Bovine milk yield is related to both intrinsic genetic and extrinsic nutritional and environmental factors. Milk composition is related more to genetic factors but is also linked, in part, to extrinsic ones.
On a short-term basis, the efficiency of nutrient use for milk production is primarily dependent on the milk production level. As milk yield increases, a lower proportion of total feed intake is used for maintenance (a non-productive requirement that is more or less constant) of the cow. A cow producing 12 kg/d of milk is using about 50% of available nutrients for milk synthesis, whereas the corresponding value is 66% when milk yield increases to 22 kg/d.
This point needs however to be reconsidered in the context of developing countries, especially in the hot and humid tropics. In these countries, several factors limit the use of high-yielding dairy cows:
highly digestible forages (and fertilizers used to produce them) are not available;
cereal and other feeds of high nutritive value are not available in excess of what is needed for human or monogastric animal consumption, or are not available at an economic price;
underfed specialised dairy cows decrease their milk production, but not enough to avoid excessive body weight loss, health and reproduction problems and even mortality;
specialised high-yielding dairy breeds are not well adapted to climatic stress, to poor management and to endemic diseases and parasitism;
zebu or crossbred dual-purpose cattle and buffaloes are well adapted to tropical conditions, produce in some cases 1000 – 3000 litres of high-fat milk per lactation and can be used as draught animals (see Preston and Leng, 1987 and Roman-Ponce, 1987, for complete analysis).
The present paper focuses on current knowledge of the physiological aspects of nutrient partitioning in lactating cows. Most data were obtained in high producing dairy cows from temperate countries. Therefore they do not apply directly to most milking cattle that are used in the tropics.
BODY GROWTH AND MAMMOGENESIS IN HEIFERS
Underfeeding of female dairy calves between birth and the first calving will decrease body growth, body reserves and will increase the rate of culling of the cows later (see also paper by J. Ugarte). Underfeeding during the first 4 months is more dangerous because there is little possibility of compensatory growth (see Johnson, 1988, and Troccon and Petit, 1989, for reviews). On the other hand, both underfeeding and overfeeding of heifers between birth and puberty can be detrimental to the milk production of the future cow (Figure 1).
Figure 1. Milk yield in 250 days of first lactation in relation to average daily gain before puberty (from 90 to 325 kg .) and after puberty (from 325 kg to first calving *). (Foldager and Sejrsen, quoted by Johnson, 1988)
The negative effect of overfeeding has been related to an excessive development of mammary adipose tissue at the expense of parenchymal tissue growth, which is allometric during this period. Overfeeding is also known to decrease age at puberty, and thereby the length of the allometric phase of mammary development. These results have also been related to a decrease in somatotropin (BST or growth hormone) secretion during overfeeding and there are data showing that injections of BST can increase the growth of mammary parenchyma in heifers before puberty. There are, however, no data showing that BST treatment of heifers will increase the future milk yield of the cow.
From puberty to first calving, an increase in feeding level is generally favourable to subsequent milk yield as well as to the overall growth and deposition of body reserves in heifers. After calving, primiparous cows are simultaneously growing and producing milk. This could explain why body lipid deposition after lactation peak is lower in primiparous than in multiparous cows (Table 1).
|Peripartum||Lactation peak||Mid lactation||Late lactation Dry period|
|Robelin and Chilliard, 1983||122||41||-||35–122|
|Butler-Hogg et al., 1985||72||40||34–72||144|
|Dilution space data3|
|Chilliard et al., 1984||81–90||51–68||90–96||-|
|Martin and Ehle, 1986||89–123||73||95||-|
|Vérité and Chilliard m||104||72||-||104|
1See Chilliard (1987) for references.
2Dissectible adipose tissues
3Lipids estimated in vivo from body weight and deuterium waterdilution space
m = multiparous cows
p = primiparous cows
PHYSIOLOGICAL ASPECTS OF MAMMARY SECRETION
Mammary secretion is a function of the number of milk secreting cells and the synthetic activity of each cell.
