Animal Reproduction Science, 31 ( 1993 ) 21-31 2 I Elsevier Science Publ ishers B.V., Ams te rdam

Short luteal phases and ovulations without oestrus in primiparous Borana (Bos indicus)

cows in the central highlands of Ethiopia

Azage Tegegne a, Abreham Geleto b and Tesfu Kassa b alnternational Livestock Centre for Africa (ILCA), P.O.Box 5689, Addis Ababa, Ethiopia

bFaculty of Veterinary Medicine, Addis Ababa University, P.O.Box 32, Debre Zeit, Ethiopia

(Accepted 22 April 1992 )

ABSTRACT

Tegegne, A., Geleto, A. and Kassa, T., 1993. Short luteal phases and ovulations without oestrus in primiparous Borana (Bos indicus) cows in the central highlands of Ethiopia. Anita. Reprod. Sci... 31:21-31.

The incidence of short luteal phases and ovulations without oestrus was investigated in 42 primi- parous Borana cows. Based on calving body weight, cows were allocated to either a control group with grass hay only or to a supplement group with a concentrate ration. Within nutritional groups, cows were further assigned to either milked or partially suckled groups. Cows were under continuous ob- servation and vasectomised teaser bulls used in oestrus detection. Blood samples were collected twice weekly commencing on Day 30 postpartum until pregnancy for determination of plasma progesterone levels. A progesterone concentration of above 1 ng m l - ~ was taken as a cut-off value between luteal and non-luteal phases and oestrous cycles of 17 days or less were considered as short. By 150 days postpartum, ovarian cyclic activity had commenced in 64.3% of the cows while 35.7% remained an- oestrous. The mean ( _+ SEM) postpartum anoestrous interval for the cyclic cows was 104 +_ 19 days. Progesterone profiles revealed that up to the first detected oestrus, 97 ovulations occurred and 72% were not accompanied by behavioural signs of oestrus, indicating that an average of 2.6 oestrous periods per cow were silent. Differences in intervals between calving to onset of first oestrus based on visual detection and progesterone concentrations ranged from 5 to 21 days in 22.2%, 24-43 days in 44.4%, 44-63 days in 11.2% and 83-166 days in 22.2% of the cows. Short luteal phases with a mean duration of 10 _+ 2 days and a range of 7-14 days were detected in 55.6% of the cows. There were no nutritional and suckling influences on the incidence of ovulation without oestrus and short luteal phases. Progesterone concentrations, however, were significantly (P< 0.01 ) lower in short than in normal oestrous cycles. It was concluded that although short luteal phases occurred in a relatively larger proportion of cows, it was not an exclusive prerequisite for the establishment of normal ocs- trous cycles. The commencement of ovarian cyclic activity in 78% of the cows before Day 63 postpar- tum also suggests that ovulations without oestrus rather than postpartum anoestrus could be more responsible for extended calving intervals in Zebu cattle. The physiological basis and significance of ovulations without oestrus and short luteal phases in the fertility of Zebu cattle thus need further investigation.

Correspondence to: A. Tegegne, ILCA, P.O. Box 5689, Addis Ababa, Ethiopia.

6-3 1993 Elsevier Science Publ ishers B.V. All rights reserved 0378 -4320 /93 /$06 .00

22 A. TEGEGNE ET AL.

INTRODUCTION

Postpartum anoestrus is one of the major contributors to extended calving intervals in Zebu cattle (Entwistle, 1983; Eduvie, 1985; Mukasa-Mugerwa et al., 199 l a). However, the postpar tum anoestrous interval is influenced by a number of both physiological and behavioural events. In Zebu cattle, the du- ration of oestrus is short and the intensity of oestrus manifestation is poor, making oestrus detection difficult (Mattoni et al., 1988; Galina and Arthur, 1990). Cows also go through normal ovarian cyclic activity without showing external signs of oestrus and receptivity to the bull (King et al., 1976; Salis- bury et al., 1978; Dawuda et al., 1989; Mukasa-Mugerwa et al., 1991b).

