Livestock Research for Rural Development 10 (1) 1998

Citation of this paper

Reproductive behaviour of commercial Zebu cows with calves under a regime of restricted suckling

*R Alderete,* H Losada, **J Cortés, M Osorio and E Aranda

Postgraduate College, Campus Tabasco, México
*Facultad de Zootecnia, Universidad Autónoma de Chihuahua, México
**Animal Production Systems Area, Department of Biology of Reproduction, Division of Biological and Health Sciences, Universidad Autónoma Metropolitana-Iztapalapa. Av. Michoacán y la Purisíma. Col. Vicentina. Iztapalpa. México DF, CP 09340.


A total of 153 commercial Zebu cows with a mean liveweight of 370 kg were allocated at random to two groups: (a) cows pregnant and/or with calves; and (b) non-pregnant cows without calves. The grazing area was made up of 50 ha of African Stargrass (Cynodon plectostachyus), divided in 20 paddocks, managed in rotation and fertilised with 200 kg of N/ha/yr. All the animals received 3.5 kg/day of a molasses/urea (2.5% urea) supplement. Confirmation of animals in heat, inseminations and diagnosis of pregnancy were made using conventional procedures. The calves were managed under a system of restricted suckling twice a day during periods of 30 minutes. The experiment had a duration of 100 days, between the months of June and September.

The percentage of animals coming into heat and conceiving were 82.5 vs. 86.0%, and 55.5 vs. 65.9% for cows with and without calves, respectively. The interval from the beginning of the mating season to first heat was 46.9 days for cows with calves, and 17.4 days for cows without calves (P<0.001). The values for the interval between calving and first heat, and calving and conception were 79.7 and 90.9 days in that order. The percentage of cows needing more than two services to achieve pregnancy was less in the treatment with cows that had calves (28.5%) than those without (43.1%). The response in reproduction behaviour obtained from the animals with calves is discussed in relation to the molasses/urea supplement.

Key words: molasses/urea, cows, reproductive performance, Zebu.


Postpartum anestrus is one of the factors which limits the efficient reproductive behaviour of cattle in the tropics (Williams 1990), reducing the productive life of the cows. This situation is aggravated when the females are managed within a programme of seasonal mounting/mating, which limits breeding to once a year (MacMillan et al 1979). Although this phenomenon has been related to possible hormonal blockages affecting the production of ovules (Nolan et al 1988), the effect has remained disguised by the nutritional demand made on the female on delivery, associated with losses in weight and the rearing of calves during breeding (Wiltbank 1976). One possibility of remedying this problem, taking into consideration the potential of the tropics for cane sugar production (Preston and Leng 1989), would be by means of a molasses/urea supplement, as well as a regime restricting the suckling time of calves (Montgomery 1982).

The objective of the present work was to study the effect of restricted suckling on the reproductive behaviour of Zebu cows grazed on highly-fertilized pasture and supplemented with 3.5 kg/dayof a molasses/urea (2.5% urea).

Material and Methods


Zebu cows (n=153) with an average live weight (LW) of 370 kg were allocated to the following categories:

(a) cows that were at least 8 months pregnant and those that had calves

(b) dry non-pregnant cows

Location and management of pastures

The experiment was carried out in the municipality of Cardenas, Tabasco. The grazing area was composed of an area of approximately 50 ha of African Stargrass (Cynodon plectostachyus), divided in 20 paddocks each of 2.5 ha, giving a density of 3 animals/ha. The grazing system was rotational, moving the animals to the next paddock in the rotation when the grass reached a height of approximately 30 cm. The grass was fertilised with 200 kg of N/ha/yr, using urea as the source of N, distributed in four equal applications during the year.

The animals and their management

The cows that were pregnant, or had recently given birth, had been inseminated during the period of June-August of the previous year. Before entering the mating season, all the animals were allowed to graze without supplement. At the start of the programme, the cows were taken from the pasture in the morning (6:00 am) to a pen where they were offered 3.5 kg of molasses/urea (2.5% urea) per animal during a period of 6 hour, after which the animals were returned to the pasture where they remained for the rest of the time. Water and mineral salts were available as desired in both the pen and the pasture.

The check on animals coming into heat, the method of insemination, and the diagnosis of gestation were carried out following the methods described by Alderete et al (1995). A standard AI routine was followed: oestrus was observed in the pen and grassland for 5 and 2 hr, respectively and detection was with two teaser bulls (fitted with chin ball and penoblock) managed in a rotational system so as to eliminate the effect of individual bulls. The animals were inseminated at an average time of 12 hr after oestrous detection, using frozen semen previously tested and with only one inseminator throughout the whole experimental stage. Conception diagnosis was through the rectum at 35 to 45 days after the last insemination.