The last phase of mammogenesis (lobulo-alveolar growth) essentially takes place during the second half of pregnancy. It is genetically determined and controlled by oestrogens, progesterone, prolactin, BST and other hormones that are also implicated in the differentiation of the mammary cells into cells that are able to make milk (lactogenesis stage I). The onset of copious milk secretion (lactogenesis stage II) at parturition is due to elevated prolactin and adrenal steroids, simultaneous to progesterone withdrawal (Forsyth, 1983).
After calving, the metabolic activity of secretory cells is dependent on: (1) neuro-endocrine stimuli which are partly linked to suckling or milking. BST seems to be particularly important for the maintenance of lactation (galactopoiesis) in ruminants; (2) availability in arterial blood of nutrients that are used for milk synthesis (in most cases blood flow variations are consequences of tissue metabolic activities, but cardiac output could also be increased by prolactin and BST in non-lactating animals); (3) regular and complete evacuation of alveolar milk, to decrease intra-mammary pressure and to remove inhibitors that are secreted into the milk (feedback inhibition) (Mepham, 1983).
The decrease in milk yield after lactation peak (that determines milk persistency) results primarily from a decrease in the number of secreting cells. There is little knowledge on the possibility of manipulating the number of secretory cells during lactation. During extended lactation in the mouse, a stronger milking stimulus caused by new younger pups was able to increase the longevity of secretory cells, thus maintaining the number of cells at peak values and milk yield at two-thirds of peak values (suggesting that better milk persistency was due to cell number maintenance, whereas their metabolic activity decreased) (Knight et al., 1988).
Milk yield of dairy cows is clearly greater (25–40 %) when they are suckling their calves twice daily than when machine-milked twice daily. This could be due to a decrease of residual milk and/or to a better response of galactopoietic hormones to suckling (see Perez et al., 1985). Interestingly, when dairy cows suckle only during the first two months of lactation, they maintain an increased milk yield (above controls) after weaning, suggesting that the number of secreting cells was increased or that there was a carry-over effect on stimulating mechanisms (Everitt and Phillips, 1971). This can be relevant to simultaneous milking and suckling in dual-purpose herds.
Hemi-mastectomy during lactation is followed by compensatory yield that is partly due to an increase in cell numbers in the remaining gland. During thrice-daily milking in goats, milk secretion was increased in the short-term (hours or days) by removal of chemical feedback inhibitor and increased metabolic activity, and in the long-term (months) by increased cell number (resulting either from increased cell proliferation or from decreased cell death rate). The latter is however in contradiction with results in cows previously milked thrice-daily over 20 weeks, in which the increased yield was not maintained when they returned to twice-daily milking. The same observation can also be made after removal of long-term BST treatment (Knight et al., 1988).
During concurrent pregnancy and lactation, there is a sharp decrease in milk yield during late pregnancy (after about 5 months in the cow), due primarily to increased oestrogen secretion that inhibits milk synthesis (and to some extent to competition for nutrients by the foetus). There is however at the same time a large proliferation of new secretory cells that will produce more milk during the following lactation (Knight et al., 1988). This proliferative phase is probably stimulated by drying-off the animals before the next lactation (Mepham, 1983). Hormonal induction of lactation in goats (without pregnancy) leads to lower milk yield and higher persistency, without change in mammary cell metabolic activities (Chilliard et al., 1986), probably due to lower mammogenesis and better cell maintenance.
Milk persistency between lactation peak and late pregnancy is also related to sustaining the metabolic activity of secretory cells (see above). It will be clearly related to adequacy of feeding (see below) and management (milking or suckling in good conditions) which enables expression of mammary cell secretory potential.
