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In 1999 rainfall in April and May was excellent and temperatures were favorable for very rapid pasture growth. Pasture growth rate and forage yields early in the season actually exceeded the rates and yields measured in 1998. Rainfall for the next three months was less than 40% of the long term norm. As might be expected, based on these conditions, pasture management was a real challenge in 1999. In this research project we have the luxury of moving cattle on and off of these grazing plots on an as- needed basis. In real world pasture management, we would have probably opted to cut quite a bit of the excess spring forage as hay at the beginning of June. Many producers did exactly this and then were faced with almost zero regrowth on hay fields which were expected to provide summer grazing. With the dry weather continuing past mid-September, the opportunity for stockpiling winter forage was very minimal. The silver lining for the cloud of 1999's drought was that most producers had an excellent early hay crop with dry weather in June allowing for timely harvest. Research procedures: This is an abbreviated description of the project protocol. A more detailed description of the project can be found at the following website: http://aes.missouri.edu/fsrc/research/divrpt.stm. Sixteen grass or grass-legume mixtures were established in a completely randomized design with four replications. The grass components were seeded on September 5, 1994 on a prepared seed bed. Grass seed was broadcast and rolled in with a cultipacker. The legume components were frost seeded on March 10, 1995. Red clover and birdsfoot trefoil establishment with frost seeding was very good while alfalfa establishment was very poor. Due to the near absence of alfalfa in the designated plots, the plots identified as TF or SB + alfalfa are being used as 0 N treatments for comparative purposes. Individual plots are 50 X 50 ft with a 10 ft bluegrass alleyway surrounding each plot. Swards were uniformly managed to encourage establishment during the 1995 growing season. Tall fescue and smooth bromegrass monocultures receive 120 lb N/acre annually as three 40 lb applications applied in March, June, and September. For interpretation of all of the figures in this report refer to the reference number beside each pasture mixture.
Tall Fescue Base Smooth Bromegrass Base
1)TF + 120 lb N/acre 9) SB + 120 lb N/acre
2)TF + 0 N/acre 10) SB + 0 N/acre
3)TF + BFT 11) SB + BFT
4)TF + RC 12) SB + RC
5)TF + A +BFT 13) SB + A + BFT
6)TF + A + BFT + RC 14) SB + A + BFT + RC
7)TF + A + BFT + RC + OG + T 15) SB + A + BFT + RC + OG + T
8)TF + SB + A + BFT + RC + OG + T 16) SB + A + BFT + RC + TF + OG
+ T + BB
In both 1998 and 1999, plots were sampled and grazed according to the following protocol. Prior to grazing, six .25m2 quadrats were clipped from each plot. At the same site, sward surface height was measured and the species composition was visually estimated. Plots were grazed with six to eight steers for four to seven hours to remove approximately 50 - 60 % of the forage biomass. Following grazing, the same measurements were made. During the first grazing cycle, all fescue-based plots were grazed first followed by smooth bromegrass-based plots. In subsequent grazing cycles, plots were grazed strictly on an as-needed basis. Total annual forage yield, yield by month, rest period to reach target grazing height, pasture daily growth rate, and forage utilization were all calculated. In 1998 average sward height at turn-in was slightly greater than our target of eight to ten inches while in 1999 the average sward height was slightly less than the target. Results and Discussion: Total annual forage yield in 1999 was about 65% of the 1998 forage yield when averaged across all pastures (Figure 2). Our first assumption might be that the pasture types which provided greater than 65% yield potential could be considered to be lower risk pastures when planning for future drought protection. This could be a safe assumption if our goal is total forage production. As an example, a single-cut hay production system that might have that objective. Total forage yield for the sixteen pastures is shown in Figure 3.
The first observation is that tall fescue receiving 120 lb N/acre was quantitatively the highest yielding pasture as well as the pasture providing the highest percentage of yield in a poor year as in the very good 1998 growing season. Statistically, four other pasture mixtures were not significantly lower yielding than TF + 120 N. The common characteristic of the other higher yielding mixtures is that they all contained birdsfoot trefoil as the primary legume. Red clover contribution to pastures was much lower in 1999 compared to 1998. While this may have been a drought effect, it may also have been a reflection of where we were in the red clover cycle. In stands managed for natural reseeding as this study is, there are generally two years of strong clover production followed by an off year. Stand measurements made in October of 1999 did find a significant presence of young red clover plants in most plots designated as red clover mixtures. Another observation is the very disappointing performance of smooth bromegrass in the dry conditions. While the addition of 120 lb-N/acre to tall fescue produced almost 3500 pounds of additional forage compared to 0 N, 120 lb-N applied to smooth bromegrass produced only 300 pounds more forage than unfertilized bromegrass. Even with the excellent growing conditions of 1998, N fertilization produced less than 1000 pounds of additional forage on smooth bromegrass pastures. Obviously, the economics of fertilization of these two species must be quite different. Smooth bromegrass responds much better to N fertilization in hay situations compared to grazing because so much of the biomass is produced above typical grazing heights. Tall fescue, on the other hand, produces an abundance of dry matter in the lower eight to twelve inches. The economics of N fertilization on grazed tall fescue can be quite good due to this high production in relatively short pastures. Coupled with high stock density which can enhance nitrogen recycling in the system, N fertilization of tall fescue looks quite good.