Short luteal phases occur in both dairy and beef cattle irrespective of the length of the postpar tum anoestrous interval (Lamming and Bulman, 1976; Schams et al., 1978; Eger et al., 1988; Mukasa-Mugerwa et al., 1991 b). About 50% of the first increases in postpartum progesterone concentrations are of short duration and the percentages decrease in subsequent cycles (Eger et al., 1988 ). The role of prostaglandin F2, and oxytocin in determining the lifespan of corpora lutea in the postpar tum cow have been investigated (Peter et al., 1989; Cooper et al., 1991 ). Lishman and Inskeep (1991) have also recently reviewed the physiological mechanisms involved in the premature regression and the subnormal secretory activity of the corpora lutea in the cow and the ewe.

The contributions of both short luteal phases and ovulations without oes- trus to postpar tum cow fertility and the physiological mechanisms involved are not as yet properly understood. Data on the incidence of ovulation with- out oestrus and short luteal phases are also inadequate in Zebu cattle in trop- ical Africa. This study examined the incidence of short luteal phases and ovu- lation without oestrus using plasma progesterone concentrations in primiparous Borana (Bos indicus) cows in the central highlands of Ethiopia.

MATERIALS AND METHODS

The study was undertaken at the International Livestock Centre for Africa (ILCA), Debre Zeit Research Station, located 50 km southeast of Addis Ababa. The research station is situated at 8 ° 44 'N and 38 o 58'E at an altitude of 1850 m. Mean annual rainfall is 864.6 m m and mean min imum and max- imum temperatures are 10.6 ° C and 25.0 o C, respectively.

A total of 42 primiparous Borana (Bos indicus) cows aged between 5 and 6 years were used for the study. Cows were kept in individual stalls in a barn and were allocated to either a control group with grass hay only or a supple- ment group with grass hay and 5 kg per head day- l of a concentrate mixture composed of 59% wheat bran, 20% cotton seed cake, 20% molasses, 0.5% bone meal and 0.5% salt. Within their nutritional group, cows were further

SHORT LUTEAL PHASES AND OVULATIONS WITHOUT OESTRUS IN COWS 23

allocated to either milked or partially suckled groups. In partially suckled cows, calves were allowed to suckle two quarters only. All cows had free access to water and mineral licks.

Following parturition, cows were rectally palpated every week for deter- minat ion of uterine involution and examination of ovarian structures. Cows were under continuous observation for sexual activities and teaser bulls used to assist in oestrus detection. Commencing on Day 30 postpartum, blood was collected via jugular venipuncture twice weekly (every other day in 16 cows ) until pregnancy was determined. Plasma progesterone levels were determined by the ELISA technique using Ovucheck kits (Cambridge Veterinary Sci- ences, Cambridge, UK) . A detailed procedure of the assay has been described previously (Mukasa-Mugerwa and Tegegne, 1989).

Ovulation without oestrus was defined as the occurrence of an oestrus pe- riod based on progesterone concentrations but not accompanied by overt be- havioural signs. The duration of a luteal cycle was defined as the period from the time when progesterone concentration increased continuously from its lowest value to above 1 ng m l - l until the following abrupt decrease to less than 1 ng ml - 1. The first luteal cycle could be preceded and followed by be- havioural oestrus. Short luteal phases were defined as oestrous cycles with durations of 17 days or less. Data on the incidence of ovulations without oes- trus and short luteal phases were analysed by the Chi-square procedure (Sta- tistical Analysis Systems Institute (SAS), 1987).

