REVIEW

Early weaning: an efficient and effective technological alternative for calf rearing corrals in grassland cattle systems in Latin America

Destete precoz: una alternativa tecnológica eficiente y eficaz para los rodeos de cría en sistemas ganaderos pastoriles de América Latina

A. Simeone,^I V. Beretta,^I

ABSTRACT

The effect of early weaning in beef cattle is reviewed. Data on the impact of early weaning for restarting the post-calving sexual activity and consequently, on pregnancy percentage, are offered.  Also the interaction of early weaning with the nutritional status of the cow, which was assessed by the body condition before mating, is underlined. Information on the nutritional management alternatives of the early weaned calf is presented classified according to grassland or confinement conditions.  The nutritional management of the early weaned calf under grazing conditions is characterized by the management based on concentrate supplementation (18 % crude protein), at a rate of 1 % of the animal live weight.  This strategy allowed attaining similar performances to those in which the calf remain with the cow. The nutritional management of the early weaned calf and managed under confinement is suggested as a strategy allowing higher profits than those attained under grazing with feed conversion around 3.5 kg of feed/kg live weight of calf produced.  It is concluded that early weaning is an efficient and effective technological alternative for improving the productivity of the calf rearing corral and that there are different ways of realizing it that are adaptable to the cattle production system in question.

Key words: cattle, suckling, growth, calf, efficiency.

RESUMEN

Se revisa el efecto del destete precoz en ganado de carne. Se presenta información del impacto del destete precoz en el reinicio de la actividad sexual post-parto y consecuentemente, en el porcentaje de preñez, destacándose la interacción del destete precoz con el estado nutricional de las vacas, que se evaluó por la condición corporal a inicio del entore. La información acerca de las alternativas de manejo nutricional del ternero destetado precozmente se presenta clasificada, según las condiciones pastoriles o de confinamiento. El manejo nutricional del ternero destetado precozmente en condiciones de pastoreo se caracteriza por el manejo basado en suplementación con concentrados (18 % de proteína cruda), a razón del 1 % del peso vivo animal. Esta estrategia permitió alcanzar comportamientos similares a los que el ternero hubiera experimentado de haber permanecido al pie de la vaca. El manejo nutricional del ternero destetado precozmente y manejado en confinamiento se plantea como una estrategia que permite obtener ganancias superiores a las que se logran en pastoreo, con conversión del alimento en torno a 3.5 kg de alimento/kg de peso vivo de ternero producido. Se concluye que el destete precoz es una alternativa tecnológica eficiente y eficaz para mejorar la productividad del rodeo de cría y que existen diferentes formas de realizarlo, que son adaptables al sistema de producción ganadero que se trate.

Palabras clave: vacunos, amamantamiento, crecimiento, ternero, eficiencia.

INTRODUCTION

Cattle rearing in Latin America countries are carried out under grassland conditions characterized by poor reproductive efficiency which conveys at the level of the production-breeder system in low weaning percentage when the number cows producing one calf is related to the amount of cows annually serviced. A very long post-partum anestrous accounts for the low weaning percentage due to the low conception rate that hinders the cows to restart the sexual activity before the removal of the bulls when used a seasonal mating system (Simeone 2000). Even though the problems of the sexual performance of cows destined for beef production during the puerperium can be tackled multifactorially, the cow’s nutritional status (Wettemann 2013) and the suckling (Williams 1990) have been identified as the two most important factors influencing on the duration of the post-partum anestrous (Short and Adams 1985).

Early weaning, understood as the definitive interruption of the cow-calf relationship 60 d post calving, has demonstrated to be a very efficient and effective technological alternative for improving the productivity of the calf rearing corrals. Its effect is mediated by the simultaneous influence of the mentioned two factors and it has been studied as technological tool for solving the problem of low reproductive efficiency of the calf rearing corrals (Simeone and Beretta 2002). The importance of the information created on early weaning in Latin America countries could vary as result of the specific peculiarities of each region which could give rise to a wide variety of approaches.

The objective of this paper is to summarize from the information generated in Latin America and Caribbean countries in general and, specifically, in Uruguay, the present awareness on early weaning, as technological tool of great impact on cattle productivity during the rearing stage, regarding its effect on cow reproduction as well as on calf development.