Under favourable conditions, unbred cows produce each month 94% of their yield during the preceding month. During a 5-year lactation, the yield of the 5th year was about 50% of that during the first (Smith, 1959). The same annual yield could be performed either with higher peak yield and lower persistency, or the other way round. Low persistency could be inherited or due to underfeeding or exhaustion of body reserves (see below) or to other unknown mechanisms that compensate for the higher solicitation of the mammary gland at peak yield. Persistency is indeed generally lower in higher producing animals, even if they are well fed (see Broster and Thomas, 1981 and Faverdin et al., 1987).
BODY RESERVES AND LACTATION IN THE DAIRY COW
Body fat at calving comprises 80–120 kg in “normally” fed Holstein x Friesian adult cows. The major part (but not all) of body lipids are stored in adipose tissues and can be lost during prolonged underfeeding. In well fed cows, the body fat that was lost after calving can be deposited again during declining lactation (Table 1).
Body proteins amount to 80–90 kg in Holstein cows, but most of them are structural components of the body and cannot be mobilized without irreversible degradation of the cow's potential. In underfed lactating cows, body protein loss estimated after slaughter or by deuteriated water did not exceed about 15 kg (20% of body protein), half of which was from muscles (Chilliard and Robelin, 1983). Body protein deposition in dry cows during fattening is lower than body protein loss in lactating underfed cows (Chilliard et al., 1987). This could explain why the muscle/bone ratio decreases in old cows (Robelin, unpublished data). A great part of body water variations is linked to body protein variations. Liver glycogen reserves are very limited and used on a short-term (day-to-day) basis.
The extent and duration of body fat and protein mobilization after calving depends on milk potential, feeding level and quality, and initial body condition.
Effect of milk potential:
When milk potential increases (above 20 kg/d at peak yield), cows eat more feed but not enough to meet the mammary needs for nutrients, even with high quality diets fed ad libitum. Consequently, body weight loss (including lipids, water and protein) increases. Energy deficit lasted about 4 and 8 weeks in cows with peak milk yield of 20 and 40 kg/d respectively (Faverdin et al., 1987). However, when comparing different breeds of cows with the same milk yield, it was suggested that body weight loss was lower in dairy breeds (Holstein, Normandy) because of their higher feed intake capacity (Journet, Colleau and Piton, unpublished data). In respiratory chamber experiments with cows receiving 95% of their theoretical ad libitum intake, body energy loss was linearly related to milk production levels between 20 and 45 kg/d (Vermorel, Rémond and Vërité quoted by Chilliard et al., 1983).
In well-fed, high-producing dairy cows (more than 30 kg/d at peak yield), body fat loss was 30–60 kg during the first two months of lactation and body protein loss was 1–8 kg. In low-producing (7 kg/d), well-fed Hereford x Friesian cows, there was no significant body reserve mobilization (review by Chilliard et al., 1987). High producing dairy cows are able to maintain their milk yield only if their calculated protein deficit is lower than 10 kg during the first two months of lactation. This is in keeping with body composition data on body protein losses during ad libitum or restricted feeding (see above).
Effect of feeding level:
The immediate milk output response to metabolisable energy intake is curvilinear and follows the law of diminishing returns (Figure 2). When intake increases above mean theoretical requirements for cow milk potential, the milk yield response is lower and supplemental energy is stored as body reserves. In underfed cows milk yield does not decrease in proportion to energy intake, due to the use of mobilized body reserves for milk synthesis. Curves in Figure 2 show that for fixed (low) amounts of feeds, high-yielding cows (that are more underfed) are more responsive to supplementary feeding than low-yielding ones. It is also probable that at a very low feeding level there is little difference in the actual milk yield of cows with different milk potentials, although further investigations concerning this point are needed.
In Holstein x Friesian cows with the same milk potential (28–29 kg/d) a feed restriction (25% below ad libitum intake) decreased peak milk yield (about-2.5 kg/d) and increased body fat (-12 kg) and protein (-5 kg) losses during the first two months. The same restriction in cows with higher potential (33 kg/d) further decreased yield (-1.0 kg/d) and increased fat (-10 kg) and protein (-7 kg) losses, when compared with restricted cows of lower potential (Chilliard et al., 1983).
