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No-tillage crop production, review and current assessment

S.H. Phillips

University of Kentucky
Lexington, Kentucky

No-tillage is defined as planting crops in previously unprepared soil by opening a narrow slot, trench, or band of sufficient width and depth to obtain seed coverage (1). Several terms are used to identify the system, including Direct Drilling, Zero Tillage, No-Till, Mulch Tillage, Stubble Planting, Eco Fallow, and Direct Seeding.

This relatively new concept has experienced continuous growth over the past 20 years with ever-increasing interest and adoption by farmers worldwide.

The USA recorded 10 million acres of no-tillage crop production in 1982, with an anticipated sharper upturn in 1983 (2), and there was continued growth in Europe, the North and South Americas, and the Asian, Australian, and African continents. It is estimated that by the year 2000, 6.2 million hectares or 45 percent of the total US crop acreage will be grown using the no-tillage system (3).

The rate of adoption by farmers is directly related to the following factors: (i) erosion problems encountered (slope of land and rainfall amount and intensity); (ii) labour availability and cost; (iii) fuel supply and cost relative to chemicals used in the system; (iv) research information to provide cultural production recommendations; (v) acreage or potential of clean, cultivated crops grown; (vi) opportunity for multi-cropping as dictated by climate.

The major advantages of the no-tillage (direct drilling) cropping systems are as follows: (i) reduced soil erosion; (ii) more intensive use of land and use of sloping land for cultivation with reduced erosion and soil degradation under conventional tillage systems; (iii) reduced energy requirement; (iv) improved planting and harvest timing by eliminating plowing and disking and the associated firm soil in no-tillage by eliminating manipulation of the soil; (v) improved soil moisture by reducing moisture losses in soil preparation, reduced runoff and evaporation; (vi) reduced labour; (vii) lower machinery investment.

As with most systems, there are disadvantages inherent with this system:

(i) rodent, insect populations, and disease incidence can be increased with the surface residues; (ii) the management ability of the grower must be better and fewer alternatives to correct problems are available; (iii) soil temperature may decrease as much as 6C at a depth of 2.5 cm in the spring in temperate zones (4). This disadvantage in the temperate zones can be an advantage in the tropical zones where soil temperatures without surface mulches can reach extremes, retarding plant growth and yields. Soil temperature can be reduced by 20-25C on sandy soils under solar radiation experience in Nigeria (5).

The increased use of pesticides in the no-tillage system has raised questions relative to the environmental impact. In countries where pesticides are normally used in conventional systems, only contact herbicides are added to the system. Most pesticides used in corn and soybeans do not move into the environment except through soil movement by erosion. Since soil erosion is vastly reduced in the no-tillage production techniques as compared to conventional systems, less movement from the production field is experienced.

Studies by Triplett et al., in Ohio, support the theory that no-tillage reduces the movement of herbicides from no-tillage fields as compared to conventional tillage (6). Other research shows pesticides are degraded to harmless compounds more rapidly in no-tillage (7).

Reduced Soil Erosion

One of the world's most important problems is reducing soil loss from wind and water erosion. The increasing population with concomitantly higher food and fibre demands is placing great stress on land resources. The most significant advantage in adopting no-tillage is the reduced erosion potential. More land can be used for crop production while reducing the costly problem. Studies have been conducted worldwide comparing the magnitude of soil losses of no-tillage and conventional tillage. McGregor et al. (8) found that on a highly erodible soil in Mississippi erosion was reduced from 17.5 mt/ha in conventional tillage to 1.8 tons/ha using no-tillage. In tropical zones with high intensity rains, losses of more than 200 mt/ha per year is recorded under conventional tillage.

Harrold and Edwards (9), Ohio, compared soil erosion and runoff on watersheds using two systems of conventional practices and no-tillage.

Table 1. Runoff and Sediment Loss from Corn Watershed (1969)

In this study, 17.5 cm of rain fell on the watershed in 7 hours. Soil erosion losses with one rainstorm were 51,477, 7,307 and 72 kilograms of soil per hectare, respectively, for the three tillage methods of plowed and disked straight rows, plowed and disked contour rows, and contour rows no-tillage. Harrold and Edwards also measured soil losses for 5 years of normal rainfall and reported erosion decreased on a 9% slope under no-tillage to 27 kg/ha from 1761 kg/ha conventionally tilled watershed.

