A reduced intake of digestible nutrients, particularly energy, is the principal cause of low milk production from tropical pastures
by T.H. STOBBS and P.A.C. THOMPSON
It is now established that well-fed Bos taurus cattle and their crosses are capable of high milk production in the tropics. Some of the best producers are found in the subtropical and tropical regions of Queensland in Australia and more dairying is carried out in this state than in any other comparable tropical or subtropical region in the world (Hayman and Radcliffe, 1973). However, yields from cows grazing solely on tropical pastures are low. Concentrate supplements are usually expensive and are often required for human consumption in many areas of the tropics. Therefore it is necessary to know the milk production per cow and per hectare that can be achieved from improved tropical pastures, to delineate the factors limiting production and to devise practical management systems to maximize production from these pastures. This article reviews these topics with particular reference to work conducted in Queensland.
T.H. Stobbs is Principal Research Scientist and P.A.C. Thompson is Information Officer at the csiro Division of Tropical Agronomy, St. Lucia, Brisbane, Australia.
Lactation yields of cows grazing tropical pastures
Milk production per cow from unsupplemented improved tropical pasture swards is markedly lower than from temperate swards fed at a similar stage of growth or from cows fed concentrate supplements. This is shown in Table 1, which summarizes the production levels obtained from Jersey cows grazing various tropical pasture swards in experiments in tropical Australia and compares them with production levels achieved from cows grazing temperate pastures and with animals receiving concentrate rations. Liveweight gain data are based on a compilation of production levels obtained in tropical environments.
Although individual cows grazing tropical pastures are capable of fairly high production, maximum daily yields of approximately 9–12 kg of milk per cow at the peak of lactation (or a total of about 100 kg butterfat through the lactation) are all that can be achieved from Jersey cows, even when grazing young, actively growing material. Friesians and other large dairy breeds are, however, capable of higher production because of their greater body size and higher feed intake. But in general the level of milk production per cow is low and below the genetic potential of the animal to produce milk.
“… there is considerable scope for selecting and planting species of higher feeding value…”
Friesian cows grazing setaria-greenleaf desmodium pasture in north Queensland
Prediction of milk production
The prediction of likely levels of milk production from tropical pastures, using estimates of their chemical composition and digestibility as calculated from laboratory and animal-house studies, is not totally satisfactory due to the following reasons:
As a consequence of these factors, actual production levels are usually rather higher than the predicted levels. But even so, the differences in milk production between cows grazing tropical and temperate pastures (and those receiving concentrate supplements) are considerably greater than the differences in beef production under corresponding feeding regimes (Table 1). This suggests that milking cows are more demanding in their nutrient requirements than growing animals. Analyses of rumen fluid from animals grazing tropical pastures show a high proportion of acetic acid, but there is little variation in the proportions of volatile fatty acids when cows graze different tropical pastures. It would thus appear that differences in fermentation cannot account for the apparent differences in beef and dairy production. Preliminary results from experiments comparing production from a range of pasture species using beef and dairy cattle rank feeds in the same order (Stobbs, 1973).
The effect of pasture maturity on milk production is recognized by every dairy farmer and, as far as is practicable, he attempts to feed young material. However, the digestibility of young regrowth rarely exceeds 70 percent. By selective grazing, cows can compensate to some extent for lower quality feed, but a stage is quickly reached at which quality affects production. It has been found that milk production from cows grazing Rhodes grass (Chloris gayana) was the same with three- and five-week regrowths, but milk production from cows grazing Kazungula setaria (Setaria anceps) was higher for the three-week regrowths (Hamilton et al., 1970). It would therefore be desirable to develop pasture species for milk production which maintain reasonable quality to a later stage of maturity, even if this means sacrificing some herbage yield.
Milk production from various tropical species and cultivars
There is considerable variation between tropical pastures in their ability to supply nutrients for milk production when excess quantities of feed at the same stage of growth are provided. For example, three-week regrowth of pangola (Digitaria decumbens) has been shown to produce approximately 10 percent more milk than Rhodes grass, with Kazungula setaria giving an intermediate level of production. A recently conducted experiment (Stobbs, 1973) which compared three-week-old regrowth of four setaria cultivars (Nandi, Kazungula, narok and splendida) with pangola and Kikuyu grass (Pennisetum clandestinum) showed that cows grazing pangola and splendida (and to a lesser extent Kazungula) produced more milk than those grazing Nandi or narok and considerably more than those grazing Kikuyu. Forage crops such as forage sorghum, bulrush millet (Pennisetum typhoides) and white panic (Echinochloa crusgalli var. edulis) provide a considerable bulk of feed, but their quality when fed at semimature stages of growth is no better than perennial tropical pastures (Stobbs, 1975).
