of a legume or grass is important, but it does not affect the two most important quality characteristics intake and digestibility. Quality of legumes and grasses is best estimated by their potential DMI and dry matter digestibility (DDM) which are determined by the NDF and ADF fractions, respectively. Both NDF and ADF increase as the plant matures causing a decline in the quality of the forage.
Relative Feed Value. Relative feed value (RFV) is an index that combines the important nutritional factors of digestibility and intake into one number which serves as an easy and effective method to evaluate the quality of legumes and grasses (8). The NDF and ADF analyses are used to calculate the potential DMI (eq. 1) and DDM (eq. 2) which, in turn, are used in the RFV equation (eq. 3). The RFV has no units. It is only used as an index to evaluate legume, grass, or legume-grass mixtures of hay or haylage. The RFV compares the feed value of these items to that of full bloom alfalfa, which has a RFV of 100. Hays ranked by RFV are assigned to a quality standard grade ranging from prime through decreasing quality of grade 1, 2, 3, 4, and 5 (Table 1). Caution is needed when comparing hays across grades as nearly similar RFVs may be placed in different grades (example: 124 = grade 2, 125 = grade 1) while hays within a grade may have a larger RFV difference (example: RFV 125 to 151 = grade 1). Hays or haylages having a RFV within 10 units of each other should be considered equal in RFV.
(eq. 1) DMI (% of body weight) = 120 / NDF % of forage DM
(eq. 2) DDM (%) = 88.9 .779 × ADF % of forage DM
(eq. 3) RFV = (DDM × DMI) / 1.29
Table 1. Quality standard grades of hay as determined by the relative feed value (RFV) equation (8).
Protein Content. Although crude protein (CP) is an important nutrient for milk production, it is not an accurate parameter for defining forage quality. Forages with a high CP content also can have high NDF and ADF content reducing intake and digestibility. For example, nitrogen fertilizer can increase CP content of grasses, but plant growth and fiber development also can be increased, especially during warm wet weather.
Fiber Quality. A more recent measure of forage quality is particle size. Besides providing nutrients, forages must provide for good rumen health and stimulate rumination. Legumes, grasses, or mixtures of legumes and grasses are the best source of the long fibrous feed material needed to stimulate rumination. Forages which have over 20% of their weight in particles over 3.8 cm long should be at least 45% of the total dietary DM.
Effect of Alfalfa/Grass Quality on Milk Production
Forage Fed At A Fixed Amount Of The Dietary DM. Because stage of maturity (or RFV) is an indicator of fiber content of a forage, there may be value in using it for predicting the production of lactating dairy cows. Several studies evaluating the effects of alfalfa/grass maturity on milk production have fed a fixed forage to concentrate ratio to determine differences in hay qualities. Rations with identical amounts of hay, yet differing in maturity, do not feed the same because mature hays are less digestible. With the less digestible forages, fewer nutrients are provided to the animal for milk production or growth. The following studies have evaluated the effects of forage quality on lactation performance when forage was fed at a fixed amount of the diet.
Kawas et al. (7) fed four hays ad libitum which differed in maturity (pre, early, mid or full bloom) with 20, 37, 54, or 71% concentrate (DM basis) to lactating dairy cattle. Stage of maturity and concentrate levels affected DMI. Cows fed pre-bloom alfalfa hay ate more DM across all four concentrate levels than cows fed the three lower quality hays. Within each stage of alfalfa maturity, DMI and DDM increased as concentrate level in the diet increased. Actual and 4% fat corrected milk (FCM) yields were highest for cows fed the pre-bloom alfalfa. Early bloom alfalfa hay diets needed to contain 54% or more concentrate to support a milk yield equivalent to pre-bloom alfalfa hay diets with less than 54% concentrate. Milk yields were lowest on the mid and full bloom alfalfa hay diets, and even addition of concentrate up to 71% of the DM did not increase milk yield to the level observed for pre-bloom alfalfa.
Turnbull et al. (12) fed four hays differing in quality to evaluate RFV and its associated effect on milk production. Hays of 174, 141, 118 and 99 RFV were fed at 45% of the dietary DM. Average daily milk yield decreased (30.3, 29.2, 29.1, and 27.6 kg) with declining hay quality (P < .001). Milk fat percentage was lowest for the 174 RFV hay but 4% FCM was not different among treatments. Cows on the highest quality hay were more persistent throughout lactation. Differences in milk yield and persistency reflect the greater available energy content of higher quality hays as the DMI of cows fed diets containing 174, 141 or 118 RFV hays was equal (3.4% of body weight). The lower quality 99 RFV hay did result in a lower (P < .05) DMI (3.1% of body weight).