EARLY WEANING AND REPRODUCTIVE EFFICIENCY IN BREEDING COWS

Theoretical bases of the early weaning effect. The early weaning technique while improving the reproductive performance is directed to the elimination of the endocrine restrictions during suckling (Williams 1990). But, mainly, leads to the improvement of the energetic balance of the post-calving cow through the elimination of the nutritional demands for milk production and the modifications in the energy division towards a recovery of the body condition (Short and Adams 1985).  This promotes a fast return to the cyclic sexual activity and, consequently, a significant increase of the pregnancy rate.  Nonetheless, and according to its way of action, results encountered demonstrate that the reproductive response to the application of early weaning is higher in those groups of animals forming part of the rearing corral, more energy demanding, that is: a) cows that calved and were mated under bad body condition and b) first calving cows.

Estimations realized by prediction equations for British breed cows (AFRC 1993) indicate that the elimination of the requirements for milk production is equivalent to an energy “savings” of 0.98 Mcal of metabolizable energy (ME) per milk liter not produced by the cow.  From this estimation it can be calculated that for one beef cow with a daily production level of 7 L of milk with 3.4 % fat, early weaning would allow the availability, without modifications in the forage basis, of 6.86 Mcal of additional ME, that could be destined to the improvement of the body condition (BC) promoting the restart of the cyclic
activity.

Effect of early weaning on the reproductive performance of breeding cows. The suckling has an inhibitory effect on early ovulation during the post-partum, but this effect is extended in bony cows (Wettemann 2013). The reproductive response to the application of early weaning has been assessed in multiparous cows in different body condition and in primiparous cows. In table 1 are summarized the main results obtained in beef cows grazing natural grassland in Uruguay.

From the data of table 1 it can be inferred that all rearing corrals submitted to early weaning register systematically, pregnancy rates around 80 and 90 %.  This represents a mean response of 30 to 40 percentile units, regarding the average weaning rate of the rearing corrals managed under extensive conditions.  However, when early weaning is applied to the most sensible groups of the rearing corral, as to cows with BC lower than 3 or to primiparous cows, this response grows to values close to 50 percenage units.  In these cases, even though there is a pregnancy improvement, the calving interval is not modified regarding the same cows without early weaning, which could even represent a delay in the next calving times.  This latter must be considered to the effects of not affecting pregnancy in the following mating. On the contrary, the application of early weaning to cows in good body condition, even though pregnancy percentage is not significantly modified, it has been demonstrated to shorten the calving interval regarding cows in the same condition and mated with its calf that can be used as management tool for the concentration of calvings.

According to these evidences, the possibility that this technique have really positive effect on the rearing enterprise depends on its application in cows showing, at the beginning of mating, a body condition 3 or lower and in primiparous cows.  For that, it is recommended, prior to mating, to form cow lots with this BC for later in these lots apply the early weaning, from the criterions related with the weight and age of the calves.

The alternative of realizing this task before mating has the inconvenient of working with the cattle with calves with the known problem of slow herd moving and other situations. For that, it is suggested to form cow lots with different BC at calving, so as at the mating time lots with different groups of cows are available: a) cows in bad BC (BC < 3.5), to which early weaning could be applied; b) cows in intermediate condition (BC between 3.5 and 4), to which temporary weaning could be applied and c) cows with good BC (BC > 4) not needing any suckling control technique.

Certainly, the classification of the breeding cows by different groups implies the availability of a specific infrastructure regarding the number of paddocks.  If at the establishment time there are not sufficient paddocks as for managing three cow lots at calving, it is suggested its classification in two lots at least from BC 3.5 as division criterion.  To the cows calving with BC < 3.5 will be applied early weaning, while in cows with BC > 35 other alternative techniques will be applied or no intervention will be made.

The economic viability of early weaning, as in any other technology, determines, of course, its adoption.  Therefore this type of analysis must be realized for each one of the particular conditions in which it is applied.  According to the previous statement, the highest bioeconomic impact will be attained on the basis of the application of temporary early weaning to thin cows (body condition between 3 and 3.5) in which the highest response is expected.  The economic results of early weaning practiced temporarily can be measured as the extra benefit obtained per cow, which is determined by the extra income, derived from the implementation of the techniques minus the additional total cost of the new management. The extra income will correspond to the net increase in the pregnancy rate, materialized in the kilograms of calves weaned conventionally (approximately at 180 d of age) in the following year to the application of early weaning (representing 15 months of return on invested capital).  In this type of evaluations, temporary early weaning is exceptionally made in a mating, returning the cows in the following to the mating without calf weaning. No changes are made on the forage bases used by the cows.  The studies carried out for assessing the effects of the economic results from the application of early weaning, suggest that the utilization of this tool produces positive results concerning the input and product prices, since even under a situation of unfavorable price relation due to low calf prices, early weaning could be equally justified in such categories in which the response in terms of increase in the pregnancy rate is higher than 40 %.  As previously indicated, primiparous cows and those arriving to mating with poor body condition (between 2.5 and 3) are two rearing categories in which there will be higher response to the application of early weaning.  In these cases increase values of 50 % can be attained.  The highest response in reproductive terms recorded in these categories will allow: a) pay rations at higher prices, or b) apply early weaning even at the perspective of low prices of the weaning calves.