Adjustment of stocking rates or changing rotations is simplified when forage growth rate is more uniform compared to widely varying growth rates. While the TF + BFT growth rate is far from being uniform, it is more consistent than TF + N pasture. Generally speaking, the fescue-based pastures with higher levels of diversity had lower spring growth rates and peak growth at midsummer. Smooth bromegrass based pastures also tended toward lower spring growth rates than fescue-based pastures and peak growth at midsummer. Many of the pasture mixes actually exhibit a unimodal growth distribution rather than the bimodal curve commonly associated with cool season forages (Figure 5). This figure contrasts a simple mixture and a more complex mixture. The slower spring growth may be due to the higher legume presence in the complex mixtures and lower total grass contribution to yield. In the early part of the grazing season, grasses are able to recover much quicker than legumes. As an example, the fescue + N plots in both 1998 and 1999 were grazed five times in April, May, and June while most of the complex mixtures were grazed only two or three times. A second factor may be greater selective grazing occurring in the more complex mixtures resulting in a patchier sward. In 1999 summer annuals did not play a major role in pasture production as they did in 1998. The high summer yields in complex and bromegrass-based pastures in 1998 was largely attributable to summer annual grasses, primarily crabgrass (Digitaria spp). While the most complex mixture does contain a perennial warm season grass, the amount of big bluestem present is not enough to create the unimodal distribution by itself. Figure 5 illustrates why it is beneficial to have more than one type of pasture on the farm or ranch. There are times of the year when one type of pasture is superior to another. Late summer and early fall is clearly the time when forage supply is most limited. Greater use of warm season grasses can help the late summer forage supply but provides minimal benefit for fall grazing. What a warm season component can contribute to fall forage supply is an opportunity to rest cool season pastures in late summer to allow regrowth on pastures that have been grazed through spring and summer.
Reducing stocking rate at midsummer when forage supply is beginning to decline is another means of increasing fall forage supply. This is one of the great advantages of combined cow/calf - stocker systems. Most of the gain put on by yearling stockers occurs before July 15 in the lower Midwest. Moving heavy stockers off the farm in mid to late July effectively lowers the overall stocking rate on the farm so that spring-summer grazed pastures have the opportunity to recover some fall regrowth. Winter grazing cows on forage stockpiled after the stockers leave the farms then minimizes hay feeding requirements. The mean daily rest period required to reach the target grazing height in 1999 was 45 days compared to 32 days in 1998 (Figure 6). Fescue with no nitrogen or legumes was the only fescue based pasture requiring more than 45 days average rest while all of the smooth bromegrass- based pasture required more than 45 days rest on average. It might be noted that the mean grazing height for most treatments was slightly less than the eight inch target while in 1998 average sward height was just over ten inches. Mean forage availability at initiation of grazing in 1999 was only about 5% less than the same measure in 1998. Forages grown under dry conditions tend to have more dry matter yield per acre- inch than forages grown in wetter conditions. In 1998 the mean dry matter yield per acre-inch for all mixtures was 221 while in 1999 it was 315. We often hear people talk about "washy grass" or that dryland range produces "strong grass". This higher dry matter yield per inch in dry grass may have a lot to do with the nutrient density of forages under different growing conditions. We know that forage intake of grazing ruminants is controlled by the three factors of bite size, number of bites, and time spent grazing. When a grazing animal pulls forage into its mouth, the physical volume of forage can become a limitation. The more dry matter that is present in a given physical volume of forage, the more nutrients the animal consumes with each bite. In another ongoing study at FSRC, steer ADG was greater in 1999 than in the three preceding years. It has been our experience over the years that grazing seasons with summer rainfall at least 20% below average usually produce our best individual animal performance. This response would seem to be related in part to the higher nutrient density of the drier forage. Slower daily growth rates are why rest periods need to be longer for some species and why summer rest periods for cool season forages are typically longer than spring rest periods. Daily growth rates in 1999 were less than 60% what they were in 1998. Only tall fescue + N exceeded 50 lb/acre/day as an average growth rate in 1999. This high average was due to daily growth rate in excess of 100 lb/acre/day early in the season. As in 1998, fescue-based pastures did tend to have higher daily growth rates than brome-based pastures. As with total forage yield, those mixtures containing birdsfoot trefoil were the only ones to exhibit growth rates above the mean of all mixtures. The variance in daily growth rates from early to late season was much greater in 1999 compared to 1998. Because of this greater variance, dry years do require a much tighter rein on grazing management. Increasing level of subdivision is one way to budget forage and provide additional days of rest during a dry year.
The Missouri Agricultural Experiment Station is the research arm of the College of Agriculture, Food and Natural Resources at the University of Missouri-Columbia Site maintained by people at AgEBB | |||||||||||