Alhadhrami and Huber (1) fed five hays differing in ADF (26-AZ, 26-CA, 28, 32 and 38%) at levels of 35 or 50% of the dietary DM. Relative feed values of the hays were 177, 171, 165, 147, and 112. Cows fed hay of 165 RFV or above averaged 3.0 kg/day more milk than those fed the 147 or 112 RFV hay. Level of concentrate did not affect milk yield. However, cows did respond to the amount of concentrate in the diet as the efficiency of feed utilization (kg milk/kg DMI) was higher for 65% than for 50% concentrate and tended to be higher for cows fed hay at 165 RFV or above.
Using a pre-bloom and 10%-bloom alfalfa (RFV 154 and 120, respectively), DePeters and Smith (4) looked at the effect of hay quality on early lactation performance of cows fed 50% or 30% concentrate. Multiparous cows receiving diets containing 50% concentrate produced more milk than those receiving 30% concentrate for both hay qualities. A similar trend was seen in the primiparous cows although it was not statistically significant. Multiparous cows fed the pre-bloom hay at 50% of the diet showed higher peaks compared to the other three treatment groups. The high quality alfalfa hay increased milk yields a total of 270.5 kg (primiparous cows) and 400.9 kg (multiparous) during the first 14 weeks of lactation. Using the high quality alfalfa with 30% concentrate resulted in milk production comparable to diets containing lower quality alfalfa with 50% concentrate. Fat content of milk was slightly higher in cows fed the lower quality hay (3.50 vs. 3.26%). Multiparous cows fed low quality alfalfa lost 22.1 kg of weight during early lactation compared to .7 kg loss for cows fed 154 RFV hay. A similar trend was seen in the primiparous cows as those fed 120 RFV hay lost 22.7 kg more weight during early lactation than cows fed the 154 RFV hay.
The effect of alfalfa maturity on milk production increases as producing ability of cows increases. Alfalfa silage (early, mid, or late cut) was fed at 55% of the diet DM (11). Cows were assigned to three treatments according to milk production; averages for the three groups were 33.9 kg (high), 27.1 kg (medium), and 20.6 kg (low). Alfalfa maturity had the greatest effect on milk production in the high production group decreasing it 1.4 and 4.1 kg/day for the mid and late cuts, respectively, relative to the early cut. Maturity had no affect on milk production when cows produced less than 25 kg/day. For high producing cows (over 27.3 kg/day) it was concluded that for each day harvest is delayed past mid-bud stage, a reduction of .10 to .15 kg milk/cow/day can be expected when alfalfa silage is fed at 55% of the diet DM.
Feeding Forages For Optimum Fiber Levels In Diets. When feeding different forage qualities, it is necessary to adjust the amount of forage in the diet to obtain the optimal amount of fiber needed for milk production, maximum nutrient intakes, and good rumination. Low quality forages will meet the optimum fiber requirements at a lower percentage of the dietary DM than high quality forages. The following studies have evaluated forage quality in relation to fiber concentration of the diet.
The level of NDF or ADF required in the diet to support the maximum yield of 4% FCM was found to be similar in studies by Kawas et al. (6) whether pre, early, mid, or full bloom hay was fed. However, at the same level of NDF in the diet, milk yield was significantly decreased (P< .05) as forage maturity increased. Milk production decreased 13, 23, and 23% from the pre-bloom alfalfa when early, mid or full bloom hays, respectively, were fed. In addition, fiber components were related to body weight change. During the period of 10 to 26 weeks postpartum, 29% NDF and 20% ADF were the maximum levels possible in diets to sustain milk yields above 35 kg/day and maintain a positive body tissue balance.
Beauchemin (2) varied forage to concentrate ratio to evaluate the effects of dietary NDF concentration and hay quality on the performance of dairy cows. An early (154 RFV) and mid (115 RFV) bloom hay were fed at 31, 34, or 37% NDF of dietary DM. Because of high NDF concentration in the mid-bloom hay, diets using the hay averaged 61% concentrate while those with the early bloom hay averaged 44% concentrate. Therefore, cows fed diets of mid-bloom hay consumed more total energy than those fed early hay. Milk production decreased as dietary NDF increased from 31 to 37% (P < .10). At similar dietary NDF concentrations, hay maturity had no affect on milk production or milk composition. Increasing the concentrate portion of the diet was able to compensate for the lower digestible energy content of hays with higher NDF.