R E S U LTS

Data on weekly rectal palpations showed that the mean ( -+ SE ) length from calving to complete uterine involution was 30 + 2 days and ranged from 28 to 42 days. By 150 days postpartum, behavioural signs of oestrus were detected in 64.3% (27/42) of the cows (Fig. 1, Cow No. 5152) while 35.7% remained anoestrous (Fig. 1, Cow No. 5184). Based on plasma progesterone concentra- tions, luteal activity had commenced in all the 27 cyclic cows before the first behavioural oestrus was detected. Plasma progesterone profiles and visual observation data revealed that in these cyclic cows a total of 97 ovulations occurred including the first behavioural oestrus and 72.2% (70/97) were not accompanied by the overt signs of oestrus, all occurring prior to the detection of the first behavioural oestrus (Fig. 2, Cow No. 7882).

All the ovulations without oestrus were subsequently accompanied by for- mation of functional corpora lutea as determined from progesterone profiles and confirmed from data on ovarian palpation. There were no nutritional or suckling effects on the incidence of ovulations without oestrus.

The mean ( +_ SE) interval from calving to first observed oestrus for cows which cycled within 150 days was 104_+ 19 days and was shorter by 50_+ 7 days when determined from plasma progesterone concentrations. There were

24 A. TEGEGNE ET AL.

e ~ e C o w No . 5 1 B 4 zo] , - 0 ~ 0 C o w No . 5 1 5 2 le t ~ _ ~ " - - & C o w NO. 5 1 5 8

18

11 &

a s i P

~. 4 le

2

_,T V ~--.. T- . . : . . : : . . . , , , , 30 4,5 60 75 90 105 120 135 150 165 180 195 210 225 240 255

Postpartum period (days),

Fig. 1. Patterns of plasma progesterone profiles in Borana cows with an extended postpartum anoestrus (No. 5184), and early postpartum ovarian cycle activity followed by conception (No. 5152 ), and with a short luteal phase preceded and followed by oestrus (No. 5158) (E, oestrus; SLP, short luteal phase).

111-

14-

E 10.

=" B, P

u

8 0.

Cow No. 7882

2 -

o I t I 30 45 60 75

q

I I 195

I I I I I I I I I I 90 105 120 135 150 165 180 210 225 240 255

Postpartum period (days)

Fig. 2. Patterns of plasma progesterone profiles in a Borana cow with ovulations without oestrus during the postpartum period (SE, ovulation without oestrus; E, oestrus).

SHORT LUTEAL PHASES AND OVULATIONS WITHOUT OESTRUS IN COWS 2 5

no nutritional or suckling effects of the postpartum anoestrous interval. How- ever, the percentage of cows cycling by 150 days postpartum was significantly (P< 0.05 ) higher in milked (48.4%) than in suckled (25.8%) cows.

The difference between the commencement of ovarian activity based on plasma progesterone concentrations and detection of first behavioural oes- trus by visual observation and/or markings by teaser bulls did not coincide in all cows which cycled. This difference ranged from 5 to 21 days in 22%, 24-43 days in 44%, 44-63 days in 11% and 83-166 days in 22% of the cows.

The first rise in progesterone concentrations above 1 ng ml-1 was consid- ered to be the beginning ofluteal activity and to represent the start of the first postpartum cycle irrespective of its duration or the interval to the next cycle. Short-lived transient progesterone peaks were detected in 55.6% of the cyclic cows prior to the establishment of normal oestrous cycles (Fig. l, Cow No. 5158 ). The mean duration of these short luteal phases was l0 + 2 days and ranged from 7 to 14 days. There were no nutritional or suckling effects on the incidence or duration of short luteal phases. Figure 3 shows pooled data on progesterone profiles of short and normal cycles from 16 cows bled every other day. Total and peak plasma progesterone concentrations were significantly (P< 0.001 ) lower in short than in normal cycles.

Plasma progesterone profiles in all cows revealed that five distinct patterns of' postpartum reproductive activities occurred. These include: (a) early cyclic activity followed by conception (Fig. 1, Cow No. 5152); (b) extended post- partum anoestrus (Fig. 1, Cow No. 5184); (c) early cyclic activity followed

18-

14-

12.