Hereford x Friesian cows (11–13 kg/d at peak yield) in good body condition that were fed at maintenance level for 6 weeks after 70 days of lactation produced 7–8 kg of milk per day, and body fat loss was estimated to be 22–23 kg. Body fat could be deposited again, and milk yield maintained, if cows were refed at 2 x maintenance requirements for 6 weeks (Topps et al., quoted by Chilliard, 1987).
Figure 2. The response of low and high yielding cows to an additional input of energy.
When cows are moderately underfed during early lactation, they are able to produce again as much milk as control cows after peak yield, if feed allowances are sufficiently high (Broster and Thomas, 1981 ; Coulon et al., 1987; Table 2), suggesting that mammary potential was not irreversibly altered. More generally, milk persistency is affected by the level of underfeeding after peak yield.
|Body condition at calving:|
Dietary energy after calving1
|Condition score at calving||3.6||3.5||1.4||1.6|
|Dry matter intake (kg/d)||19.8||15.1||18.8||14.7|
|Milk yield (kg/d)||30.4||28.9||28.9||26.0|
|Milk fat content (g/kg)||40.8||43.6||39.7||39.1|
|Milk protein content (g/kg)||32.2||30.4||32.4||31.0|
|Energy balance (Mcal/d)||-0.55||-8.71||+1.21||-|
|Plasma free fatty acids (mM)||0.48||0.94||0.47|
|Body weight change (kg)2||-32||-59||+11||-11|
|Condition score change||-0.2||-0.6||+0.7||0.0|
|Milk yield (kg/d over weeks 19–44)3||15.6||16.6||16.2||14.7|
Data are from 51 cows (11 to 14 per group) between weeks 1 to 8 of lactation.
1 Protein (PDI, protein digestible in the intestine) concentration wasincreased in“low”energy diet in order to achieve similar PDIintakes in “high” and “low”groups.
2Corrected for differences in dry matter intake.
345 cows only (8 to 14 per group)
Effect of body condition at calving:
Body condition scoring allows the assessment of subcutaneous fat variations by manual palpation of the tail head and the loin areas. With a 0–5 scale, in 49 Holstein x Friesian slaughtered cows an increase of one unit of condition score was equivalent to 35 kg of body weight (r=0.69) and 28 kg of body lipids (r=0.85) (Rémond et al., 1988).
Body condition at calving is the result of body reserve mobilization and deposition cycles during the life of the cow, and more particularly during the previous lactation and dry periods. Increasing the level of feeding before calving generally increases subsequent milk yield (Broster and Thomas, 1981). This could be due to better mammogenesis and lactogenesis during the last weeks of pregnancy and the first days of lactation, to better digestive adaptation, as well as to better body condition (availability of body reserves) of the cow.
In well fed cows, body condition at calving has little effect on milk production, if the condition score is above 2. Fat cows generally have lower voluntary feed intake but produce the same amount of milk, due to body lipid (and probably protein) mobilization (review by Garnsworthy, 1988). These cows are however more susceptible to metabolic disorders (see following sections) and reproductive problems.
Differences between fat and lean cows are more pronounced during underfeeding. Underfed fat cows maintain their milk and fat yields due to a very high body lipid mobilization (Table 2). The decrease in milk protein yield and content is probably related in part to the low ability of body proteins to be mobilized. On the other hand, underfed lean cows still mobilize body reserves but not sufficiently to maintain their yield of milk, fat and proteins. In our trial they presented no higher incidence of health problems. When fed ad libitum from the 9th week, they reproduced normally and produced as much milk as fat cows from the “high”group during the second part of the lactation (Table 2). The excellent performances of lean cows that were well fed with high concentrate diet should also be underlined.
