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VII. The Relationship between Forage Quality and Forage Yield

VII. The Relationship between Forage Quality and Forage Yield

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66



W. F. RAYMOND



ferent stages of maturity, as given by Minson et al. (1960, 1964) in association with the digestibility data shown in Fig. 1 . The use of in vitro

digestibility techniques has made possible the compilation of yielddigestibility relationships for many different forage materials; these are

most usefully reported in the form used by Green ( I 966) and Aldrich and

Dent (1967) for grasses and legumes, and by Rogers (1967) for arable

forage crops (Fig. 5 ) . From these graphs different forage species and



2



diqestible orqanic matter content



\



75



655500



I



I



I



Ib./ocre YIELD



I



!

I



10,000-



8,000-



I



1

0 20



Aprd



i



3 0 10 2 0 3 0 10 2 0

May



June



FIG. 5. The changes in yield and digestibility of the first growth of S.24 ryegrass during

first growth in the spring. (From Green, 1966.)



varieties can be compared in terms of their yields at a given level of digestibility. Figure 6 shows the yields per acre of different grass varieties

during first spring growth when they all contain 65 percent of digestible

organic matter (Green, 1966). This figure illustrates clearly the higher

yields of ryegrass varieties than of other grass species harvested at the



THE NUTRITIVE VALUE O F FORAGE CROPS



67



same level of digestibility, and the interval of 23 days between the dates

at which the digestibility of the earliest (S.24) and the latest ryegrass

variety (VERTAS HARDGRAZING) falls to 65 percent digestible organic

matter content.

The interpretation of such results is complicated, however, by the

fact that the harvest of a perennial forage, unlike that of an arable crop,

is also a treatment for the following regrowth. A forage harvested a t an

Ib./acre YIELD OF DRY MATTER



I0,OOO



1



eS 321

Q

RvP



s 22



6,000



1



Q

.



QTetronc



OS 152



20

MAY



oPetro



0523



524



0 5.215



Ib



ORcveille



5.53

5.48



30



A



TOII fescue



A



Cockrfmt



4



I?



JUNE



FIG.6. The date at which the content of digestible organic matter in the dry matter falls

to 65 percent during first growth of different grass varieties. (From Green, 1966.)



immature, high digestibility stage will regrow more rapidly than if

harvest is delayed until a more mature stage; as a result, the total yield,

of the first growth plus regrowth, from early or late harvested forage

may differ much less than would be indicated from Fig. 5. In terms of

digestible organic matter yield the difference may be even less. Very few

data are available on the comparative annual yields from different timesequences of harvesting a forage through a complete growing season, yet

such data are essential if harvesting or grazing programs are to be planned

on an objective basis.



68



W. F. RAYMOND



VIII. Forage Breeding for Improved Nutritive Value



The first objectives in a forage breeding program must concern agronomic characters -yield, persistence, disease resistance, growth habit

in relation to planned method of use-and only when these are achieved

does it seem justified to select specifically for improved nutritional

features. But such further selection can already be considered within the

numerous forage materials of high agronomic quality now available.

Until recently the main criteria used in selection for nutritive value

have been those accepted for many years, namely crude protein content

and leaf stem ratio. Crude protein content assumed less importance with

the recognition that it was only casually related to the digestibility and

energy content of forages, but the usefulness of leafstem ratio was

accepted until the more recent studies (Section IV, B) which demonstrated that, in immature plants, stem may in fact be more digestible than

leaf.

The importance of digestibility as a measure of energy value, and also

as an index of voluntary intake characteristics, and the availability of

in vitro techniques for estimating forage digestibility, have emphasized

digestibility as a promising selection parameter (Rogers and Whitmore,

1966). Cooper et al. ( 1962) found considerable differences in digestibility

between individual plants of S.37 cocksfoot, all the plants being at a

young leafy stage. Furthermore, the progeny of crosses between the genotypes of highest digestibility showed a mean value higher than the main

population, with a heritability of 0.5. This early study, showing the possibility of objective selection for high digestibility in herbage plants, has

been confirmed by other workers. Julen and Lager ( 1966) found that the

more digestible plants within a cocksfoot population maintained this

higher digestibility at different growth stages, and that both the leaves

and the stems of some plants were more digestible than in others. The

same authors later reported similar differences in digestibility between

plants of other grass species, as well as with several legumes and fodder

rape, which might form the basis for a breeding program, and Mowat

(1 969) reported similar results from Canada. Knight and Yates (1 968),

however, found no consistent digestibility ranking for cocksfoot plants

grown in a Mediterranean environment in Australia. They concluded that

under these conditions responses to temperature, resistance to rust

diseases, and the observed 4-fold differences in yield between genotypes

were much more important than possible marginal improvements in digestibility. This concept is most important. For if one genotype is of

higher yield than another when both are at the same level of digestibility,



THE NUTRITIVE V A L U E O F FORAGE CROPS



69



then the first genotype is likely to be of higher digestibility when both

are harvested at the same yield-and improved yield is in other respects

a rewarding objective.

Significant genetic variation has been found in other nutritionally important characters of forages, in particular of minerals, including sodium

(de Loose and Baert, 1966). This illustrates an important aspect of the

responsibility of the nutritionist in advising the plant breeder on nutritional objectives, for selection for improved sodium content may well be

irrelevant in comparison with direct sodium supplementation of the animals that are to be fed.

This means that the nutritionist must consider most carefully before

advising the plant breeder to start a selection program. In particular he

must recognize that a breeding program takes many years, so that the

nutritional objectives he defines must be relevant to the likely feeding

systems of the future, rather than to those presently operating (Raymond,

1968). Within the context of the nutritional developments outlined in

other sections, including forage processing, feed combinations, and

mineral and urea supplementation, the impact of selection for improved

digestibility or for higher contents of soluble carbohydrates or minerals

could well be marginal. In particular a more effective use of nonprotein

nitrogen supplements would relieve the breeder of the necessity to ensure

“feeding standard” levels of crude protein in forages.

The most important developments may lie in improving the intake

characteristics of forages, and in eliminating toxic components. Selection for improved palatability within Phalaris tuberosa (Section V, B, 2 )

and for reduced levels of estrogens witin subterranean clover and of

coumarins in sweet clover (Section V, F) are examples of breeding objectives that are unlikely to be made obsolete by other advances in nutritional science.

IX. The Effects of Processing on the Components

of Forage Nutritive Value



The previous sections have considered some of the basic nutritional

features of forage crops. However, many forage crops are cut and

processed before they are fed; it is clear that such processing may at

times markedly modify the nutritive features found in the fresh crop, and

an understanding of these changes is essential to the development of improved processing techniques.

A. DEHYDRATION

The effects of dehydration on the digestibility and voluntary intake of



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