E 10-

¢D ¢- 8- o

6"

n 4-

2-

o ( 2 4 6 B 10 12 14 16 18 20 22

Day of oestrous cycle

I 24

Fig. 3. Patterns of plasma progesterone profile of normal oestrus cycle (a; n = 16) and short luteal phase (b; n = 6 ) in Borana cows during the postpartum period (error bar indicates SEM ).

18 -

,e I 14

12

¢- v

q) C 8 - o

w 6 ¢D 0 1

P n 4

C o w No. 5 1 5 4

14-

12-

10- &

~- 4. E E

2.

0 - T I I I I I I I I 30 45 60 75 go 105 120 135 150 165 180 lg5 210 225 240 255

P o e t p a ~ u m per iod (days)

Fig. 4. Patterns of plasma progesterone profiles in a Borana cow which had embryonic mortality (E, oestrus; EM, embryonic mortality; SE, ovulation without oestrus).

0 30

I I I I 105 12o 135 15o

C o w No. 7 8 7 7

I I I 4,5 80 75 go

26 A. TEGEGNE ET AL.

E I

185 18o lg5 21o 225 240 255

Postport.um period (days)

Fig. 5. Patterns of plasma progesterone profiles during the postpartum period in a repeat breeder Borana cow (E, oestrus).

by embryonic mortality (Fig. 4, Cow No. 5154); (d) early cyclic activity fol- lowed by repeat breeding (Fig. 5, Cow No. 7877); (e) late cyclic activity followed by conception (Fig. 6, Cow No. 5156 ).

SHORT LUTEAL PHASES AND OVULATIONS WITHOUT OESTRUS IN COWS 2"?

16. Cow Noo 5 1 5 8

14.

.~ 12.

& 1o.

4. SIP

2. F"

0 . . . . . . . . . . . . . . . I I I 3D 45 GO 76 90 105 1:!0 135 16D 165 180 195 210 1~5 240 ~!55 270 ~lE5 300

PosLporturn period (doys)

Fig. 6. Patterns of plasma progesterone profiles in a Borana cow with an c×tended postpartum anoestrus followed by pregnancy (SLP, short ]utea] phase: E, oestrus).

DISCUSSION

The interval from calving to complete uterine involution was considered normal and was in agreement with the findings of Eduvie ( 1985 ). The latter study showed that short luteal phases and ovulation without oestrus are im- portant components of postpartum reproductive activity in Borana cattle. The high incidence of ovulations without oestrus found in the present study is in agreement with earlier findings (Rakha and Igboeli, 1971; Ozsar et al., 1986; Dawuda et al., 1989). In large herds of Bos indicus cows, 44.3% of the ovula- tions which occurred during the first 60 days postpartum were not accom- panied by behavioural signs of oestrus (Kidder et al., 1952 ). In Zebu cattle, Dawuda et al. ( 1989 ) and Mukasa-Mugerwa et al. ( 1991 b) reported that 29% and 33%, respectively, of the cows studied had ovulations without oestrus during the first 90 days postpartum.

Short luteal phases were detected in 55.6% of the cows and ovulations with- out oestrus occurred in all cows which cycled. Extended postpartum anoes- trus was also detected in 36% of the cows. A high incidence of ovulations without oestrus and long anoestrous periods were therefore the major con- tributors to the extended calving intervals previously reported for Zebu cattle (Oyedipe et al., 1982; Dawuda et al., 1988b; Mukasa-Mugerwa et al., 1991 a ). A number of factors such as nutrition, suckling, season of calving, age and parity of the cow influence the interval from calving to onset of first oestrus

28 A. TEGEGNE ET AL.

(Eduvie, 1985; Dawuda et al., 1988a,b; Randel, 1990; Williams, 1990; Mu- kasa-Mugerwa et al., 1991 a).