Voluntary legume intake
The voluntary intake by animals fed tropical legumes indoors is usually higher than that of tropical grass and hence higher milk production per cow would be expected. Thus, Lablab purpureus is able to maintain a relatively high production in contrast to tropical grasses (Hamilton et al., 1970). Good milk production has been obtained from Leucaena leucocephala (Stobbs, 1972). Trifolium semipilosum-based pastures have proved to be quite outstanding, with milk yields from Jersey cows averaging 16 kg/cow/day over extended periods (Stobbs, 1973). However, lower milk production has been obtained from cows grazing pure stands of siratro (Macroptilium atropurpureum) and greenleaf desmodium (Desmodium intortum) than from nitrogen-fertilized pangola (Stobbs, 1971), possibly due to the difficulty cows have in harvesting the leafy fraction of the legumes. Pure stands of such trailing legumes are obviously not typical of farm practice and better production has been obtained from grass/legume mixtures, the legume being particularly beneficial in the early dry season.
Relatively free of toxins
As a group, the tropical pasture species are relatively free of toxins and other undesirable compounds (Hutton, 1971). The only tropical legumes known to have caused bloat are Lablab purpureus and Trifolium semipilosum and there is little evidence of oestrogenic activity. However, the oxalic acid content of a number of tropical grasses is high, particularly some Setaria spp. and some deaths from oxalate poisoning have been recorded. High oxalate in feed can depress butterfat content of milk. Leucaena leucocephala contains mimosine which reduces cell division and also acts as a depilatory, but with carefully controlled feeding it is not considered a major problem. Minor toxic effects have also been recorded on cattle grazing Kikuyu, and cyanogenic glucosides can cause death on Cynodon plectostachyus and sorghum pastures. Cows grazing some tropical legumes can produce milk with objectionable odours and flavour, the strongest occurring when animals graze Lablab purpureus and Leucaena leucocephala. Fortunately, these taints are lost on pasteurization. Some difficulty has been experienced in making cheese from milk produced when cows graze siratro, but feeding energy supplements overcomes this difficulty.
The poor nutritional quality of most tropical pastures not only results in low milk production per cow but also results in important changes in milk composition. When milk yield is depressed, the percentage of butterfat increases, and the percentage of solids-not-fat — particularly protein and casein — and short-chain fatty acids in milkfat decreases. Although these changes in milk composition are less important in tropical countries where fresh milk is usually the major requirement, such measurements are valuable for selecting herbage plants of high nutritive value.
Production per hectare from tropical pastures
Payne (1963) suggested that it should be possible on “good humid tropical pasture” to maintain five dairy cows per hectare, each producing at least 2720 kg of milk annually. In fact, high-carrying capacities can generally be obtained on tropical pasture swards and the target of 13 600 kg/ha of milk was greatly surpassed by cows grazing heavily fertilized (672 kg N/ha), irrigated pangola pastures (Thurbon et al., 1973) when mean yields of 17 400 kg/ha of milk were produced by Jersey cows and 22 400 kg/ha by Friesian cows, without any supplemental feed. High milk production per hectare has also been obtained from nonirrigated nitrogen-fertilized tropical pastures, particularly at high stocking rates (Colman and Holder, 1968; Caro-Costas and Vincente-Chandler, 1969). However, Swain (1971) expressed doubts as to whether a viable dairy industry could be maintained in the tropics on legume-based pastures because of the low production per cow and per hectare. Although carrying capacities are generally reduced on grass/legume pastures compared with nitrogen-fertilized pastures, milk yields between 6 000 and 8 000 kg/ha have been recorded (Stobbs, 1972; Byford and O'Grady, 1973).
Reasons for low milk production
A reduced intake of digestible nutrients, particularly energy, is the main cause of low production. The protein content of tropical pastures is generally low by temperate standards and the concentration falls rapidly with pasture maturity. Trials in which energy and protein supplements are fed to cows grazing tropical pastures have shown that protein is less limiting than energy (Hamilton et al., 1970). Studies of the composition of milk from cows grazing tropical pasture swards also suggest that the intake of digestible energy is the major factor limiting production.