Body fat mobilization is also related to protein feeding. An increased supply of limiting amino-acids to the mammary gland can indeed increase milk yield and therefore the energy requirement of the cows. This can lead either to increased feed intake (with highly digestible forages) or to increased body fat mobilization that will supply fatty acids for fat synthesis and oxidative energy in the mammary gland (Journet and Rémond, 1981). These hypotheses are confirmed by the fact that milk response to an increased protein supply was higher in fat than in lean cows (Garnsworthy, 1988). Peak yield increase could be, however, followed by decreased persistency (Broster and Thomas, 1981).
A knowledge of body stores is very important for understanding the long-term effects of underfeeding during several lactations (see Broster et al.,1984).In wiktorsson's trials, underfeeding during the first year did not affect peak yield but increased body weight loss and decreased persistency. On the contrary, continued underfeeding during the second year decreased peak and total yield, without further body weight loss (due to body reserve exhaustion, cf. Table 2). When cows were refed during the third year, nutrients were directed primarily to body gain and milk yield was not fully returned to control level.
REGULATION OF NUTRIENT PARTITIONING
Both exogenous and endogenous nutrients are used by the mammary gland. They are more available during lactation because of increased intake and digestion, as well as increased endogenous nutrient mobilization, or decreased competition, by other tissues and organs. The liver plays a key role in glucose production. Adipose tissue (muscles) can release or take up fatty (amino) acids, glucose and acetate. Mineral metabolism in the bones and gut is also involved.
During lactation, mammary metabolism is stimulated by galactopoietic hormones, among them somatotropin (BST) plays a central role. BST is also involved in the coordination of extra-mammary metabolism in order to ensure the priority of the mammary gland for nutrients (teleophoresis, see Bauman and Currie, 1980). Knowledge on the mechanisms of BST action has increased very rapidly thanks to the treatment of cows with recombinant BST. Such a treatment rapidly increases milk yield but the feed intake response is delayed for 6-8 weeks. During this period body reserves are mobilized, but can be deposited again after several months of BST treatment in adequately fed cows (review by Chilliard, 1988).
The primary effect of BST is to stimulate the mammary gland, probably via stimulation of somatomedin production. BST also decreases glucose and amino acid oxidation, at the expense of adipose tissue long-chain fatty acids, and stimulates liver gluconeogensis. Part of these adaptations is due to BST counteracting insulin effects in various tissues. Lowered somatomedin secretion is partly responsible for “BST resistance” in underfed animals (Gluckman et al. 1987).
Insulin secretion and tissue responses to insulin decrease in early lactating animals whereas glucagon secretion is maintained or increased. This favours liver glucose production and adipose tissue mobilization (that is also favoured by higher beta-adrenergic sensitivity) and decreases glucose and amino-acid utilization in adipose tissues and muscles, but not in the mammary gland (see Chilliard, 1987). Thyroid hormones are also lowered during early lactation, possibly decreasing basal energy expenditure and protein turn-over (Aceves et al., 1985). The respective effects of teleophoretic hormones such as BST, and of the mammary drain of nutrients in metabolic and endocrine adaptations to lactation are not completely understood.
High-producing dairy cows have been selected for their ability to give a high metabolic priority to the mammary gland. If teleophoretic mechanisms overdo homeostatic regulation, several metabolic disorders can occur (milk fever, ketosis, steatosis, infertility, etc.), even in cows receiving high quality diets in temperate countries. This can partly explain why specialised dairy breeds are more subject to health problems under tropical conditions in developing countries, contrary to local or crossbred cattle whose milk yield decreases more rapidly when they are underfed (Preston and Leng, 1987). Low milk potential could be considered as one facet of genetic and phenotypic adaptation to unfavourable conditions. Adaptation to heat stress is accompanied by changes in numerous lactogenic, galactopoietic or homeostatic hormones (Aceves et al., 1985; Johnson, 1987 and present meeting).