In Georgia, Langdale et al. in 1978 (10) on a 6% sloping land found no-tillage with a killed rye cover crop left as surface residue and planted to soybeans without tillage reduced soil losses from 45 mt/ha to 0.1 mt/ha. No-tillage can be an effective system to reduce soil erosion losses.

Crop Yields - Soil Moisture Conditions and Utilization by Crops

An interesting soil moisture relationship has been reported by Blevins et al. (11). In a 4-year period in central Kentucky on a Maury silt loam at 0-15 cm depth during the corn growing season, a 29.5% moisture on a volume basis was recorded for the no-tillage system compared to 24.4% for conventional tillage. This difference was responsible for significantly higher yields. The higher moisture under no-tillage and a much higher level at the surface probably accounts for more efficient phosphorus use as well. Decreased evaporation was recorded at 15.0 cm less water to the atmosphere than did conventional tilled soil.

A comparison of corn grain yields from well-drained soils in Kentucky is shown in Table 2 (12).

Table 2. Average Corn Grain Yields from Well-Drained Soils under the No-Tillage and Conventional Tillage Systems

When 0 or low amounts of nitrogen are applied, no-tillage yields are lower. Delayed application of nitrogen must be made or higher rates of N (28-30 kilograms per ha) applied on no-tillage corn. Soybeans, a nitrogen-fixing crop, will require only potassium and phosphorus, which eliminates this problem area. It is apparent that several problems are involved in nitrogen efficiency in the relatively undisturbed soil in no-tillage production: (i) soil surface residues create larger amounts of organic matter at the surface, contributing to denitrification when the soil contains a surplus of moisture; (ii) increased leaching is caused by less evaporation of water and undisturbed channels through which water and nitrogen move undisturbed into and through the soil profile; (iii) inherent nitrogen in the soil mineralizes more slowly in an undisturbed soil than a tilled soil.

Table 4 presents efficiency data from several years' study with nitrogen, phosphorus and potassium (combined Kentucky and Maryland data (14-16)).

On poorly-drained soils no-tillage yields are reported lower in the northern extremities of the USA (13). Soil temperatures have greater effect than in most regions with soils remaining cold in the early part of the growing season; surface mulches contribute to cooler soil temperatures and slower soil surface drying.

Fertilizer Use and Efficiency in No-Tillage

One of the greatest concerns expressed in the early years of the adoption of no-tillage involved fertilizer use and efficiency. This concern was based on the historical incorporation and mixing of fertilizer in the conventional tillage system. The research relative to fertilizer use was based on this mixing in the surface to 20 cm depth with little information on surface application except in forage production. In the past 20 years, experiments have shown that there is little, if any, difference in potassium availability between the surface and incorporation method of application. Phosphorus application at the surface has an advantage over either banding or incorporation. Because the soil moisture is much higher at the surface, less phosphorus fixation with increased water supply associated with no-tillage improves the diffusion rate of phosphorus to the roots, which heavily proliferate at the 0 to 20-25 cm depth of soil. Kentucky studies by R.E. Phillips have shown fertilizers labelled with phosphorus 32 have more phosphorus available to plants when surface applied under no-tillage conditions than conventional tillage.

Nitrogen fertilizer efficiency presents a different situation from that of phosphorus and potassium. On well-drained soils the amount of corn yield obtained per kilogram of nitrogen was greater for no-tillage, as shown in Table 3.

Table 3. Efficiency of Different Increments of Nitrogen Fertilizer, Expressed as Kilograms of Grain per Kilogram of Nitrogen, on Corn Grown on Well-drained Soils in Kentucky and Maryland

Table 4. Fertilizer Efficiency in No-tillage Corn

Lime requirements remain the same between the two systems. The most acid soil zone will be the surface to 2 cm because of higher organic matter and placement of acid-forming fertilizer materials.

Energy Requirements in No-Tillage

Fuel cost and availability has increased the interest in no-tillage. In 1974, D. Griffith et al., Purdue University, calculated the horsepower and fuel requirements for no-tillage, disking, chisel, and conventional minimum tillage (15).

