The objective of intensively managed grazing is to have high quality forage available to cows through the entire growing season. The nutritional value of forages (forage quality) decreases as plants mature. The concentrations of digestible energy, protein, and vitamins are substantially less in mature plants than in immature plants. For example, cool season grasses harvested in the vegetative stage may contain 18 to 20% crude protein (CP) and .72 to .78 Mcal net energy for lactation (NEL)/lb. Grasses harvested in the full head stage of maturity usually have 10 to 12% CP and .55 to .60 Mcal NEL/lb. Milk production is directly proportional to the quality of forage the cows are consuming.
In an intensively managed pasture system, a combination of mechanical harvesting and grazing is used to prevent the forage from becoming overly mature. A strip grazing procedure was used in this project to maintain forage quality. Strip grazing is not only the most intensive form of controlled grazing, but it also provides the highest quality forage to animals when done properly with forages in the negative growth stages. Cows graze only the tops of the plants and then are moved to the next strip. The top portion of plants contains a large proportion of highly nutritious, immature leaves.
When high quality forage is provided to cows, the need for concentrate is reduced; however, some concentrate is needed to achieve high levels of milk production. Overfeeding concentrate results in reduced consumption of forage, reduced digestibility of the forage, and usually increased costs. Intensively managed pasture provides an exceptional nutritional base which should be complemented by the concentrate.
The pastures at the Mahoning farm were predominantly cool season grasses (mainly tall fescue). Composition (dry basis) of the pasture (averaged over years and months) was 18% CP,57% neutral detergent fiber (NDF), 30% acid detergent fiber (ADF), and .60 Mcal NEL/lb. Minerals averaged 2.5% potassium, .8% calcium, .35% phosphorous, and .3% magnesium.
Overall, the nutrient profile was about what would be expected for cool season grasses harvested in the vegetative state. Month-to-month and year-to-year variation in composition was substantial. The CP content of the pastures was less than 15% several times over the duration of the project. Milk production generally decreased when cows were consuming poorer quality forage. Because of the variation in nutrient composition, pasture samples should be taken frequently so that the diet can be adjusted appropriately.
Several different concentrate feeding systems were tried during the five-year project. During the first year, Jersey cows were fed concentrate at the rate of 1 lb (0.45 kg) concentrate for every 4 lb (1.8 kg) of 4% fat-corrected milk; Holstein cows were fed at a 1:5 ratio. During the second year of the project, all cows were fed concentrate at a rate of 1 lb (0.45 kg) for every 4 lb (1.8 kg) of actual milk, but all cows were fed at least 10 lb (4.5 kg) of concentrate regardless of milk production. During the third year, the ratio was 1 lb (0.45 kg) of concentrate for every 3.5 lb (1.6 kg) of actual milk with a 10 lb (4.5 kg) minimum. The concentrate feeding strategy changed for the last two years of the project. All cows were fed 17 lb (7.7 kg) of concentrate per day for their entire lactation. The daily concentrate allotment was divided into two equal parts fed at milking.
Because of the severe drought experienced during 1991, supplemental hay had to be provided. The concentrate mix varied over years, but was based on corn grain and soybean meal and was formulated to contain about 15% CP (dry basis) and provide adequate minerals to meet National Research Council (NRC) (1989) recommendations. Strip-grazing usually provides high protein forage to cows; therefore, the concentrate does not need to have a high concentration of CP. Fresh, green forage also contains high concentrations of B-carotene (a vitamin A precursor) and vitamin E. Grazed cows also are outside in the sunshine, so much less supplemental vitamin D needs to be provided. The concentrate must provide adequate amounts of calcium, magnesium, phosphorous, salt, and trace minerals.
