Manure Application Evaluation
Cumulative average quantities of N available to the crop and N harvested in grass silage were estimated after each cutting for the three treatments (Fig. 1) . Soil nitrate levels before each cutting were also plotted (Fig. 1). The amount of manure ammonia N applied per hectare over the growing season increased for the 2x rate, three times treatment and for the 2x rate, four times treatment by approximately 128 and 205 kg of ammonia N ha–1, respectively, and increased N harvested in the grass crop. The amount of N harvested over the growing season increased by approximately 9 kg ha–1 for the 2x rate, three times treatment, and increased by approximately 80 kg ha–1 for the 2x rate, four times treatment. Soil nitrate levels tended to be greater (than the control treatment) by the last cutting for the 2x rate, three times treatment, and were greater during the last three cuttings for the 2x rate, four times treatment. The increase of soil nitrate levels with the extra manure applications indicates that some of the additional N from the manure was not being utilized by the grass.
Forage nitrate N and CP also began to accumulate in the grass for the 2x rate, four times treatment compared with the control (Table 1). At Cuttings 4 and 5, the CP concentrations were 4.2 and 4.6% units greater for the grass from the 2x rate, four times treatment compared with the control (Table 1). Forage nitrate N was 4.5-fold greater in Cutting 4 and 10-fold greater in Cutting 5 than the control plots for the 2x rate, four times treatment (Table 1).
Dry-matter yields tended to increase with additional manure applications even though nitrate N accumulated in the soil (Table 1). Over the entire growing season, there was a 3% increase in yield for the 2x rate, three times treatment (Table 1). The increased yield was even greater (12%) for the 2x rate, four times treatment (Table 1). The greater yield benefit of the 2x rate, four times treatment versus the 2x rate, three times treatment may have resulted from two factors: (i) the increased amount of N and other nutrients added to the crop (an additional 128 kg manure ammonia N ha–1 for the 2x rate, three times treatment and an additional 205 kg manure ammonia N ha–1 for the 2x rate, four times treatment); and (ii) earlier N addition in the growing season when there was a greater potential for additional growth.
Dry-matter yields were measured on 11 Dec. 2001 to evaluate the amount of growth that occurred over the fall after the last cutting. There were 70 d of growth between the last cutting and the grass yield estimates in December. The plots that received extra manure (2x manure, three times = 1.03 Mg dry matter ha–1 and 2x manure, four times = 0.90 Mg dry matter ha–1) during the growing season had greater yields over the fall than the control plot (0.67 dry matter Mg ha–1). These results suggest that there is some growth during the fall, and the plots receiving additional manure during the 2001 growing season achieved greater grass production through the fall. The ability of the grass to grow through the fall indicates that some of the N from the soil is available in the nitrate form even when decreasing soil temperatures limit N mineralization and heavy fall rainfall should have flushed available nitrate away from the root zone.
Quality and Quantity of Old Seeding versus New Seeding Grass Silage
Crude Protein and Nitrate Concentration in Fresh Grass Silage
Cumulative amounts of N harvested in the new-seeding grass fields (Fig. 2) were greater than or equal to N amounts harvested in the old-seeding grass fields (Fig. 1a), despite 8 to 10% smaller yields from the new-seeding fields than the old-seeding fields (Tables 1 and 2). Cumulative N harvested from the new-seeding grass fields increased because grass CP concentrations in the new-seeding grass fields exceeded CP concentrations in the old-seeding grass fields (Tables 1 and 2).
Soil nitrate concentrations were generally greater in the new-seeding grass fields than the old-seeding grass fields (Fig. 1a and 2). This occurred even though N available from manure, commercial fertilizer, and soil organic matter for the new-seeding fields was less than or equal to the old-seeding grass field (Fig. 1a and 2). Forage nitrate N levels were also greater in the new-seeding grass (Table 2) than the old-seeding grass (Tables 1 and 2). Forage nitrate N concentrations tended to increase with successive cuttings through August for the new seeding (Fig. 3) .
Increases in soil and forage nitrate concentrations in the new-seeding fields compared with the old-seeding fields may have resulted from increased N mineralization in the soil due to tilling. Microbial activity in the soil increases when soil is tilled. One of the results of increased microbial activity is increased N in the nitrate form. Therefore, new-seeding grass does not need additional N from commercial fertilizer in fields that have received manure for many years. In addition, nitrate N is the form of N that plants readily take up. Nitrate is not toxic to the plant. However, it can lead to health and production problems for lactating dairy cattle when fed at high levels (especially if there is no adjustment period before feeding forages with high levels of nitrate). Therefore, applying high levels of commercial N fertilizer in the spring on fields that have received manure for many years can increase the risk for nitrate accumulation in the plant during the first few cuttings. This higher nitrate concentration in the grass can be of concern when balancing rations for lactating dairy cattle.