Table 5. Fuel Use and Horsepower Comparisons of Tillage Systems

These data support no-tillage requirements for smaller tractors and 70% reduced fuel requirements for land preparation on planting as compared to conventional tillage. W. Frye and J. Walker, Kentucky, (1978), report a savings of 36.6 liters of diesel fuel per ha. The extra pesticides recommended for no-tillage above conventional was offset by 2.65 liters additional fuel equivalent. Corn production is reported as 7% less and soybeans 18% in favor of no-tillage. Using these figures it can be calculated that in the year 2000 in the US, with 65% of the corn and soybeans grown no-tillage, the potent4al annual saving would be 14.9 x 1012 Kcal or 1.5 billion liters of diesel fuel equivalent (16).

No-Tillage Multicropping Opportunities

Multicropping may develop as the most important and universally used system developing from no-tillage. The advantages of no-tillage become more important with multicropping, including: (i) reduced labor and cost; (ii) elimination of moisture loss that occurs at planting by soil preparation, which insures quicker germination and stand in the second and third planting in

the cropping sequence; (iii) increased land use by growing two or more crops on the same land; (iv) reduction or near-elimination of soil erosion; (v) improved or maintenance of soil structure and organic matter; (vi) time saved in planting the second or third crop and more timely establishment of these crops; (vii) improved cash flow for farmers by having 2-3 crops for market during the year; (viii) increased use of machinery spreading cost over two or three crops; and (ix) interseeding into pastures without soil preparation.

Maintaining organic matter in the tropics has been a major concern when multicropping is used. No-tillage offers opportunity for continuous cropping while maintaining organic matter. E.S. Roman and M.R. Baker measured the organic matter after seven consecutive corn crops, comparing no-tillage and conventional tillage (17).

Table 6. Organic Matter Levels after Seven Consecutive Maize Crops Comparing No-Tillage and Conventional System on Santo Angelo Soil Series. (Udox soil, USDA Classification,Rio Grande do Sul, Brazil)

In addition to improved organic matter, soil structure was maintained.

The most important multicropping system worldwide is small grain (wheat- barley) followed by soybeans. In the tropical area triple cropping of soybeans-wheat-grain sorghum offers potential. As the climate becomes more temperate, 3 crops in two years (central USA) has become standard. This system involves corn-winter wheat-soybeans over a two-year period and is profitable and dependable for growers.

Multicropping possibilities include:

  • corn-corn
  • corn-grain sorghum
  • grain sorghum-ratoon-grain sorghum
  • wheat-soybeans
  • wheat-corn
  • wheat-millet
  • wheat-grain sorghum
  • wheat-grain sorghum-soybeans (triple cropping)
  • barley-soybeans barley-corn
  • wheat (silage) - corn (silage)
  • winter legume (for nitrogen) - corn
  • wheat-legumes for forage
  • grass-clover wheat-cotton
  • wheat-sugar beets
  • vegetables (2-3 crops)
  • sugar beets-sorghum
  • rice-ratoon-rice
  • green pea-sweet corn
  • annual rye grass (forage) - soybeans
  • winter small grain - interseeded into grass for forage
  • summer annuals - (forages) interseeded into cool season grasses.

Multicropping systems can be generally categorized into groups:

  • Winter-summer double cropping (180-day frost-free period)
    Ex: wheat-soybeans, wheat-sunflower, wheat-grain sorghum
  • Summer-summer double cropping (250-day frost-free period)
    Ex: corn-soybeans, soybeans-soybeans, sorghum-sorghum (ratoon 2nd crop)
  • Winter-summer-summer (wheat-vegetable-cool season vegetables)
  • Tropical - wheat-soybeans-grain sorghum.

Care must be exercised in herbicide selection to prevent injury to following crops in the multicropping system.

Aerial seeding of wheat into standing soybeans, timed with senescence and leaf drop, is used in much of the central and south-eastern USA to reduce time lag in seeding wheat double cropped after soybeans and is an adaptation used in multicropping.

The development of multi-cropping systems will depend largely on economics, climate, and management ability of growers but it does offer a wide range of possibilities with the adoption of no-tillage systems.

Direct Drilling Forage Production

Much of the early research work on direct drilling of crops was directed toward forage production. The introduction of legumes into grass pastures, interseeding of small grain into summer or cool season species, and many other forage species provided information used in no-tillage systems for grain production. T.H. Taylor and W.C. Templeton, Kentucky, have studied the introduction of legumes into grass swards since the early 1950's.