However, the length of the postpartum anoestrous interval also depends on the method and efficiency of oestrus detection. Although a high incidence of ovulations without oestrus has been reported in Zebu cattle by other authors (Rakha and Igboeli, 1971; Dawuda et al., 1989; Mukasa-Mugerwa et al., 1991 b), most data in the literature on the postpartum anoestrous interval are based on the interval from calving to onset of the first behavioural oestrus (Eduvie, 1985; Dawuda et al., 1988a,b; Mukasa-Mugerwa et al., 1991 a). This may be significant from a management point of view, but could be misleading in physiological aspects.

In the present study, plasma progesterone profiles revealed that Borana cows were capable of commencing ovarian activity as early as Day 30 postpartum. However, in some cows, detection of the first behavioural oestrus was delayed as late as Day 120 postpartum or even longer. A relatively higher percentage of oestrus periods was missed even after employing continuous visual obser- vation by herdsmen and use of teaser bulls. This could be owing to failure to detect oestrus, complete absence of manifestation of behavioural oestrus, low intensity of oestrus manifestation or low efficiency and poor libido of the teaser bulls used to detect cows in oestrus.

All the ovulations without oestrus detected in this study were accompanied by normal functions of corpora lutea as revealed by plasma progesterone pro- files. However, the complete absence of external signs of oestrus and lack of receptivity to the bull in these cows show the extent of the difficulties in- volved in postpartum oestrus detection in Zebu cattle. Moreover, the high incidence of ovulations without oestrus could be a physiological adjustment whereby the animal may defer acceptance of the bull until the time that she is physiologically and physically capable of conception and maintenance of pregnancy. Even if detection of ovulations without oestrus were made possi- ble through the use of more sophisticated and reliable methods as suggested by Dawuda et al. ( 1989 ), data on the success of matings on ovulations with- out oestrus are not available and further studies of this aspect are necessary from both physiological and economic points of view.

Hansel and Alia (1984) reported that inadequate oestrus detection is one of the main causes of anoestrus in Zebu cattle in tropical regions. The oestrus period in Zebu cows is often of short duration and some cows even refrain from repeated mounting while others do not manifest external signs of oes- trus at all (Mattoni et al., 1988 ). Our observation that all cows had at least one regular oestrous cycle prior to the first observed oestrus is supported by an earlier report by Schams et al. ( 1978 ) who found that at least 50% of Bos indicus cows had ovulation without oestrus before showing the first behav- ioural oestrus.

The rate of occurrence of short luteal phases is in agreement with earlier

SHORT LUTEAL PHASES AND OVULATIONS WITHOUT OESTRUS IN COWS 2 9

reports (Schams et al., 1978; Eger et al., 1988 ). Moreover, the lower proges- terone concentration in short luteal phases than in normal cycles indicates inadequate corpus luteum formation as suggested earlier (Schams et al., 1978). Although the early demise of the first corpora lutea has been attrib- uted to insufficient blood supply or luteinising hormone (LH) availability (Niswender et al., 1976) or the inability of the corpora lutea to respond to gonadotropin stimulation (Manns et al., 1983), a number of studies have shown that it results from the effect of premature release of prostaglandin F2,~

from the uterus (Peter et al., 1989; Lishman and Inskeep, 1991; Cooper et al., 1991 ). However, short luteal phases appear to be a normal physiological phe- nomenon and do not indicate ovarian disorders as suggested by Eger et al. (1988).

The various patterns of postpartum reproductive characteristics identified in the present study using plasma progesterone profiles agree with the work of Mukasa-Mugerwa et al. (199 lb) who monitored progesterone profiles in Arsi (Zebu) cows. In these cows, inadequate oestrus detection, extended postpartum anoestrus, repeat breeders and early embryonic mortality were identified as the major contributors to reproductive wastage. Similarly, Wells et al. (1986) reported that the extended postpartum anoestrous interval is a major obstacle in attempts to improve Zebu cow reproductive efficiency. Moreover, Sreenan and Diskin (1983) showed that early embryonic mortal- ity and foetal deaths account for 75% of reproductive wastage following a single breeding in cattle.