The CP and nitrate N concentrations of fresh grass samples before each cutting are plotted in Fig. 4 . As the CP content increased, forage nitrate N also increased (Fig. 4). New-seeding grass tended to accumulate nitrate N to a greater extent than old-seeding grass (Fig. 4b). From an animal health and production standpoint, it is important to note that as the CP levels in grass increase above 21%, there is potential for more nitrate accumulation in the grass plant. Therefore, it is important to have new-seeding grass silages above 21% CP analyzed for nitrate before balancing dairy cattle diets.
New Seeding versus Old Seeding Evaluation in 2002
In 2002, the new-seeding fields (Fields 2 and 6) were compared with the old-seeding fields (Fields 1a, 1b, 1c, 4, and 5; Table 3). Field 2 was reseeded from grass in 2001 to grass in 2002, and Field 6 was reseeded from corn in 2001 to grass in 2002. In 2002, similar trends were observed for increased N harvested in new- (Field 2) versus old-seeding fields (Table 3) when compared with observations from 2001. This was due to higher CP concentrations in the new-seeding grass than the old-seeding grass (Table 3).
Forage nitrate N concentrations were also greater in the new-seeding grass compared with the old-seeding grass in 2002 when Field 2 had been reseeded from grass in 2001 to grass in 2002 (Table 3). Soil nitrate concentrations also tended to be greater from 8 July through 1 Sept. 2002 in the field that had been grass in 2001 and was reseeded into grass in 2002 (Field 2) than an old-seeding grass field (Fig. 5) . Tilling grass sod in the spring tends to increase N mineralization rates in the soil during the following growing season. The increased levels of nitrate N in the grass and soil in the new-seeding field that had been tilled out of grass sod (Field 2) compared with old-seeding fields (Fields 1a, 1b, 1c, 3, 4, and 5) is probably due to increased organic N mineralization in the soil due to tilling. This observation allows the opportunity to understand N cycling between the manure, soil, and plant, which allows for reduced import of N in the form of commercial fertilizer on fields that have been tilled out of grass sod.
Nutrient Content in Grass Silage
Three silages from the grass harvested in 2001 and 2002 were analyzed for many nutrients important in dairy cattle ration balancing. The three samples consisted of grass silage from an established grass stand (old seeding planted in 2000 and 2001), grass silage from a new seeding (planted in spring 2002), and malfermented grass silage. The new-seeding grass silage had a greater CP concentration (27.1%) than the established stand of grass silage (16.9%; Table 4). However, the other protein fractions (soluble CP and ammonia N) were similar between the new and old seeding (Table 4). High protein in the new seeding can be an indication of high nitrate concentrations in the forage, as was the case in this comparison (Table 4). Acid and neutral detergent fiber and lignin concentrations were lower in the silage of new-seeding grass compared with the old seeding (Table 4). The lower lignin suggests a lower stem to leaf ratio in the new seeding than in the old seeding. Sugar levels (not measured) in the new-seeding silage were inferred to be greater than the old-seeding silage because the lactic acid concentration was greater in the new-seeding grass silage (Table 4). Lactic acid–producing bacteria consume sugar in the forage to make lactic acid as an end product. More sugar present in the grass provides more energy for the lactic acid–producing bacteria.
The malfermented forage showed the classical characteristics of a forage that had undergone a clostridial fermentation. The dry matter content was Table 4), which provides an environment for clostridial bacteria to grow (McDonald et al., 1991). The ash content was also higher than that of the other grass silages (Table 4), which suggests some soil contamination in the forage when it was ensiled. Clostridial bacteria can live in the soil, and therefore the soil may have inoculated the forage with clostridial bacteria. The high pH and ammonia, acetic acid, and butyric acid concentrations (Table 4) and low lactic acid concentration also suggest a clostridial fermentation (McDonald et al., 1991).
Nutrient Content in Lactating Dairy Cow Rations
The differences in nutrient profile of the three grass silages described in the preceding section deserve attention when feeding lactating dairy cattle. If the differences in quality are not accounted for when switching from one grass silage type to another, performance characteristics such as milk production can be affected. In the following section, two scenarios were used to evaluate the grass silages in the Cornell–Penn–Miner (CPM) version of the Cornell Net Carbohydrate Protein System (Fox et al., 1990) ration evaluator. The results from the two scenarios summarize how ration formulation is affected by grass silage quality, and the effect that forage quality can have on lactating dairy cow performance characteristics.