W.C. Templeton lists the advantages of adding legumes to cool season grass swards (18) as follows: (i) the N fixed by legumes is utilized in plant tissue production and, compared to grasses without N fertilizer, legumes and legume-grass mixtures are considerably more productive; (ii) legumes with grasses add a quality factor to the herbage. This is especially true in the case of tall fescue. Legumes are digested more rapidly and, normally, intake of legume or mixed grass-legume herbage is higher than that of grass. The protein content of legumes is, of course, considerably higher than that of unfertilized grass and they are normally higher in calcium; (iii) summer productivity of legumes and legume-base pastures usually exceeds that of cool-season grasses. T.H. Taylor, W.C. Templeton and E.M. Smith, Kentucky, have continued the early 1950 studies and have added the dimension of narrow strip tillage and the use of Paraquat as a contact herbicide to control or retard existing vegetation (19). Table 7 provides legume stand and seedling size as determined by differential of band widths of Paraquat.

Table 7. Part I. Legume Stands and Seedling Sizes under Different Herbicide Treatments 60 Days after Sowing*

* All seed placed below the soil surface. '' Alfalfa and clover data are combined for March trial. Only alfalfa was sown in September. + Means within a column having a common letter do not differ at the 5% level of probability.

Australian research is addressing the forage direct drilling and the findings will be most useful in the future. B.R. Watkin and P.J. Vickery have compared time and method of sowing using legume species on a granite soil in New South Wales (20). Table 8 presents data (Experiment A) indicating sod-seeded white clover, subterranean to be superior to broadcast and disked broadcast methods of sowing.

Table 8. The Effect of Method of Sowing on the Yield of Two Legumes December 1962 (Experiment A)

Data from the USA and Australia support the need to suppress existing vegetation by herbicides, grazing, and other cultural practices or combinations and to place seed into tilled area with proper depth and proper seed to soil contact.

When the vast acreage under forage production and the impact on animal production are considered, direct drilling of pastures should occupy the highest priority. Generally, the deterrent to adoption has been more by economic forces than by technology.

Review of Australian Direct Drilling : Introduction

An intensive study of current research was undertaken in a relative short period of time (June 22 - July 8) with visits to West and South Australia, Queensland and New South Wales. Interaction with Department of Agriculture, CSIRO on a limited basis, Soil Conservation and College research staff provided the base for conclusions reached. Information provided by extension, industry representatives and farmers was an important item relative to assessing the value and adequacy of research in addressing needs at the producer level.

The reviewer wishes to recognise the research, extension, college staff, industry representatives and farmers who gave freely of their time, shared data and objective comments on the tillage research work in Australia.

The review attempts to evaluate the current research, changes occurring presently, and to reconstruct the problems and research activities in the late 1960's and early 1970's, which provide the data base for present and future projects and programmes.

Time restrictions prevent individual institutional reviews and all comments are made on a nation-wide basis. Institutional research review comments would have been very positive as the reviewer was highly impressed with the quality and quantity of research programmes at each institution visited.

World Tillage Observations in Comparison with Australian Research

One excellent criterion for evaluation of research activities is to make comparison with those items impacting on the country's agriculture and the reaction of the scientific community and others influencing the direction of agriculture in terms of addressing these problems.

It is important to note several factors that tend to be worldwide, producing profound changes in agricultural production and the associated pressure on human and natural resources. The major reference continents within the scope of the reviewer's experience include North and South America, Asia, Africa and now Australia.

Perhaps the reduction of rotations, with more acres in grain crops, will produce the most significant change and impact more on use of resources than any other force observed. Farmers are shortening rotations to meet economic pressures, which tend to be more favourable with grain production in all countries, and this was observed in Australia.

It is most fortunate Australian scientists recognised this trend and developed research to offset the land degradation possible with intensive cropping systems moving on lands with erosion hazards, limited yield potential or other agronomic problems. This group has been instrumental in developing a national concern for protecting the land resources which was reflected by farmers in all the Australian States.

The Australian reduced tillage research adoption and, in particular, direct drilling had reduced erosion potential, increased total yields and made significant contributions to protecting productive lands.

The increased cropping has triggered other changes including:

  • movement of livestock geographically;
  • rapid adoption of direct drilling (from 30 to 90 per cent) in Australia;
  • direct drilling is now a permanent system in the Australian agrarian scene;
  • need for additional research for the next phase of direct drilling which will include still less tillage and mulch management;
  • need to improve farmer managerial skill to cope with a complex system of production that affects both the crops and livestock enterprise on his farming unit.