EFFECT OF EARLY WEANING ON CALF DEVELOPMENT

The application of early weaning in rearing corrals implies assuming the compromise of feeding the calves so as to attaining, at least, the same gain rates they would have reached if maintained with the cow until six months of age.  Studies carried out by Monje and Hoffer in Argentina (INTA 1996) on the application of early weaning in rearing corrals under extensive cattle production and the creation of proposals for nutritional management of calves have been pioneers on this subject matter and have significantly contributed to the promotion of this technique.  Therefore, the profile of the investigation developed in countries such as Uruguay has been the assessment of management alternatives of the calf feeding, adjusted to the specific production conditions.  In studies of Simeone and Beretta (2002) great part of the information on this topic has been synthesized.

Synthesizing, results derived from this investigation line on the management of early weaned calves can be grouped in two areas:  nutritional management of early weaned calves under grassland conditions and b) nutritional management of early weaned calves under confinement conditions.

Management of grazing early weaned calves. Research on the nutritional management of early weaned calves and managed under grazing conditions is directed towards development nutritional strategies that will allow obtaining in calves live weight gain rates similar to those that would be reached if maintained with the cow.  In table 2 is shown a synthesis of the main results obtained at IBPU with the nutritional management of the early weaned grazing calf.

Data demonstrate the productive viability of the early weaning under grazing conditions, since animal performance is similar to that reached in the suckling calf. For attaining this objective, experimental data suggest calf grazing management with forage supply (FS) of 8 kg DM/100 kg live weight and supplementing with an energy-protein concentrate (18 % protein, 80 % digestibility).  It has been identified a supplementation level of 1 % live weight as that optimizing gains (Simeone et al. (1997c).

Certainly the nutritional management of 80 kg live weight calves involves an important operative challenge at production system level.  Thus, emphasis has been made in the development of a very detailed protocol guaranteeing the before mentioned productive objectives (Simeone and Beretta 2002).  Early weaning must be divided into two stages: I) weaning and adaptation of the calf to the new diet (10 – 15 d) and II) grassland management and supplementation (60 to 90 d).

During the first period, calves must be recovered from the weaning stress and learn consuming a high quality solid diet that will stimulate ruminal development. The definitive result of the technique at five or six months of age depends directly from the success in this stage. Regarding the weight and age of the calf, it is suggested considering a minimum critical weight of 70 kg, considered as a reliable value, in case calving date is known, 60 d as minimum age for weaning. It is recommended a quick weaning in the establishment corrals, early in the morning.  Cows must be immediately transferred to a far paddock for assuring the definitive interruption of the cow-calf relationship.  Aspects related with calf health must be guaranteed and the application of antiparasitic treatments and vaccinations for preventing diseases as kerato-conjunctivitis, clostridiosis, infectious bovine rhino-tracheitis and viral diarrhea (bovine) among others. The adaptation of the calf to the solid diet is the key and this period must last 10 d, approximately, although under problematic situations can be extended to 15 d.  Data created at the IBPU suggested dividing this stage in four phases:

- Phase 1 (day 1). It corresponds to the day when weaning is carried out.  Calves are maintained in the corrals only with shade and water, without access to any type of feed.  Calves move around and bleat until they stay calm.

- Phase 2 (day 2).  In the morning feed is offered, placing manually the stripped down bale, as floor cover of the feeding trough at a rate of 0.500 kg/animal. The concentrate is distributed on the bale at a rate of 0.250 kg/animal. It is essential to allow the animals the access to the feeding troughs once finished feed distribution so as there is equal consumption opportunities for all calves. To these effects it is recommended the availability of other short passage next to the adaptation corral or to put a temporary sub-division with electric wire at the corner of the corral, only while feeds are placed in the feeding troughs.

- Phase 3 (day 3 to 9).  From the third day it starts to increase daily the supply of concentrates at a rate of 0.250 kg/animal, until reaching 1 % of the live weight.  At the last hour of the afternoon the removal of the surplus feed is conducted. If there is no feed, the bale can be gradually increased until attaining 1 % live weight.  At this moment all animals must eat uniformly.