The calves stayed permanently with their mothers for a period of seven days after birth, after which they were managed under a system of restricted suckling, allowing the cows access to the calves twice a day for thirty minutes. The separated calves were kept in a shaded area where they were offered a commercial concentrate. The cows and calves were weighed after calving and at weaning at 100 days.. The experiment had a duration of approximately 100 days, between the months of June and September.

Statistical analysis

There was no control group. The data were analysed using a method of variance analysis of completely random design, with different numbers of repetitions (Steel and Torrie 1985). The interval between the start of the mating season and the first heat was determined by the time that elapsed in days, starting from the beginning of the mating season until the animal coming into first heat, while that for the interval from start of mating season to fertilisation was considered to be the time from the start of the mounting/mating season until the last insemination - excluding the animals that did not come into heat as well as those that had been gestating - for the first and second parameters respectively. In both cases, gestating animals and those that had been in heat were assigned a value of 1, and those of dry animals and animals that did not come into heat a value of 0. The measurements obtained were decimal fractions that were converted into percentages by multiplying by a value of 100 (Buck et al 1976).


The presence of the calf delayed the onset of oestrous (Figure 1) and conception (Figure 2) although after 100 days of the mating season the difference between cows with and without calves was no longer significant.

Figure 1: Effect of presence of the calf on incidence of heat in Zebu cows (Differences between treatments were significant for the first 25, 50 and 75 daysof the mating season [SE ±4.5, ±4.3, ±4.1, respectively; P<0.01] but not for 100 days [SE ±3.6]) Figure 2: Effect of presence of the calf on incidence of conception in Zebu cows (Differences between treatments were significant for the first 25, 50 (SE±4.0, 4.8, P<0.01) and
75 days of the mating season (SE±4.9, P<0.05) but not for 100 days, SE±4.9)

The intervals from the start of the mating season to first heat and to conception were longer for cows with calves (Table 1). .

Table 1: Effect of presence of calf on interval from: (i) start of mating season to first heat and to conception and; (ii)(for cows with calves) from calving to start of mating season, to first heat, to conception and to weaning

BLGIF.GIF (44 bytes)

With calves

Without calves


BLGIF.GIF (44 bytes)

No of animals



Interval (days) start of mating season to:

First heat

46.9 (52)

17.4 (80)



55.1 (35)



Interval (days) calving to:

Start of mating season

36.7 (53)


First heat

79.7 (34)





90.9 (34)





99.7 (53)

BLGIF.GIF (44 bytes)

Numbers in parenthesis correspond to the number of animals in each category

The proportion of animals that had to be served a second time was lower among cows that suckled calves (Figure 3). This was reflected in birth rates which appeared to be higher for first services in cows with calves (Figure 4) although the difference was not significant.

Figure 3: Effect of presence of the calf on numbers of services
during the mating season. Difference for 2nd insemination was significant(SE±4.8; P<0.05)
Figure 4: Effect of presence of the calf on birth rates for successive inseminations. SE±3.9; P=0.12 for birth rate following the 2nd insemination.


The use of supplements during the mounting/mating season has been discussed by various authors (Dunn and Kaltenbach 1980; Richards et al 1986). In most cases there is a positive effect of supplements in increasing the percentage of animals coming into heat and conceiving in beef cattle herds. Nevertheless, one of the factors which has occasioned much discussion is the effect of the calves in causing possible hormonal blockages which regulate the production of milk, as well as the ovarian activity of the mother (Short 1976; Lamming 1978; Edgerton 1980; McNeilly 1988; Williams 1990). In accordance with the results obtained in this experiment, although there was a tendency to obtain at the 100 day stage (end of the mating period) a similar rate for both heat and conception (82.5 vs. 86% and 55.5 vs. 65.9% for cows with and without calves respectively), during the earlier phase of the mating period, both in relative and absolute terms, the tendency favoured the cows that did not suckle calves. The data also indicate that the effect of suckling in the early phase of the mating season was primarily manifested in reduced expression of heat and not in efficiency of conception.

To try and discover which was the more important - calf management (restricted suckling) or the supplement - was not possible in the present study as both categories of cows (those with and those without calves) received the supplement. Nevertheless, it is suggested that the molasses/urea supplement (3.5 kg/day) probably helped to offset the negative effect of suckling on oestrous activity and conception rate thus leading to a situation at the end of the 100 day mating period in which heat manifestation and conception rate were similar for cows with and without calves. The excellent growth rates of the calves (over 700 g/day) and the fact that their dams did not lose bodyweight in the first 100 days of lactation (Table 2) provide evidence of the high nutritional status of the cows as a result of supplementation.

Table 2: Mean values (±SE) for changes in the liveweight of calves and their dams during the study
BLGIF.GIF (44 bytes)
Cow liveweight, kg
BLGIF.GIF (44 bytes)

At calving


At weaning


Calf liveweight, kg

At birth


At weaning


Days to weaning


BLGIF.GIF (44 bytes)

The fact that reproduction depended on heat detection followed by artifical insemination could have contributed to the observed differences in heat expression apparently induced by suckling. It is well known that in dual purpose (milk and meat production) herds the negative effect of restricted suckling on reproductive rate is mainly due to a reduced expression of heat and not to impairment in conception rate per se. Thus, natural mating systems with a fertile bull permanently with the cows support higher reproductive rates than heat detection followed by artifical insemination (Naidoo et al 1981), a management factor consistently applied by farmers practicing dual purpose milk and meat production with integrated restricted suckling of the calves.