Efficiency of nutrient use for milk yield depends on the balance between glucogenic, lipogenic and aminogenic nutrients that are absorbed from the digestive tract and on absolute needs of the mammary gland for each particular nutrient (see Preston and Leng, 1987 and present meeting). It depends also on the effects of the absorbed nutrients on the endocrine state, particularly the insulin/BST ratio, and on the physiological needs of the cow to recover its “normal” body condition (see Chilliard, 1987). The efficiency of digestible energy utilization is decreased by heat stress, resulting in increased nutrient requirement at the absorptive level under tropical conditions (Roman-Ponce, 1987). As ambient temperature increases above 21°C, the cow lowers heat production by decreasing feed intake and milk yield, although some individuals are more resistant (Johnson, 1987).
Maintenance requirement represents a major portion of energy needs for cattle production and is consequently a major factor in overall energetic efficiency. Maintenance requirement is higher in high-producing dairy cattle breeds during weight maintenance periods without lactation. This is probably linked in part to the greater development of visceral organs with high protein turn-over (digestive tract, liver, heart, etc.) that are involved in handling larger amounts of feeds, nutrients and blood during lactation. This could also partly explain an increased maintenance requirement in the same animal during lactation, or according to previous higher feeding level (see Ferrell and Jenkins, 1985).
Knowledge of high-yielding dairy cow physiology and nutrition has rapidly increased during the last decades. Although not directly useful for milk production in tropical conditions, data on the effects of suckling, milking, underfeeding, body reserves, endocrine regulation and nutrient partitioning could probably be used as a basis for the planning and discussion of applied research, as well as for developing basic research, directly related to tropical conditions. In term of priorities, a particular attention has to be payed to feeding and management of late pregnant-early lactating cows, since they are more responsive at this physiological stage.
Aceves, 1985 C., Ruiz-J, A., Romero, C. and Valceroe-R, C. Homeorhesis during early lactation. Euthyroid sick-like syndrome in lactating cows. Acta Endocrinologica 110 : 505–509.
Bauman, 1980 D.E. and Currie, W.B. Partitioning of nutrients during pregnancy and lactation: a review of mechanisms involving homeostasis and homeorhesis. Journal of Dairy Science 63: 1514–1529.
Broster, 1984 W.H., Clements, A.J. and Broster, V.J. Multiple lactation feeding of the dairy cow. World Review of Animal Production 20: 61–69.
Broster, 1981 W.H. and Thomas, C. The influence of level and pattern of concentrate input on milk output. In: Haresign W. (Editor). Recent Advances in Animal Nutrition. Butterworths: London, pp. 49–69.
Chilllard, 1987 Y. Revue bibliographique: Variations quantitatives et métabolisme des lipides dans les tissus adipeux et le foie au cours du cycle gestation-lactation. II Chez la brebis et la vache. Reproduction Nutrition Développement 27: 327–398.
Chilliard, 1988 Y. Review. Long-term effects of recombinant bovine somatotropin (rBST) on dairy cow performances. Annales de Zootechnie 37: 159–180.
Chilliard, 1986 Y., Delouis, C., Smith, M.C., Sauvant, D. and Morand-Fehr, P. Mammary metabolism in the goat during normal or hormonally-induced lactation. Reproduction Nutrition Développement 26: 607–615.
Chilliard, 1987 Y., Rémond, B., Agabrlel, J., Robelin, J. and Vérité, R. Variations du contenu digestif et des réserves corporelles au cours du cycle gestation-lactation. Bulletin Technique C.R.Z.V. de Theix, I.N.R.A. 70: 117–131.
Chilliard, 1983 Y., Rémond, B., Sauvant, D. and Vermorel, M. Particularites du métabolisme energétique. In “Particularités nutritionnelles des vaches à haut potentiel de production”. Bulletin Technique C.R.Z.V. de Theix, 1.N.R.A. 53: 37–64.
Chilliard, 1983 Y. and Robelin, J. Mobilization of body proteins by early lactating cows measured by slaughter and D2O dilution techniques. IVth International Symposium Protein metabolism and nutrition (Clermont-Ferrand), European Association for Animal Production. Publication no. 31, Volume II, 195–198.