- Phase 4 (day 10).  The supply of the bale is discontinued and the concentrate offered in the paddock where the animals will be managed with the utilization of a feeding trough similar to that of the corral. Once the supplement is consumed, calves are released to the pasture.

Regarding the management, during the second stage in the field, it is recommended that during the first three days of concentrate consumption, the staff observe attentively animal performance to know which animals do not consume concentrate or is dispersed when supplied, registering the problem cases.  It is suggested to offer the concentrate early in the morning avoiding the high temperature hours.  For attaining that all animals consume orderly, one next the other, it is essential to maintain a strict routine and to respect the supply hours of the concentrate.  In this way the access of the animals to the feeding trough is avoided until the concentrate distribution is finished.  Just like that during the adaptation to the feed, it can be available at some paddock corner an electric wire for establishing a temporary sub-division where the calves wait, while the workers place the feed in the feeding trough. The staff must observe daily the animals for identifying eventual sanitary problems in which the veterinary doctor must examine (actinomycosis, respiratory problems, severe kerato-conjunctivitis and deaths of unknown cause, among others).  Also they must realize the normal management that takes place during establishment (treating miasis and identifying dominance problems between animals, among others).

Management of early weaned calves under confinement conditions (EWC). The research concerning the nutritional management of early weaned calves under confinement has followed a different logic regarding the early weaning under grazing, since different productive objectives are considered. While, the management under grazing states as central axis to obtain the same performance that a suckling control, the stabled management is presented as a correcting option and accepts the challenge of exceeding significantly that performance.

Results achieved demonstrate the possibility of attaining performances around 1.2 kg/d in early weaned calves, managed under confinement with high concentrate diets.  A conversion efficiency of 3.5 kg of dry matter of feed/kg live weight gain is obtained.  This opens an interesting perspective for improving the productivity of the rearing phase, since it assures the possibility of attaining a calf of six month of age with 230 kg live weight and, at the same time, fulfill with high gestation in its dams.  The excellent calf conversion during confinement makes viable the application of this option from the economic point of view in a broad scenario of feed and calf prices.

Some aspects related with the application of EWC merit particular attention on analyzing this technological alternative for cattle production systems.  Among these outstands the evaluation of the effect of animal welfare by the decrease of the caloric stress and of the type of protein used in the diet formulation.

Simeone et al. (2012) reported an experiment realized at the North Littoral of Uruguay, directed to assess the effect of the shade on the performance of early weaned calves (86.6 ± 11.5 kg) fed under confinement conditions during the first three months post-weaning: a) management in corrals with free access to artificial shade in the corral and b) management in corrals without shade.  Weight gains attained were high, in absolute terms and when compared to mean gains obtained in suckling calves or early weaned and supplemented under grazing (Simeone and Beretta 2002).  This response was expected given the higher energy concentration of the diet and the ad libitum supply of the feed. However, even under conditions of expression of the animal’s potential it was verified that calves with access to the shade registered average daily gains of live weight significantly higher than those with no access to it (1.45 vs. 1.30 kg/d P < 0.05).  Feed consumption tended to be higher in animals that had access to the shade (5.3 vs. 5.0 kg of dry matter/d; P = 0.1034).  Nonetheless the highest daily weight gain determined better feed conversion efficiency (3.68 vs. 3.91 kg DM/kg LW gained; P = 0.07).  These evidences suggest that the environmental conditions registered during the summer predispose to the heat stress in early weaned calves.  The possibility of having a shade area reduces this effect.  Animals with access to the shade showed lower stress symptoms, mainly, lower respiratory rate and better productive performance (Simeone et al. 2012).  This could have important implications for countries of tropical cattle production where temperatures can be higher.

Concerning the type of protein to be used in the diet formulation for early weaning calves, characteristics of this animal category are determinant factors. Early weaned calves present high protein demand and low daily consumption of dry matter.  The contribution of metabolizable protein (MP) from the microbial protein can result insufficient for covering the daily ME requirements.  In this case, the contribution of protein source supplying with higher diet contribution of non degradable protein in the rumen (NDPR) could improve the performance of early weaned calves.  Likewise, higher contribution of NDPR will also offer the possibility of adjusting more precisely the requirements of some metabolizable amino acids which are essential.  This could improve the weight gain or the conversion efficiency and even, both.  Some background show that in iso-nitrogenous diets, the supply of increasing levels of fish meal (+ 80 % NDPR) (NRC 2000) to crossbred Frame steers half-confined (231 kg) increased amino acid absorption at duodenal level and modified the plasma amino acid profile regarding the use of urea as supplementary source (Zinn and Shen 1998), improving the energetic efficiency and the contribution of net energy of the feed.