In conclusion, although short luteal phases occurred in a relatively larger proportion of cows, it was not an essential part of ovarian function for the establishment of normal oestrous cycles during the postpartum period. Fur- thermore, the commencement of ovarian cyclicity in 78% of the cows before Day 63 postpartum suggests that ovulations without oestrus rather than post- partum anoestrus could be more responsible for extended calving intervals in Zebu cows. The physiological basis and significance of short luteal phases and ovulations without oestrus in Zebu cow reproduction need further investiga- tions with an objective of improving overall herd reproductive performance.

ACKNOWLEDGEMENT

The authors thank Hirut Mulugeta and Kassahun Zewdie for data and sam- ple collection, Girma Abele for performing progesterone assays and J. Sher- ington and Amare Atale for the statistical analysis.

REFERENCES

Cooper, D.A, Carver, D.A., Villeneuve, P., Silvia, W.J. and lnskeep, E.K., 1991. Effects of pro- gestagen treatment on concentrations of prostaglandins and oxytocin in plasma from the posterior vena cava of post-partum beef cows. J. Reprod. Fertil., 91:411-421.

30 A. TEGEGNE ET AL.

Dawuda, P.M., Oyedipe, E.O., Pathiraja, N. and Voh, Jr., A.A., 1988a. Serum progesterone concentrations during the postpartum period in indigenous Nigerian Zebu cows. Br. Vet. J., 144:253-257.

Dawuda, P.M., Eduvie, L.O., Esievo, K.A.N. and Molokwu, E.C.I., 1988b. Interval between calving and first observable oestrus in postpartum Bunaji cows. Br. Vet. J., 144:258-261.

Dawuda, P.M., Eduvie, L.O., Esievo, K.A.N. and Molokwu, E.C.I., 1989. Silent oestrus mani- festation in Nigerian Bunaji Zebu cows. Anita. Reprod. Sci., 21:79-85.

Eduvie, L.O., 1985. Factors affecting postpartum ovarian activity and uterine involution in Zebu cattle indigenous to Nigeria. Anim. Reprod. Sci., 8:123-128.

Eger, S., Shemesh, M., Schindler, H., Amir, S. and Foote, R.H., 1988. Characterization of short luteal cycles in the early postpartum period and their relation to reproductive performance of dairy cows. Anim. Reprod. Sci., 16:215-224.

Entwistle, K.W., 1983. Factor influencing reproduction in beef cattle in Australia. Proc. Aust. Soc. Anim. Prod., 13:61.

Galina, C.S. and Arthur, G.H., 1990. Review on cattle reproduction in the tropics. Part 4. Oes- trous cycles. Anim. Breed. Abstr., 58:697-707.

Hansel, W. and Alia, H.W., 1984. Causes of postpartum anoestrus in cattle in the tropics. Proc. Consult. Meet. Int. Atomic Energy Agency 13-16 June 1983, Vienna, International Atomic Energy Agency, pp. 21-40.

Kidder, H.E., Barrett, G.R. and Casida, L.E., 1952. A study of ovulations in six families of Holstein-Friesians. J. Dairy Sci., 35:436-444.

King, G.J., Hurnik, J.F. and Robertson, H.A., 1976. Ovarian function and estrus in dairy cows during early lactation. J. Anim. Sci., 42:688-692.

Lamming, G.E. and Bulman, D.C., 1976. The use of progesterone radioimmunoassay in the diagnosis and treatment of subfertility in dairy cows. Br. Vet. J., 132:507-517.

Lishman, A.W. and Inskeep, E.K., 1991. Deficiencies in luteal function and reinitiation of cyclic breeding activity in beef cows and in ewes. Rev. S. Afr. J. Anim. Sci., 21 (2):59-76.

Manns, J.G., Humphrey, W.D., Flood, P.F., Mapletoft, R.J., Rawlings, N. and Cheng, K.W., 1983. Endocrine profiles and functional characteristics of corpora lutea following onset of postpartum ovarian activity in beef cows. Can. J. Anim. Sci., 63:331-347.