Alfalfa hays were harvested by Kaiser and Combs (5) at the early vegetative (RFV 177), late bud (RFV 105), and full bloom (RFV 103) stage of maturity and fed in diets formulated to contain equal concentrations of NDF (29.4%) and NDF from forage (22.0%). The forage to concentrate ratio of the early vegetative diet was 68:32 compared to 53:47 for the late bud diet and 45:55 for the full bloom diet. No differences in milk production, milk composition, or DMI were observed between treatments when fed to cows in early lactation. Results from this study indicate high quality forages can be incorporated into diets at a greater percentage of the DM than low quality forages without any loss in animal performance.
Cleale and Bull (3) harvested mixed grass and legume silages at early and late maturities to determine the effect of forage maturity in isocaloric and isonitrogenous diets on performance of lactating cows. Because of greater digestibility and protein content, early cut silage was incorporated into the diet at 40% of the DM compared to 22% of the DM for late cut silage. Dry matter intake as a percent of body weight did not differ for the diets. Milk yield, 4% FCM, and milk fat percent did not differ. Concentrate needed in the diet increased by about one percentage unit for each day harvest was delayed past early cutting as defined in this study. This study again shows the addition of concentrate to the diet can compensate for the nutrient and digestibility declines of forages with increasing maturity.
When orchardgrass and alfalfa silage contain similar ADF concentrations, no differences in milk production were observed when fed in diets containing 40 to 60% concentrate (13). Differences in silages were apparent when fed at 80% of the dietary DM as cows fed orchardgrass produced less (P < .05) milk and consumed less DM than those fed 80% alfalfa silage. Dry matter intakes increased (P < .01) as concentrate level increased, and were higher (P < .01) for alfalfa diets than orchardgrass diets.
Combined Studies Legumes/Grasses
Dry matter intake and milk production data from the studies cited above were combined and used to evaluate the effect of forage quality on milk production. Relative feed values were calculated and a quality grade assigned to all forages. The percent of Net Energy-Lactation (Mcals) supplied by the alfalfa/grass in the ration was calculated and plotted against milk production (Figure 2). Feeding prime quality alfalfa/grass forage resulted in the most milk yield when forage was included at 35 to 65% of the total ration net energy. With lower quality forages (grades 1 and 2), more grain supplementation was needed and, hence, less energy from forage were fed to obtain the highest milk productions. The window for maximizing milk production from feeding prime quality forage was narrower (35 to 65% of diet Mcals) than with lower grades of forage (35 to 75% for grade 1 and 25 to 60% for grade 2). This narrower window for prime forages emphasizes the importance of balancing rations. With the high quality forages, fiber is less. Feeding less high quality forage than needed to meet fiber requirements in the diet can lead to acidosis whereas feeding more forages than needed will limit intake of other nutrients and feeds.
Feed Costs And Legume/Grass Quality
Besides directly affecting milk production, forage quality also affects the cost of milk production. As the quality of forage decreases, more protein and energy supplements need to be purchased for the ration. Ration costs are dependent on the price of forages and other ingredients available for feeding. A common method of pricing legume/grass forages in Minnesota is by the equation: $/ton of DM = RFV 25. This equation is based on nine years of data from quality tested hay auctions.
The four rations in Table 2 illustrate cost changes when forage is either set at a fixed amount in the ration, or variable, but balanced to a specified forage-NDF concentration. Alfalfa haylage was held at 40% of the DMI in rations 1 and 2. Rations 3 and 4 were balanced for 22% NDF from forage. All rations were balanced for 38.6 kg of milk and 21.8 kg of DMI. Haylage (50% DM) price was determined from the RFV equation. Corn silage (35% DM) price was set at $30/ton as fed. Other ingredients were priced at their current market values.
When 40% of the DMI is from haylage, the ration containing 120 RFV haylage (Ration 2) was $.11/cow/day lower than Ration 1 with 160 RFV. In this scenario, high quality haylage appears to be over-priced by the RFV pricing equation relative to the other feeds available. However, if haylage, corn silage, and corn are available on the farm, off-farm feed purchases totalled $1.15/cow/day for Ration 1 with 160 RFV haylage compared to $1.58/cow/day for Ration 2. If all costs except haylage are figured, total daily cost for Ration 2 is $2.45 compared to $2.14 for Ration 1. This $.31/day difference equates to 160 RFV forage being worth $32.63 more per ton of DM than 120 RFV forage.