In the first scenario, three forages were entered into the CPM ration evaluator as part of a typical diet fed to high-producing lactating dairy cattle on the dairy farm in this evaluation. The exercise was conducted to evaluate the effect of not adjusting lactating dairy cow diets when grass silage of substantially different nutrient content was replaced in the ration on the effect of dairy cow performance indicators. The grass silage represented 15% of the diet on a dry-matter basis, and the forage to concentrate ratio was 39:61. Other forages included corn silage (approximately 9% of diet dry matter) and alfalfa hay (approximately 14.6% of diet dry matter). The major ingredients in the concentrate mix included flaked corn, beet pulp, corn distillers, soybean meal, whole cottonseed, and canola meal.
The CP concentration of the diet was 1.6 percentage units greater for the diet containing the new-seeding grass silage compared with the diet containing the old seeding of grass silage (Table 5). The greater CP concentration in the diet increased the metabolizable protein and the amount of peptides and ammonia present in the rumen (Table 5). In turn, the efficiency of using metabolizable protein for milk protein synthesis decreased (Table 5). Both diets contained excess protein, but the diet containing the new-seeding grass silage was in greater excess. Microbial protein production and efficiency of microbial protein production were slightly lower for the diet containing the new seeding also (Table 5). Only carbohydrates and products of carbohydrate fermentation provide energy at rates sufficient for growth of most ruminal bacteria. Therefore, the amount of protein derived from bacteria depends primarily on the amount and fermentability of feed carbohydrates. The lower microbial protein synthesis and efficiency (Table 5) may be due to the slightly lower fermentability of carbohydrates in the diet containing the new compared with the old seeding.
The additional CP content of the new-seeding silage diet was in excess of the dairy cow nutritional requirements for CP and was not needed. This was demonstrated by increased urea cost and milk urea N concentrations (Table 5). Urea is formed by animals during metabolism and circulates through the body. It takes energy to convert feed CP to urea. Therefore, when CP is fed in excess of an animal's nutrient requirements it takes additional energy to process the extra feed CP into urea, and the additional urea is excreted in the urine unused by the animal. Predicted milk urea N concentrations were also elevated in the animals fed diets containing new-seeding grass silage (Table 5). This is another indication that CP was overfed, and the increased levels of CP in the new-seeding grass silage were not needed.
The diet containing the malfermented grass silage did not have a CPM nutrient profile that differed greatly from the diet containing the old seeding of grass silage. Microbial CP yield was lower, but that was mainly due to lower fermentable carbohydrates (Table 5). Metabolizable protein allowable milk was also approximately 0.9 kg different between rations (Table 5). However, feeding malfermented grass silage at 15% of the diet dry matter would cause many problems with health and production in the dairy cow (Erdman, 1993, p. 210–219). Thus, visual observation of the forages being fed is important. The clostridial grass silage became suspect by sight and smell. The silage was sent to a laboratory for a silage fermentation profile, which confirmed that the forage had probably undergone a clostridial fermentation.
In the second scenario, three diets were optimized for each of the three grass silages, and all of the nutrient constraints within the CPM optimization program were met. This exercise was conducted to evaluate the feeding of grass silages of different nutritional value on dairy cow production indicators when the diets were balanced to meet the nutrient requirements of animals before being fed. The three diets tended to have similar nutrient profiles and microbial protein production (Table 6). The biggest difference between the three diets was the forage to concentrate ratio. The diet containing the malfermented grass silage required a higher level of concentrate (forage to concentrate ratio = 40:60) compared with the other diets (forage to concentrate ratio = 53:47; Table 6). This suggests that more concentrate had to be fed to compensate for the poor-quality forage.
The amount and type of protein present in grass silage affects protein balance and excretion in the dairy cow. It is important to be aware of the amount of CP as well as the amount of soluble CP, ammonia N, and nitrate N present in the grass silage when balancing a ration. Overfeeding protein leads to excess N excreted in the urine, which has negative environmental implications and represents an energy cost to the animal. When soluble N and ammonia N are overfed there is less efficient use of metabolizable protein for milk synthesis, and when nitrate N is overfed it can be a health risk for the animal (Erdman, 1993, p. 210–219).