In other words, you, the scientist, have responded well but face new challenges in the future if Australia is to continue its important role in competing in world food and fibre markets.

Comments on Australian Tillage Research Component and Data Base Generated Presently

Australian scientists have been highly prolific in generating a broad data base on tillage in a relative few years. It is unfortunate much of the data and conclusions have not been published in the world journals. Fortunately, within the country many excellent conferences have been conducted to provide a forum for information exchange and discussion. This is highly commendable and more important to the economic improvement of Australia than worldwide distribution of technical information.

In a country as large as Australia with diverse soils, climatic and cropping systems, generalisations can be hazardous. However, in this paper it will be necessary to generalise comments and to recognise the reduced accuracy of conclusions by using this approach. This section is an analysis of the tillage research data base with specific comments relative to the conclusions reached over the four geographical areas visited.

The Australian data are similar to other research from other parts of the world, including such diverse climatical environments as the humid temperate and the humid tropical zones. The similarities of direct drilling research results within Australia and with the other zones include:

  • reduced soil erosion losses from soil by water and wind erosion;
  • improved soil physical condition over time with direct drilling;
  • increased chemical degradation in direct drill;
  • increase in the number of macro- and micro-organisms in the soil;
  • either increased organic matter or decreased organism matter loss;
  • equal or improved crop yield in direct drilling, with higher yield response with summer grains and legumes more nearly equal yields of wheat direct-drilled;
  • decreased soil temperature, a problem in temperate zone yet favourable in warmer climates;
  • increased water infiltration;
  • improved moisture within the soil profile; and
  • electrical conductivity lowered.
  • Minor variances were observed in Australian data relative to fertilizer uptake (phosphorus primarily) and rainfall infiltration, which appears limited, and specific to a farm soil type.

One of the more interesting plant interactions reported in Australia and other countries is related to disease and insects.

The data show higher levels of pathogenic organisms and insects in direct drilling and each researcher's report indicates concern. However, the yields rarely reflect the increased risks as would be expected. Reasons for the inverse relationship between expected loss and actual reduction has not been supported by data at this time in Australia or other countries.

Australian direct drilling research results compared with conventional tillage data is parallel with those from other countries in agronomic factors, which lend greater confidence in interpretation of direct drilling recommendation from areas within and outside the country. The major difference will be in magnitude rather than diverse or contrary conclusions.

A reconstruction of the farmers' problems and research being conducted in the 1960's and 70's would support several conclusions. The early work was tillage system comparisons, simple comparisons of herbicides, fertilizer application, with the end-product to refine farmer recommendations. The wide range of soils and rainfall patterns across the country created an atmosphere for applied type of research. Much to the credit of research staff these demands influenced most of the projects and the geographical soil and climatical differences were addressed.

A new research direction is now apparent in Australia to bring more science into the research programmes. Additional instrumentation is in place to allow for this dimension and for more critical scientific measurements.

Machinery needed to plant direct-drilled crops poses a severe limitation to adoption. The equipment industry should relieve the research institutions of equipment development within the near future. This important industry can react to the problems more efficiently than can research in completing the direct drill linkage from the laboratory and field research to the farmland of Australia.

Recommendations for Future Research

Addressing the future of direct drilling is not possible without a brief look at the past. There is a regional and national need to preserve the oldest plots with no-tillage history. At best, the science of no-tillage is less than 30 years, with the preponderance of work in Australia falling within the 10-20 year period. These longer-term plots and areas will be needed for benchmark information and to measure changes over time.

All new field plots should be examined and benchmark data recorded for measuring effects of tillage treatments.

The following recommendations are similar to those that could be made in any country relative to tillage studies and are not specific or limited to Australia. They include:

  • next generation research should be more science-orientated and provide the answers to what is happening as a result of tillage changes and soil-plant interaction;
  • increase research on the effect of mulch in a direct-drill system;
  • continue to emphasize the multidisciplinary "team" approach;
  • plant breeding and varietal selection research reorientated to direct-drill conditions;
  • evaluation of new crops, which could be a product of flexibility allowed by direct-drilling systems or new systems of husbandry for plant and animal agriculture;
  • continue and enlarge evaluation of multi-cropping potential as a potential with farmers changing to direct-drill farming systems;
  • improved weed science research from biological and environmental view-point;
  • develop direct-drilled computer models to refine and predict inter-actions inherent with the system; and
  • continue communication within the scientific community to reduce duplicative research effort.

