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Effect of selective consumption on voluntary intake and digestibility of tropical forages
105
or accessing Nutrition Abstracts and Reviews (a sub-file of CAB Abstracts) via a computer terminal linked to one of the U.S. or European on-line suppliers. This bibliography can only be recommended to long-suffering newcomers to the field without ready access to either of the above sources of information. R.I. HOWKER
(Bedford, Gt. Britain)
TROPICAL FORAGES
Effect of selective consumption on voluntary intake and digestibility of Tropical Forages, by G. Zemmelink. Agricultural Research Reports 896. Centre for Agricultural Publishing and Documentation, Wageningen, The Netherlands, 1980, 100 pages, paperback, Dfl. 25.00, ISBN 90-220-0729-4. The nutritional value of a feed is ultimately to be assessed by the animal production it will sustain, and production is determined at least as much by the quantity of the feed the animals will eat as by its composition, especially its digestible energy content. There is a positive relationship between the digestibility and the ad libitum intake of a forage by ruminant animals, but the relationship differs substantially between forages, and their proper evaluation must include studies of both variables. However, what are the ad libitum conditions in which intakes should be measured? This problem has been examined by the author, when in Colombia, with 19 growths of tropical forage from legume species (Stylosanthes, Desmodium and Centrosema) and two species of Brachiaria. A comprehensive review of reports on intake measurements shows that these have rarely been made with 15% excess feed on offer, a standard procedure introduced by K.L. Blaxter and colleagues, even when this was the intention. Another approach, used by D.J. Minson and colleagues, is maintenance of about a 500-g excess of tropical forage over 10 day measurement-periods with sheep; that is a total refusal of 5--7% of total feed offered and not the 50--70% suggested on page 9. This approach minimizes selection by the animals, which is also reduced by chopping the forage. The often jungly pastures of tropical grasses and legumes are usually grazed, not harvested, and the grazing animals select leaf sometimes to the extent that virtually no stem is eaten. One possible way of determining intakes with penned animals, that may be more relevant to grazing conditions, is to separate leaf and stem fractions and measure the digestibility and intake of each fraction. The author has developed another technique for determining dry matter (DM) and digestible DM intakes, in which the feed is given unchopped and the amounts offered differ widely among the test animals. Response curves are derived
106 from regressions of intakes on total amounts offered, not on levels of excess (refusals) as such, which is shown to give biased results. The curves obtained for the various forages intersected, that is their rank order for intake varied with level of excess. The reason was that the stem of some forages was well accepted, and with a small excess of feed the intakes were greater than with other forages where mainly leaf was eaten and stem rejected. In the latter instance, the relatively low intakes reflected a restricted supply of leaf in the material offered, b u t with a large excess the intake ranked relatively high because, though stem was still rejected, there was then an abundant supply of leaf. It is not certain to what extent studies of ingestive behaviour made by this technique will truly define the relative merits of plants under grazing conditions, b u t it will provide a more comprehensive and less arbitrary characterization of the feeding value o f morphologically heterogeneous forages, especially tropical forages, than has been provided by a variety of other procedures. J:L. CORBETT (Armidale, N.S. W., Australia)
or accessing Nutrition Abstracts and Reviews (a sub-file of CAB Abstracts) via a computer terminal linked to one of the U.S. or European on-line suppliers. This bibliography can only be recommended to long-suffering newcomers to the field without ready access to either of the above sources of information. R.I. HOWKER
(Bedford, Gt. Britain)
TROPICAL FORAGES
Effect of selective consumption on voluntary intake and digestibility of Tropical Forages, by G. Zemmelink. Agricultural Research Reports 896. Centre for Agricultural Publishing and Documentation, Wageningen, The Netherlands, 1980, 100 pages, paperback, Dfl. 25.00, ISBN 90-220-0729-4. The nutritional value of a feed is ultimately to be assessed by the animal production it will sustain, and production is determined at least as much by the quantity of the feed the animals will eat as by its composition, especially its digestible energy content. There is a positive relationship between the digestibility and the ad libitum intake of a forage by ruminant animals, but the relationship differs substantially between forages, and their proper evaluation must include studies of both variables. However, what are the ad libitum conditions in which intakes should be measured? This problem has been examined by the author, when in Colombia, with 19 growths of tropical forage from legume species (Stylosanthes, Desmodium and Centrosema) and two species of Brachiaria. A comprehensive review of reports on intake measurements shows that these have rarely been made with 15% excess feed on offer, a standard procedure introduced by K.L. Blaxter and colleagues, even when this was the intention. Another approach, used by D.J. Minson and colleagues, is maintenance of about a 500-g excess of tropical forage over 10 day measurement-periods with sheep; that is a total refusal of 5--7% of total feed offered and not the 50--70% suggested on page 9. This approach minimizes selection by the animals, which is also reduced by chopping the forage. The often jungly pastures of tropical grasses and legumes are usually grazed, not harvested, and the grazing animals select leaf sometimes to the extent that virtually no stem is eaten. One possible way of determining intakes with penned animals, that may be more relevant to grazing conditions, is to separate leaf and stem fractions and measure the digestibility and intake of each fraction. The author has developed another technique for determining dry matter (DM) and digestible DM intakes, in which the feed is given unchopped and the amounts offered differ widely among the test animals. Response curves are derived
106 from regressions of intakes on total amounts offered, not on levels of excess (refusals) as such, which is shown to give biased results. The curves obtained for the various forages intersected, that is their rank order for intake varied with level of excess. The reason was that the stem of some forages was well accepted, and with a small excess of feed the intakes were greater than with other forages where mainly leaf was eaten and stem rejected. In the latter instance, the relatively low intakes reflected a restricted supply of leaf in the material offered, b u t with a large excess the intake ranked relatively high because, though stem was still rejected, there was then an abundant supply of leaf. It is not certain to what extent studies of ingestive behaviour made by this technique will truly define the relative merits of plants under grazing conditions, b u t it will provide a more comprehensive and less arbitrary characterization of the feeding value o f morphologically heterogeneous forages, especially tropical forages, than has been provided by a variety of other procedures. J:L. CORBETT (Armidale, N.S. W., Australia)