The protein supplement varies, not only in the protein contribution (% DM) but also in the NDPR (% CP) and in the amino acid profile of this fraction (NRC 2000). The sources of true protein most commonly used in the formulation of rations for early weaned calves include soybean meal or sunflower expeller which NDPR varies between 20 and 35 % (NRC 2000). In this context, the substitution of this type of supplement by other protein sources of poorer ruminal degradability could improve the performance of early weaned calves and fed in corrals.  From this hypothesis at the IBPU was carried out an experimental study to assess the effect of various protein sources differing in the ruminal degradability of the protein contributed as ration ingredients for early weaned calves (Beretta et al. 2012).

Results obtained indicate significant effect of the protein source on daily live weight gain of the calves (P < 0.05).  There was lower gain in calves receiving non protein nitrogen as supplementary protein source (0.859 kg/d) regarding those receiving soybean (0.990 kg/d) or fish meals (0.995 kg/d) which did not differ (P < 0.05). Dry matter intake of the diet did not vary between treatments. This was expected since the study was realized with fixed feed supplies.  As consequence, the conversion efficiency (kg DM of feed consumed/kg live weight gained) tended (P = 0.070) to be better for the treatments with soybean or fish meals compared to that of urea.  As average, calves receiving urea required 13.4 % and 15.8 % more of feed for each kilogram of live weight gained regarding those receiving soybean or fish meals, respectively. These results suggest there is positive response in live weight gain when the non degradable protein in the rumen is increased, probably associated with the augment of the intestinal absorption of MP.  This can be explained since, in a certain way the NDPR will compensate a lower microbial protein synthesis in an animal category with a development rumen.

Early weaning is a management practice allowing the increase of the reproductive performance of the cows. Background point that in British cattle, continental, Indian or in its crosses, it is reasonable to expect pregnant percentages of cows early weaned around 85 – 90 %.  The effect of this practice is higher in second mating cows and with poor body condition (CC < 3). Body condition at the beginning of mating can be an adequate criterion for deciding when the early weaning must be applied.

It is necessary to create more information allowing the characterization with greater precision of the existing relationships between the nutritional stage of the animal and the response to early weaning, as well as the existing interaction with other management practices as stocking rate adjustment (INTA 1996), season and mating length (Bermúdez et al. 2005), management of the natural grassland and intervention of hormonal treatments (Menchaca et al. 2013). The quantification of these relationships will positively contribute to increase the productivity of the rearing phase.

Diverse options of nutritional management for early weaned calves have been developed under grazing conditions as in confinement.  Data gathered in that regard indicate that under grazing, with supplementation strategy with energetic-protein balanced feeds at 1 % live weight, it is possible attaining the same gain rate as in the suckling calf, while under confinement, with high proportion of concentrate in the diet, it is even possible to duplicate that performance.  The definition of one or other option will depend on the characteristics of the production system in question.

REFERENCES

Alderman G. & Cottrill B. R. 1993. Energy and protein requirements of ruminants: an advisory manual prepared by the AFRC Technical Committee on Responses to Nutrients. UK: CAB International, 159 p., ISBN: 978-0-85198-851-1, Available: <http://www.cabdirect.org/abstracts/19941400396.html;jsessionid=7B833711E508B6F8D52EC3A87AD61F7E>, [Accessed: February 24, 2016].

Beretta V., Simeone A., Elizalde J., Gamba D. & Terzian A. 2012. ‘‘Effect of protein source on feedlot performance of early-weaned beef calves’’. In: Book of Abstracts of the 63rd Annual Meeting of the European Association for Animal Production, Bratislava, Slovakia: Wageningen Academic Pub, p. 118, ISBN: 978-90-8686-206-1.

Bermúdez S., Canon F. & Capurro A. 2005. Efectos de la época de servicios sobre la evolución del estado nutricional en tres momentos críticos del ciclo productivo y la eficiencia reproductiva de vacas Hereford destetadas precozmente. Graduate Theses, Facultad de Agronomía, Universidad de la República, 84 p.