Mattoni, M., Mukasa-Mugerwa, E., Cecchini, G. and Sovani, S., 1988. The reproductive per- formance of East African Zebu (Bos indicus) cattle in Ethiopia. 1. Estrous cycle length, du- ration, behaviour and ovulation. Theriogenology, 30:961-970.

Mukasa-Mugerwa, E. and Tegegne, A., 1989. Peripheral plasma progesterone concentrations during pregnancy in Bos indicus (Zebu) cows in Ethiopia. Reprod. Nutr. Dev., 29: 303-308.

Mukasa-Mugerwa, E., Tegegne, A., Mesfin, T. and Teklu, Y., 199 la. Reproductive efficiency of Bos indicus (Zebu) cows under artificial insemination management in Ethiopia. Anim. Re- prod. Sci., 24:63-72.

Mukasa-Mugerwa, E., Tegegne, A. and Ketema, H., 1991b. Patterns of postpartum oestrus on- set and associated plasma progesterone profile in Bos indicus cows in Ethiopia. Anim. Re- prod. Sci., 24:73-84.

Niswender, G.D., Reimers, T.J., Diekman, M.A. and Nett, T.M., 1976. Blood flow: a mediator of ovarian function. Biol. Reprod., 14:64-81.

Oyedipe, E.O., Buvanendran, V. and Eduvie, L.O., 1982. Some factors affecting the reproduc- tive performance of White Fulani (Bunaji) cattle. Trop. Agric. (Trinidad), 59:231-234.

Ozsar, S., Guven, B., Izgur, H., Emre, Z. and Cosar, S., 1986. Some managerial factors affecting the reproductive performance of dairy cows in Turkey. In: Nuclear and Related Techniques in Animal Production and Health. Proc. Symp., Vienna, 17-21 March 1986, FAO/IAEA, Vienna, pp. 621-622.

Peter, A.T., Bosu, W.T.K., Liptrap, R.M. and Cummings, E., 1989. Temporal changes in serum

SHORT LUTEAL PHASES AND OVULATIONS WITHOUT OESTRUS IN COWS 3 I

prostaglandin F2,~ and oxytocin in dairy cows with short luteal phases after the first postpar- tum ovulation. Theriogenology, 32:277-283.

Rakha, A.M. and Igboeli, G., 1971. Effects of nutrition, season and age on the oestrous cycle of indigenous Central African cattle. J. Anim. Sci., 32:943-945.

Randel, R.D., 1990. Nutrition and postpartum rebreeding in cattle. J. Anim. Sci., 68:853-862. Salisbury, G.W., VanDemark, N .L and Lodge, J.R., 1978. Silent or quiescent estrus. In: Physi-

ology and Reproduction and A.I. of Cattle, 2nd edn. Freeman, San Francisco, ('A, pp. 661 662.

Schams, D., Schallenberger, E., Menzer, Ch., Stangl, J., Zottmeier, K., Hoffmann, I3. and Karg, H., 1978. Profiles ofLH, FSH and progesterone in postpartum dairy cows and their relation- ship to commencement of cyclic functions. Theriogenology, 10:453-467.

Sreenan, J.M. and Diskin, M.G., 1983. Earl~ embryonic mortalit~ in the cow: Ils relationship with progesterone concentration. Vet. Rec., 112:517-521.

Statistical Analysis Systems Institute, 1987. SAS Users Guide: Statistics. SAS Institute. (ar~_. NC.

Wells, P.L, Holness, D.H., McKabe, C.T. and Lishman, A.W., 1986. Fertility in the Africaner :-ow 3. Once a day suckling and its effect on the pattern of resumption of ovarian activit', and conception rate in early lactation. Anim. Reprod. Sci., 12:1-12.

Williams, G.L., 1990. Suckling as a regulator" of postpartum rebreeding in cattle. -X review ,r ~Xnim. Sci., 68:831-852.