Factors Affecting Rate Of Growth In Ruminants

Press, Volume C, Issue 29524, 27 May 1961, Page 6

Factors Affecting Rate Of Growth In Ruminants

In a thought-provoking paper to the Lincoln farmers’ conference last week Dr. A. T. Johns, director of the Plant Chemistry Division of the Department of Scientific and Industrial Research, put forward a probable explanation of why sheep made greater five-weight gains on short-rotation ryegrass, and particularly short-rotation and white clover, than on perennial ryegrass, and also why finely ground foodstuffs have proved superior in stimulating growth in ruminants to similar long bulky material.

In trials which have been carried out during the last five yean on the Massey College sheep farm under the auspices of the sheep husbandry department of the college in collaboration with the Plant Chemistry and Grasslands Divisions of the D.5.1.R., sheep have been grazed on four types of pastures perennial ryegrass, short-rotation ryegrass and each at these species with white clover. From time to time sulphate of ammonia was applied to the pure swards, the annual dressing being about 15cwt„ and all pastures received 3cwt of super and lOcwt of lime annually. Two trials have been conducted on the 16 acre padaock with ewe hoggets being set stocked from May , a minimum carrying capacity of six ewes to the acre. The ewes were mated in the second year and their lambs were reared to the weaning stage. All animals were slaughtered in December of the second year. In both trials differences in the growth rates of the ewes on the various pastures became apparent at an early date and were sustained throughout. After only nine months in the second trial the mean liveweight of animals grazing the short rotation and clover was 251 b greater than for those on perennial which showed poorest weight gains. The first trial data for 78 ewes and 55 lambs showed that lambs on short-rotation and clover gained 0.471 b a day over the period, on perennial and clover 0.401 b, on short-rotation 0.381 b, and on perennial ryegrass 0.301 b a day. There was a difference of )331b between the average live-weight of ewes on short-rota-tion and clover compared with those on perennial and a margin of more than 151 b in carcase weight. In the second trial using figures for 60 animals the same trends were maintained with live-weights of ewes on short-rotation and clover being 401 b greater than on perennial alone after 18 months, and there being a difference of 281 b in carcase weights. In each trial perennial and white clover and Eure short-rotation gave simiir results. In his paper Dr. Johns advanced an explanation for this result. The stomach of the ruminant is divided into four compartments, the first two of which form a large fermentation vat where conditions are ideal for microbial digestion of food. It is known that any pasture will supply the cow or sheep with carbohydrates ranging from the simplest sugars to the complex fibrous celluloses. These are broken down by the micro-organisms to the same limited end products, although at rates which differ greatly with different carbohydrates. With diets of hay, which is relatively high in fibrous cellulose, the mixture of acids found in the rumen usually consist of about 70 per cent, acetic, 17 per cent, propionic.

10 per cent butyric with 3 per cent, traces of higher acids. On the other hand where diets are rich in soluble sugars or starch the proportion of acetic falls and that of propionic rises. So with a cow eating 21b of hay and 241 b of concentrates the proportions may be 41 per cent, acetic and 39 per cent propionic. With increasing amounts of soluble carbohydrates, as provided in concentrate feeding, the acids also tend to be produced more rapidly in the fermentation process. A feature of the ruminant stomach is that to a marked extent the products of diges-

tion are absorbed directly into the bloodstream—a process facilitated by the papillae on the inner surface of the rumen, and it has been shown that the chemical products of digestion—the volatile fatty acids—exert a much greater influence on the development of these than does the physical form of the food itself. Now it has been found that acetate is used by the animal in the production of fat, and particularly milk fat by the dairy cow, but when the three volatile fatty acids, acetic, propionic and butyric are used for fat synthesis in the fattening animal it has been found that acetic has an efficiency of 33 per cent., proprionic 55 per cent, and butyric 62 per cent. “Greater weight gains have been obtained in growing animals where diets have been manipulated to produce fermentation which gives a relatively high proportion of propionic acid,” said Dr. Johns. Fine Feeds This can be done by feeding a diet relatively high in soluble sugars and low in fibre and also by finely grinding hay. This process is being widely used in the United States where ground hay is made into pellets. The rumen bacteria can more easily ferment the finely divided fibre than they can ferment the unground hay. The result is that not only is there an increase in the rate of digestion of ground hay but the proportion of propionic acid is increased.

In the United States eight steers on a diet of coarsely chopped hay, ground corn and linseed oil meal gained an average of 2.051 b for 56 days. Another eight received the same weights of food per 1001 b body weight, but the hay was ground and pelleted. This group gained 2.551 b daily. Examination of their rumen contents showed a marked fall in acetate and an increase in propionate. In general, said Dr. Johns, pelleting of hay results in an increase in intake over that of unground forage. In a Californian experiment, with pelleted lucerne hay and long hay fed to lambs, the pelleting of hay resulted In an increase of intake from 2.81 b to 3.591 b per day with weight gains of .241 b and •381 b per day respec-

tively. In the rumen. Dr. Johns said, as the food is digested and broken down into a finely divided state it passes on to the third and fourth stomachs. Hence the more rapidly the food is digested the more rapidly it passes on down the digestive tract. Rapidly digested concentrates and finely divided hays pass on more rapidly than the long fibrous hays and with the passage of the food the animal has the desire to eat more. The results of experiments

with sheep showed that the voluntary intake of fodders was related to digestibility and rate at passage of foods, attributes which were hardly consonant with their “acceptability to the palate or taste.” This was, of caurse, a very simplified concept giving the general picture which could be modified by both animal and plant factors. Dr. Johns said that it was unlikely that results would be the same with all animals. There was a very considerable difference in the rate of growth between individual animals in a flock on the same feed and there were also differences in breeds. Returning to the superiority of short-rotation ryegrass and white clover in inducing weight gains at Massey. Dr. Johns noted that the animals which had grown the fastest—those on shortrotation and clover —actually ■had the smallest forestomachs and the highest level of fermentation.

In the case of shortrotation ryegrass and clover not only would there be more acid produced but it would have to be absorbed more rapidly from the smaller organ. Although there was very considerable variation in each group it had been found that the papillae, which provide the absorption area, were in general better developed in the animals on this type of pasture than in those on perennial ryegrass. So far only the individual volatile acids produced in the rumens of animals on the perennial ryegrass and shortrotation plus clover groups have been determined. It was found that the short-rotation and clover group was significantly lower in acetic and significantly higher in both propionic and butyric acids than the perennial ryegrass group.

“It is evident that the short-rotation ryegrass and clover produces, besides a more intense fermentation in the rumen, a mixture of acids that are more efficient in producing weight gain than are those formed from perennial ryegrass,” states Dr. Johns. “From this evidence it appears that vre have the same differences between perennial and short-rotation ryegrass as we have between hay and concentrates or long hay and ground hay. Perennial behaves as a more fibrous plant, more slowly digested, giving a higher proportion of acetate, while short-rotation is more rapidly digested with less of the fermentation products coming from cellulose. Clover with its low fibre and high soluble carbohydrate also increases the intensity of the fermentation.” It seemed probable, he added, that a particular pasture mixture that was ideal for lamb fattening might not be ideal for milk production. In this respect it has now been fully established that diets which bring about a fall in acetate and an increase in propionate in the rumen (diets low in roughage) cause a fall in milk tat production. I