The concentrate compositions used for lactating cows and heifers the final year of the project were as follows:
Lactating Cow Feed. This complete feed was used to supplement the pasture feeding program for lactating Holsteins and Jerseys which all freshened in the spring. During pasture season, cows were fed pasture as the only forage as long as conditions permitted. If pasture growth became inadequate, cows were supplemented with alfalfa hay and/or corn silage. After pasture season, cows were fed alfalfa hay and corn silage. The concentrate was pelleted but need not be. If not pelleted, the grain should contain approximately 2.5% liquid molasses as fed to control dust. This feed was ordered in 3 ton lots. Specifications: 100% dry matter basis.
Additional trace minerals met minimum NRC (1989) recommendations.
Supplemental fat sources were limited to soybeans, calcium salts of fatty acids, bleachable fancy tallow, or hydrolyzed animal vegetable blend.
Heifer Feed. Complete feed for replacement heifers from 400-1,300 lbs. Heifers were fed primarily grass hay and possibly some corn silage during the winter. During pasture season, heifers were fed only pasture as long as conditions permitted. This feed was given at the rate of 4-6 lb/day. This feed provided 200 mg of monensin per head/day when fed at 5 lbs. This feed was ordered in 1 ton lots supplied in approximately 100 lb bags. Specifications: 100% dry matter basis.
Additional trace minerals met NRC (1989) recommendations.
Corn silage from the Mahoning Experiment Station was stored in bags near the cow barn for winter feed. About the end of November, as pastures were no longer growing, grazing was reduced to half-days and silage feeding began. About the middle of December, depending on weather conditions, the cows were taken off pasture to be fed in confinement on silage and medium quality hay. The herd was turned dry about December 22 each year. During the dry period, the cows were fed as usual for confinement dry-cow rations.
Feeding concentrate based on milk production, as was done the first three years, was not the ideal supplementation strategy. All cows in the herd calved in early spring and usually reached peak milk production in early May. Correspondingly, when concentrate feeding was based on milk production, maximum concentrate intake occurred at this time. The quality of cool season grasses is highest during the spring and then declines during the hot summer months. Cows were at their peak production, fed large amounts of concentrate, and forage quality was highest at the same time of the year. The large amount of concentrate offered (many cows received 20 to 22 lb per day) during this time probably reduced consumption and digestibility of the forage. On the other hand, concentrate intake was lowest when forage quality also was low (late summer). This resulted in thin cows. The reason for the 10 lb (4.5 kg) minimum for concentrate offered was to prevent losses in body condition during late lactation. Setting a minimum, however, did not eliminate overfeeding concentrate during the spring.
Feeding a fixed amount of concentrate daily throughout lactation resulted in less concentrate being fed during the spring and more being fed during late summer than when concentrate feeding was based on milk production. Seventeen pounds (7.7 kg) was chosen based on forage quality during the previous years and projected milk production. Another consideration in choosing 17 lb (7.7 kg) was that cows were fed concentrate twice daily (during milking). Conditions within the rumen are adversely affected when more than about 8 lb (3.6 kg) of concentrate are consumed during a single meal. When concentrate is fed only twice daily, 17 lb (7.7 kg) is close to the maximum amount of grain that should be fed.
During each lactation, Holstein cows were fed approximately 4,500 lb (2,045 kg) of concentrate during each of the last two years of the project and 4,000 lb (1,818 kg) of concentrate during each of the first three years of the project. Considering the amount of concentrate fed per lactation, and the overall nutritional quality of the forage, milk production was not exceptionally high. Forage species were probably the cause for the relatively low production. Tall fescue was the predominant species of forage consumed. Most unimproved tall fescue pastures contain an endophytic fungus which is related to poor consumption, decreased digestibility, and reduced production. Forage was not tested for the endophyte, but the probability is high that the pastures were infected. Concentrate feeding cannot overcome all the problems associated with the feeding of endophyte infected tall fescue.
1. Nutrient quality of cool season grasses can be maintained with an intensively managed grazing system.
2. Feeding concentrate based on milk production is not the ideal system with a spring calving, intensive grazing system. Rather, a constant feeding rate throughout lactation is favored.
3. A pasture system based predominantly on tall fescue that probably was infected with endophyte is not conducive to high levels of milk production even when substantial amounts of concentrate are fed.