While more details about the forage features of the Mahoning Project are reported in Chapters 8 and 9, this Chapter is dedicated to discussion of the grazing principles. Excellent treatises on grazing and fencing can be found in books by Murphy (1991) and Smith et al. (1986). Fundamental to the use of pastures, "intensive" implies high stocking rates of cattle for a short period of time on each plot. Time is defined as ranging from 12 hours to three days. Intensive management calls for rotation of cattle to new areas every 12 to 24 h. If the cattle are left in a single area, generally spacious, more than three days, the management is described as extensive.
For dairy cows in production, forage intake is closely correlated with the volume of milk produced. Sustaining high intake levels of consistently high-quality forage is best accomplished by rapid rotation rates of 12 or 24 hours per plot. With non-lactating stock, more relaxed rotations of up to four days can suffice if daily rotation is difficult to do. Extension bulletins available on grazing include those authored by Michigan scientists Bartlett (1991), Moline and Plummer (1991), and Moline et al. (1991). Another important source of grazing information is a bulletin by Emmick and Fox (1993).
Generally, with regard to forage requirements, a farm in the U.S. Midwest will annually support one cow per acre (.4 hectare). The daily plot allowance is variable with plant growth conditions, but in the spring an acre will feed about 50 cows for 24 h. Cows should go to the pasture in the spring as soon as grasses are about six inches (15 cm) high. The frequency of returning to a plot ranges from 14 to 35 days, depending on the climate.
The agronomic principles obeyed with the rapid rotation depend upon using a high stocking rate to cause the avoidance of selective grazing of mixed forage species and the avoidance of grazing new regrowth of forages before roots have been replenished through photosynthetic processes. The effect achieved is uniform competition of all forages for sunlight, space, nutrients, and moisture, while allowing roots to become stronger and deeper. In the process, the soil is stabilized and moisture retention is increased as root expansion opens up the soil. Meanwhile with grazing, soil organisms, worms, and insects rapidly degrade organic matter and manure for rapid recycling for plant growth. Earthworms amplify the moisture holding capacity and aeration of soil when their tunnels are not disturbed by tillage - another attribute of grazing.
Pastures that are intensively grazed as compared to extensively grazed are generally characterized as having a favorable balance of forage species that are variously responsive to climatic changes. The cool season grasses are predominant in the spring and late fall, while legumes such as Lovers are more active during the summer. Also, due to greater root strength and depth, such pastures are more drought resistant and more resistant to insect and disease damage. These pastures are permanent, lasting decades, as long as they are intensively managed and soil fertility is maintained.
Because the forage growth rate is largely dependent on moisture and temperature, pastures will regenerate at varying rates during the year. Where the number of cattle is relatively constant, the size of the plot offered for grazing at each rotation must be adjusted to most effectively match the stocking density to the expected daily intake of the cows. This is easily done with portable electrified fencing. Polywire was used at the Mahoning Project to govern the 24-hour allowance of pasture by defining pie-shaped plots of pasture at each rotation. A follow-up back fence is needed to prevent regrading of new shoots which begin to appear at about four days.
The yearling heifers were generally grazed behind the cows during the second 24 h. The cows were thus only required to top graze the most select 60% of forage because the heifers would clean up the rest of the standing forage. High levels of milk production can best be sustained when the cows have the best feed (top grazing) which encourages robust appetites for the pasture. The heifers were removed from the cycle when drought conditions limited the amount of pasture growth and when one cow was thought to have Johne's disease which could have been transmitted to heifers. During these times, the yearlings were grazed elsewhere. Post-weaning, baby calves were reared for their first year in a small pasture near the barn where they could be fed supplemental grain.
Bloat control was not utilized. Little alfalfa was available and no incidents of bloat were observed. Those pastures where alfalfa had been overseeded, which were used only during the second year, had an abundance of grassy species to temper the bloating effect of the alfalfa.