Small differences in cost per cow per day were found in Rations 3 and 4 when NDF from forage (100% of NDF in haylage and corn silage and 50% of the NDF in cottonseed) was held constant at 22% of the DM. Again, off-farm purchases of feeds were $.49/cow/day greater for the 120 RFV haylage ration compared to the 160 RFV haylage ration. Including all costs but the haylage gives daily costs of $2.33 for Ration 3 and $2.82 for Ration 4. At the haylage feeding rate in Ration 4, the difference of $.49/day indicates that the 120 RFV forage should cost $75.40 per ton of DM less than the 160 RFV forage to be an economical buy.
The problem with computer generated paper rations is their lack of biological sensitivity. The impact of lower quality forages in a ration on feed intakes, digestion, and milk production are unknown until the diet is actually fed. The truest evaluation of overall forage quality will be in animal performance and feed costs. Whether high quality forage lowers feed cost per cow per day or feed cost per cwt of milk is dependent on the price of other feeds in the ration and the price assigned to the forage. However, based on the forage quality studies summarized and ration cost estimates, dairy producers raising their own forages will have the most profitable and greatest milk production when they harvest and feed high quality forages.
Table 2. Forage quality and ration costs.1
|Feeds (as fed basis, kg/day)|
|Haylage - 50% DM||17.2||17.2||15.0||11.8|
|Corn silage - 35% DM||7.7||2.7||9.1||9.1|
|Nutrient Composition Ration (DM basis)|
1 Ration balanced for 38.6 kg milk, 612 kg cow, and 21.8 kg DMI.
2 Feed prices ($/ton): 160 RFV haylage ($67.50), 120 RFV haylage ($47.50), corn silage ($30), corn ($89.30), cottonseed ($190), 38% protein ($264), tallow ($490), and mineral ($334.25).
3 Farm feeds include haylage, corn silage and corn. Off-farm feeds are all others.
Eating Behavior And Forage Quality
Eating behavior is affected by forage quality. The more fibrous a forage is, the longer it takes for an animal to consume a given amount. Alhadhrami and Huber (1) found that as hays increased from 26 to 38% ADF, cows tended to eat more meals per day, spent more time eating per day and the time spent eating per meal increased. The amount of hay in the diet also affected eating activities with more time spent eating when hay was included at 50% of the dietary DM compared to 35%. Total DMI per day was not affected by ADF content of the hay or level of concentrate in the diet. In diets containing either mid-bloom or early bloom hay, Beauchemin (2) found the total time spent chewing per day increased and the number of chews per day increased linearly with increasing NDF concentration in the diet. This increase was caused by more time spent eating rather than a change in rumination.
Implications of this feeding behavior may be important in freestall barns where bunk space is inadequate. When forage quality is low, more time will be required to consume feed. In over crowded situations, competition for the feed will restrict the time cows have to eat; thus, milk production will decrease as feed intake declines.
Cows can also sort out fibrous particles. A large increase was found in the ADF and NDF concentration of feed refusals when hays utilized in the diet were greater than 30% ADF compared to higher quality hays of 26 and 28% ADF (1). Because of tendencies to sort out the fibrous particles, feeding low quality, long stem forage separately may result in less than desirable fiber intake. Sorting can result in consumption of rations different from those designed to be fed. Total mixed rations and chopping may alleviate some of the sorting.
Forage quality is important for lactating dairy cows. High quality forages offer greater DMI and digestibility than low quality forages. Their inclusion rate in diets is higher and the contribution of nutrients to the diet is greater than low quality forages. The impact of forage quality on feed costs is variable, but as quality increases, less complementary protein and energy supplementation needs to be purchased.
Differences in forage quality when diets are maintained at a constant percent of forage are readily apparent. Milk production decreases when forage quality declines if a similar forage to concentrate ration is maintained. This concept is important in field application where producers keep the forage at a constant amount in the diet from year to year, regardless of quality.
When diets are balanced on forage-NDF levels, the effect of forage quality is not usually a production response. Good quality forage can be included in the diet at higher levels when compared to low quality forages. As a result, more of the total energy needed for milk production comes from the high quality forage.
In the examples of balanced rations, amount of purchased feeds increased in rations utilizing a lower quality forage. When rations were balanced to include equal amounts of forage, or equal forage-NDF levels, rations containing the 120 RFV hay resulted in higher off-farm feed costs as compared to the 160 RFV haylage. The true animal performance response is not reflected in the computer generated rations.