In contrast to the negative effect of suckling on expression of (or ease of detecting) oestrous, there was an apparent positive effect of suckling on conception rate to first insemination (Figures 3 and 4). It has been shown that cows which suckle their calves have higher levels of oxytocin and of androgenic hormones and lower levels of cortisol compared with cows whose calves were removed soon after parturition (Berit Samuelsson and Kerstin Svennersten, cited in Mistra 1996). It could be hypothesised that an improved hormone status stimulated by suckling could be conducive to more efficient fertilization.

The tentative conclusions drawn from the results of this study are that, even when dam nutrition is at a high level, calf suckling:

These conclusions emphasise the need for more detailed studies, especially in dual purpose herds, of the interaction of the chosen method of reproduction (eg: heat detection followed by AI or natural mating with a fertile bull running with the cows) with suckling management (extent of the separation of cow and offspring) and dam nutrition.


The authors wish to thank the authorities of the Universidad Autónoma Metropolitana (Autonomous Metropolitan University) for the facilities and to Miss R Pealing, a researcher from UK sponsored by the interchange CONACYT-British Council, for language manuscript corrections.


Alderete R, Losada H, Osorio M and Aranda E 1995 Effect of molasses/urea and blood meal suplementation on the reproductive performance of commercial Zebu heifers. Cuban Journal of Agricultural Science 29: 29.

Buck N G, Light D, Rutherford A, Miller M, Rennie T W, Pratchett D, Capper B S and Trail J C M 1976 Environmental factors affecting beef cow reproductive performance in Botswana. Animal Production 23:357.

Dunn T G and Kaltenbach C C 1980 Nutrition and the postpartum interval of the ewe, sow and cow. Journal Animal Science 51(Suppl. 2): 29.

Edgerton L A 1980 Effect of lactation upon the postpartum interval. Journal Animal Science 51 (Suppl.2):40.

Naidoo G, Hulman B and Preston T R 1981 Effect of artificial insemination or natural mating on calving interval in a dual purpose herd. Tropical Animal Production. Volume 6, Number 2:188

Lamming G E 1978 Reproduction during lactation. In: Control of ovulation (Editors: D B Crighton, N B Haynes, G R Foxcroft and G E Lamming). Butterworth, London.

McNeilly A S 1988 Suckling and the control of the gonadotropin secretion. In: The physiology of reproduction (Editors: E Knobil and J D Neill). Raven Press, New York.

MacMillan K L, Allison A J and Struthers G A 1979 Some effects of running bulls with suckling cows or heifers during the premating period. New Zealand Journal of Experimental Agriculture 7:121.

Mistra 1996 Food 21. Sustainable food Production. A Mistra Program. Uppsala, Sweden (pp:84)

Montgomery G W 1982 Influence of suckling frequency and bromocryptine treatments on the resumption of ovarian cycles in postpartum beef cattle. Theriogenology 17:551.

Nolan C J, Bull R C, Sasser R G, Ruder C A, Panlasigui P M, Schoenemann H M and Reeves J J 1988 Postpartum reproduction in protein restricted beef cows: Effect on the hypothalamic-pituitary-ovarian axis. Journal of Animal Science 66:3208.

Preston T R and Leng R A 1989 Adecuando los sistemas de producción pecuaria a los recursos disponibles: aspectos básicos y aplicados del nuevo enfoque sobre la nutrición de rumiantes en el trópico. Condrit Ltd: Cali, Colombia.

Pérez J, Beavers J, Meléndez J and González A 1976 Efecto de la fertilización nitrogenada del pasto Pará en la producción de carne. Agricultura Tropical 1:35.

Richards M W, Spitzer J C and Warner M B 1986 Effect of varying levels of postpartum nutrition and body condition at calving on subsequent reproductive performance in beef cattle. Journal of Animal Science 62:300.

Short R B 1976 Lactation-The central control of reproduction. In: Breast-feeding and the mother. Ciba Foundation Symposium. No. 45. p 73. Elsevier, Exerpta Medica, Amsterdam and Oxford.

Steel R G D and Torrie J H 1985 Principles and procedures of statistics with special reference to the biological sciences. McGraw-Hill Book Co. New York.

Williams G L 1990 Suckling as a regulator of postpartum rebreeding in cattle: a review. Journal Animal Science 68:831.

Wiltbank J N 1976 Managing beef cows to get them pregnant. Memorias. Seminario internacional de ganadería tropical en México. Gro. México. SAG. FIRA.

Return to top