Griffith M. K. & Williams G. L. 1996. ‘‘Roles of maternal vision and olfaction in suckling-mediated inhibition of luteinizing hormone secretion, expression of maternal selectivity, and lactational performance of beef cows’’. Biology of Reproduction, 54 (4), pp. 761–768, ISSN: 0006-3363, 1529-7268, DOI: 10.1095/biolreprod54.4.761.

Menchaca A., Núñez R., de Castro T., Pintos C. G. & Cuadro F. 2013. ‘‘Implementación de Programas de IATF en rodeos de cría’’. Seminario de Actualización Técnica: Cría Vacuna, (208), pp. 229–246, ISSN: 1688-9266.

National Research Council. 2000. Nutrient Requirements of Beef Cattle. 7th ed., Washington, D.C.: National Academies Press, 248 p., ISBN: 978-0-309-06934-2, Available: <http://www.nap.edu/catalog/9791>, [Accessed: February 24, 2016].

Orcasberrro R. 1991. ‘‘Estado corporal, control del amamantamiento y performance reproductiva de rodeos de cría’’. In: Carambula M., Vaz M. D. & Indarte E. (eds.), Pasturas y Producción Animal en Áreas de Ganadería Extensiva, 2nd ed., Montevideo: INIA, pp. 158–163, ISBN: 9974-38-009-X.

Short R. E. & Adams D. C. 1988. ‘‘Nutritional and Hormonal Interrelationships in Beef Cattle Reproduction’’. Canadian Journal of Animal Science, 68 (1), pp. 29–39, ISSN: 0008-3984, DOI: 10.4141/cjas88-003.

Simeone A. 2000. ‘‘Destete temporario, destete precoz y comportamiento reproductivo en vacas de cría en Uruguay’’. Estrategias para acortar el anestro posparto en vacas de carne, (108), pp. 35–39, ISSN: 1688-9266.

Simeone A. & Beretta V. 2002. Destete Precoz en ganado de carne. Montevideo: Hemisferio Sur, 119 p., ISBN: 9974-645-23-9.

Simeone A., Beretta V., De León M., Silvera E., Torres S. & Widmaier G. 1997a. ‘‘Efecto del nivel de suplementación en pasturas sobre la ganancia de peso de terneros destetados precozmente’’. Revista Argentina de Producción Animal, 17 (Supl. 1), p. 60, ISSN: 0326-0550, 2314-324X.

Simeone A. & Lobato J. F. P. 1996. ‘‘Efeitos da lotação animal em campo nativo e do controle da amamentação no comportamento reprodutivo de vacas de corte primíparas’’. Revista Brasileira de Zootecnia, 25 (6), pp. 1216–1227, ISSN: 1806-9290.

Simeone A., Trujillo A. I., Córdoba G., Gil J., Rodríguez M., Bejérez A., Zanoniani R., Baccino F. & Umpiérrez M. 1997b. ‘‘Efecto del destete precoz y de dos sistemas de alimentación post-destete sobre la ganancia de peso de terneros Hereford hasta los 15 meses de edad’’. Revista Argentina de Producción Animal, 17 (1), pp. 58–59, ISSN: 2314-324X, 0326-0550.

Simeone A., Trujillo A. I., Córdoba G., Gil J., Rodríguez M., Zanoniani R., Botello A., Bejerez A. & Fonseca F. 1997c. ‘‘Efecto del destete precoz sobre el estado corporal, la ganancia de peso y el comportamiento reproductivo de vacas Hereford pastoreando campo natural’’. Revista Argentina de Producción Animal, 17 (1), p. 258, ISSN: 0326-0550, 2314-324X.

Wettemann R. P. 2013. ‘‘Nutritional influences on reproduction of beef cows’’. Seminario de Actualización Técnica: Cría Vacuna, (208), pp. 247–264, ISSN: 1688-9266.

Williams G. l. 1990. ‘‘Suckling as a regulator of postpartum rebreeding in cattle: a review’’. Journal of animal science, 68 (3), pp. 831–852, ISSN: 0021-8812.

Zinn R. A. & Shen Y. 1998. ‘‘An evaluation of ruminally degradable intake protein and metabolizable amino acid requirements of feedlot calves’’. Journal of Animal Science, 76 (5), pp. 1280–1289, ISSN: 0021-8812, DOI: /1998.7651280x.

Received: November 25, 2015 ]]> Accepted: March 31, 2016

A. Simeone, Facultad de Agronomía. Universidad de la República. Ruta 3, km 363, Paysandú, Uruguay. Email: asimeone@adinet.com.uy