The low digestibility of tropical pasture species at a relatively early stage of growth is one of the major causes of low milk production. Even temperate species grown in hot humid conditions have been shown to have a lower digestibility (Minson and McLeod, 1970) and therefore have a lower milk production potential.
The quality of dry matter voluntarily eaten by an animal is the most important factor controlling the productive value of a feed. The intake of pasture herbage is largely controlled by bulk in the rumen; the more rapidly the breakdown and digestion of feed proceeds, the faster the rate of passage (Thornton and Minson, 1973). It is therefore understandable that tropical herbages which are relatively low in digestibility have a low voluntary intake. Dairy cows grazing some tropical pasture swards, with low leaf yields and inaccessible leaf, can have difficulty in harvesting sufficient feed to maintain stable production. Grazing times on tropical pastures, particularly when trailing legumes and tall stemmy plants are present, have been shown to be excessively long (10–12 hr/24 hr) compared with an average of 6–7 hr/24 hr on higher quality temperate pastures. Animals are obliged to take small bites from some tropical pasture swards and they increase biting rate in an attempt to increase herbage intake. When cows have difficulty in satisfying their nutrient requirements, grazing is extended into the night; at certain times of the year over 50 percent of total grazing time is recorded between evening and morning milkings. The proportion of night grazing increases with the degree of heat stress. This emphasizes the need for good night-paddocks when grazing tropical pastures and is confirmed by increases in milk production achieved in practice.
How to get the best out of tropical pastures
With few exceptions, tropical pastures provide low to medium quality feeds for milk production and it is important to utilize these feeds to the best advantage. There is considerable scope for selecting and planting species of higher feeding value and, provided that these species are high yielding and persistent, considerable increases in production are possible. There is potential for improving milk production with better grazing management, particularly by grazing less mature herbage.
Table 1. A summary of research findings on temperate and tropical pastures showing differences in digestibility and productivity
| Diet | Dry matter digestibility | Maximum milk production1 | Beef production liveweight gain |
| Percent | Kg/cow/lactation | Kg/day | |
| Tropical pasture | |||
1. Immature | 60–65 | 1 800–2 200 | 0.7–0.9 |
2. Semimature | 50–55 | 1 000–1 400 | 0.4–0.5 |
| Temperate pasture | 70–80 | 3 300–3 800 | 0.9–1.2 |
| Concentrate ration | 80–85 | 4 400–4 900 | 1.2–1.4 |
If increased output of milk from cows is desired, it will be necessary to feed concentrate supplements, although careful consideration of the economics of such feeding will be necessary. Energy supplements such as sorghum or molasses are mainly required, although in some circumstances protein supplements may be necessary. There is a tendency to say that a cow must have a certain amount of feed on a certain day because she is giving a certain amount of milk on that day. Since cows are continually building up body reserves or are milking off condition it is suggested that this process should be exploited in achieving the best use of feeds. Feeding should be considered over the whole lactation because there are periods when high quality feed is required and others when lower quality feed is adequate. There is no doubt that early lactation is the most important phase of the whole cycle: the cow's potential is being established and every attempt should be made to obtain a high peak yield. Tropical pastures are not sufficiently nutritious to meet the cow's energy requirements and it is highly desirable to feed concentrate supplements at this time. The cow's ability to respond to extra feed by giving extra milk falls gradually as the lactation proceeds. Well-managed tropical pastures are capable of reasonable levels of production once peak production has been achieved. Good quality tropical pastures do, however, allow animals to build up body reserves for the next lactation. Some steaming up with concentrates may be necessary for cows in poor condition and gains of 0.4 kg/cow/ day or more are desirable to allow for growth of the foetus.
Jersey cows grazing Lablab purpureus in southeastern Queensland
It can be concluded that cows grazing improved tropical pastures are capable of milk production up to about 2 000 kg per lactation. Despite being fibrous feeds of low digestibility, tropical pastures are capable of producing good liveweight gains and there is considerable scope for integrating such pastures into dairy feeding programmes.
REFERENCES
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Caro-Costas, R. & Vincente-Chandler, J. 1969. Milk production with all grass rations from steep intensively managed tropical pastures. J. Agric. Univ. P. Rico, 53: 251–258.
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