In summary, Australian tillage research has been productive because of the innovative, response, well-trained scientific group. There is evidence of shifts to meet new challenges and problems and to add to the scientific body of knowledge needed to support the dynamic agriculture of Australia.

A rough estimate of returns to the Australian economy from investment in tillage work would support a ratio of $7 to $9 return for each $1 invested in the form of increased crop and livestock production and protection of Australia's valuable land resource.

The future of Australian agriculture is in good hands, with her research, extension staff, farmers and industry co-operating in the future as has been evident in the direct-drilling story to this date in 1982.

Summary

In summary, the science of no-tillage is relatively new, with increasing interest and adoption rate accelerating. The concept combines production methods as old as recorded history and as new as the most recent chemical development for crop production. As the demand for food and fiber increases, the system will allow more acres to be cultivated, yet reducing erosion to acceptable levels. No-tillage acreage increases tend to be higher in geographical areas with high concentration of maize and soybean production. The need for no-tillage is paramount and most important in the geographic regions of the world where combinations of rainfall and cultivation of sloping land occur. No-tillage can be used with a wide variety of crops and allows the opportunity to expand and develop multi-cropping systems. The interest in no-tillage has spread into all countries of the world and is adaptable to all systems of production from small subsistence farmers using hand tools to the larger commercial farmers using tractors and combines. The future of no-tillage will largely be determined by development of technology for different crops and climatic conditions.

References

1. S.H. Phillips and H.M. Young. No-Tillage Farming. Milwaukee, Wisc.: Reiman Associates, 1973.

2. F. Lessiter, Editor, No-Till Farmer, Milwaukee, Wisc. (Personal communication, 1982).

3. US Department of Agriculture, Office of Planning and Evaluation (Governmental Printing Office Publication No. 57-399, Washington, DC, 1975).

4. R.E. Phillips. No-Tillage Research Conference Proceedings. (College of Agriculture, University of Kentucky, 1974).

5. R. Lal. IITA, Ibadan, Nigeria. (Personal communication, 1976).

6. G.B. Triplett, B.J. Conner, W.M. Edwards. Ohio Report on Research and Development. (Ohio Agricultural Research and Development Center, Wooster. September-October 1978, pp 70-73).

7. C.H. Slack, R.L. Blevins, C.E. Rieck. Weed Science 26: 145.

8. K.C. McGregor, I.D. Green, G.E. Gurley. American Society of Agriculture Engineering. Trans. 18: 918 (1975).

9. L.O. Harrold and W.M. Edwards, Jr. Journal of Soil and Water Conservation 27: 30 (1972).

10. G.W. Langdale, A.P. Barnett, J.E. Box, Jr. Proceedings of the First Annual Southeastern No-Till Systems Conference. J.T. Touchton and D.G. Cummings-, Eds. (Ca. Experimut Station Spec. Pub. No. 5, 1978).

11. R.L. Blevins, D. Cook, S.H. Phillips, R.E. Phillips. Agronomy Journal 63: 593 (1971).

12. R.E. Phillips, R.L. Blevins, W.W. Frye, S.H. Phillips. G.W. Thomas. Science (Volume 208, No. 4448: 1108-1113, June 1980).

13. G.B. Triplett, Jr., and D.M. VanDoren. Scientific American 236: 28 (1977).

14. V.A. Bandel, V.P.I., Blacksburg, Va. (Personal communication).

15. D. Griffith. Purdue University Research Report (1974).

16. R.E. Phillips, G.W. Thomas and R.L. Blevins, Eds. No-Tillage Research: Research Reports and Reviews, University of Kentucky (1981).

17. E.S. Roman, M.R. Baker. Anais do II Encontro Nacional de Pesquisa Sobre Conservacao do Solo Passo Fundo, Brazil 3=47 (1978).

18. W.C. Templeton, Jr. Southern Association Agricultural Scientists. Southern American Society of Animal Science, Memphis, Tenn. (1974).

19. T.H. Taylor and W.C. Templeton, Jr. Agronomy Journal (Vol. 61, pp- 761-766 (1969).

20. B.R. Watkin and P.J. Vickery. Australian Journal of Exp. Agriculture and Animal Husbandry: Vol. 5, pp. 23-28 (1965).

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