Forages have excellent levels of important vitamins while they are in the vegetative state. Cows consuming fresh forages gain the advantage of high intake of such vitamins, particularly vitamins A and E. When forages are machine harvested and preserved, substantial deterioration of vitamins in these forages occurs which leads to expensive supplementation for the herd. Also, the protein supply of fresh forages is retarded by mechanical harvesting and storage processes. In terms of quality factors, the value of the cows' forage ration is thus maximized by grazing.
Mechanical harvesting of forages is expensive in terms of equipment costs. Furthermore, there is an agronomic cost to the soil through compaction when machinery is used. Harvesting losses such as leaf shattering are reduced by grazing. Finally, manure handling costs are reduced substantially in a grazing program since most of the defecation occurs in the fields. This could eliminate the need for a lagoon or other manure storage facilities.
Mechanical harvesting of forages is highly weather dependent. Wet conditions create hazards of greater soil compaction and losses in mass and nutritional quality of crops. Grazing is free of weather constraints during the usual season. Soil compaction by cows during intensive grazing is negligible, even under very wet conditions. The cows are in one plot only a few hours. They are nearly always moving, not trampling in one spot. While walking, they help press seeds into the ground for germination. The grasses are sometimes helped when lightly trodden into the mud, it seems, by helping runners take root. Clovers may be damaged, but they are easily restored by no-till seeding.
Grazing does not totally eliminate the need for mechanical harvesting of forages on a dairy farm. In the spring, plant growth is typically very rapid. Unless more cows can be acquired to increase the stocking density for a few weeks, there will be more feed than the cows can graze. It is important for the grazing plan to not get behind on the forage harvesting such that plants go to maturity. If that happens, feed quality diminishes and the plants do not regrow as rapidly following harvest. Normally, the excess spring pasture must be harvested mechanically at the correct stage of growth to preserve its nutritive value. This preserved feed can be fed during late fall and winter.
A useful analysis of grazing economies in Pennsylvania was reported by Parker et al. (1992), who found the gross margin was $121 per cow higher with intensive grazing compared to a drylot feeding system.
The cows spread manure nutrients as they graze. Thus, very practical matching of harvesting and fertilization is accomplished every grazing rotation. Soil nutrients are replenished as they are taken up by the plants. This is a very natural balance. Soil insects, microbes, and worms are fed in the process.
Ecologically speaking, a dairy grazing operation is nearly perfect. Soil nutrients taken up by plants are consumed by cattle which partition them into milk, manure, and sometimes, growth. The manure is returned to the soil right away. Milk and meat leave the farm. In replacement, the cows will receive purchased concentrates along with bedding and supplementary forages for use during the winter months which were derived elsewhere. The net effect is nearly balanced extraction and replenishment of soil nutrients for the farm over the long term without much commercial fertilizer being used.
Soil pH is the most likely condition requiring management with purchased fertilizer. Soil testing is recommended to monitor other nutrients. Commercial plant growth fertilizers will seldom be needed. However, forage micronutrient deficiencies for the cows will probably need correction either through soil fertilization or in the concentrate ration provided for the cows.
Cows require great volumes of water to produce milk. A common expectation is that a lactating cow will drink about 35 gal (132 L) of water per day during the summer. When cows are grazing, the moisture content of the ration is naturally greater than would be found with drylot rations, but cows still need great amounts of drinking water. It is far better to supply drinking water in the pasture rather than have the cows return to the barn to drink. If they stay in the pasture, they will graze longer. The cows in the Mahoning Project were in the pasture all the time except when they were brought in for milking twice per day. Water was provided for them in the pastures.
A brief exception to the above statement regarding cows being in the
pastures all the time occurred about ten days per year when the heat
was overwhelming. On such days, the cows were allowed to come to the
shade of the barn at about 10:00 a.m., since they were not interested
in grazing at that point. Flies were also more persistent on such
days. Shades were not important in the pasture because the cows were
generally grazing or resting in a dispersed arrangement in the
pastures. It is better to encourage the cows to be dispersed to keep
manure more evenly spread and also to avoid excessive trampling of
plants. Mastitis is less likely when cows are not lying in one spot
which may become contaminated.