Coulon, 1987 J.B., Petit, M., D'Hour, P. and Garel, J.P. The effect of level and distribution of concentrate supplementation on performance of dairy cows. Livestock Production Science 17: 117–133.
Everitt, 1971 G.C. and Phillips, D.S.M. Calf rearing by multiple suckling and the effects on lactation performance of the cow. Proceeding New-Zealand Society Animal Production 31: 22–40.
Faverdin P., 1987 Hoden A., Coulon J.B. Recommandations alimentaires pour les vaches laitiéres. Bulletin Technique C.R.Z.V. de Theix, I.N.R.A., 70: 133–152.
Ferrell, C.L. and Jenkins, T.G. Cow type and the nutritional 1985 environment: nutritional aspects. Journal of Animal Science 61: 725–741.
Forsyth, 1983 I.A. The endocrinology of lactation. In Biochemistry of lactation, 309–349 (Mepham, T.B., Ed.) Elsevier Science Publishers B.V.
Garnsworthy, 1988 P.C. The effect of energy reserves at calving on performance of dairy cows. In “Nutrition and lactation in the dairy cow” (edited by P.C. Garnsworthy) pp. 157–170. Proceedings of the 46th University of Nottingham Easter School in Agricultural Science.
Gluckman, 1987 P.D., Breier, B.H. and Davis, S.R. Physiology of the somatotropic axis with particular reference to the ruminant. Journal of Dairy Science 70: 442–466.
Johnson, 1987 H.D. Bioclimates and Livestock. In “World Animal Science, B5. Bioclimatology and the adaptation of livestock” pp. 3–16. Edited by Johnson H.O., Elsevier, Amsterdam.
Johnson, 1988 I.D. The effect of prepubertal nutrition on lactation performance by dairy cows. In “Nutrition and lactation in the dairy cow” (edited by P.C. Garnsworthy) pp. 171–192. Proceedings of the 46th University of Nottingham Easter School in Agricultural Science.
Journet, 1981 M. and Rémond, B. Response of dairy cows to protein level in early lactation. Livestock Production Science 8: 21–35.
Knight, 1988 C.H., Wilde, C.J. and Peaker, M. Manipulation of milk secretion. In “Nutrition and lactation in the dairy cow” (edited by P.C. Garnsworthy) pp. 3–14. Proceedings of the 46th University of Nottingham Easter School in Agricultural Science.
Mepham, 1983 T.B. Physiological aspects of lactation. In “Biochemistry of Lactation” (edited by Mepham T.B.) pp. 3–28. Elsevier Science Publishers B.V.
Perez, 1985 O., Jimenez de Perez, N., Poindron, P., Le Neindre, P. and Ravault, J.P. Relations mére-jeune et réponse prolactinique à la stimulation mammaire chez la vache: lnfluences de la traite et de l'allaitement libre ou entravé. Reproduction Nutrition Développment 25: 605–618.
Preston, 1987 T.R. and Leng, R.A. Matching livestock production systems to available resources in the tropics and sub-tropics. Penambul Books, Armidale.
Rémond, 1988 B., Robelin, J., Chilliard, Y. Estimation de la teneur en lipides des vaches laitiéres Pie Noires par la méthode de notation de l'état d'engraissement. INRA Production Animale 1: 111–114.
Roman-Ponce, 1987 H. Lactation of dairy cattle in humid tropical environments. In “World Animal Science, B5, Bioclimatology and the adaptation of livestock”, pp. 81–92. Edited by Johnson, H.D. Elsevier. Amsterdam, Netherlands.
Smith, 1959 V.R. Physiology of lactation. Fifth Edition. Iowa State University Press, Ames, Iowa, USA.
Troccon, 1989 J.L. and Petit, M. Croissance des génisses de renouvellement et performances ultérieures. INRA Production Animale 2: 55–64.