Management and nutrition strategies to reduce the breeding season in beef cows

Management and nutrition strategies to reduce the breeding season in beef cows

Centro de Energia Nuclear na Agricultura CP 96 - CEP 13416-000 Piracicaba, Sao Paulo, Brasil

* Department of Animal and Poultry Science, University of Guelph, Ontario, N1G 2W1 Canada

Effects of temporary calf removal and phosphorus supplementation on conception rate in 60 or 120 day breeding seasons were observed in several trials conducted over three years on substantial numbers of Nellore cows. Experiment 1 involved 47 acyclic Nellore cows with suckling calves ranging in age from 55 to 70 days. Calves were temporarily removed from their dams for 48 h at the beginning of the trial or stayed with the dams throughout the trial. The proportion of cows that cycled during the breeding season was 5 out of 25 (20%) in the control group and 15 out of 22 (68%) in the group whose calves had been removed (X2=4.66, P<.05). In Experiment 2, 66 acyclic Nellore cows, averaging 60 days postpartum were allocated at random in three groups as follows: A=calves temporarily removed from their dams for 48 h on the first day of the breeding season; B=similar to group A, except that calves were removed for 72 h; and C=control group (no calf removal). The percentage of pregnant cows at the end of the breeding season was 54.6 in the control treatment compared with 50.0 in the 48 h removal group and 63.6 in the 72 h removal group. In Experiment 3, 75 pregnant cows in the final trimester of gestation were allocated at random in four groups: A=the cows received free choice of a mineral mixture with 12% P during three months before and three months after the calving season with the calves temporarily removed on the first day of the breeding season for 96 h; B=group A, except that the calves were not removed; C=group A, except that the mineral mixture had 8.8 % P; and D=group C, except no calf removal. More cows receiving 12% P were cycling (P<.05) at 30, 60 and 90 days into the breeding season. Under the conditions in these trials, restricted suckling for 48, 72 or 96 h prior to breeding season caused inconsistent results on pregnancy rates. However, the restricted suckling of cows with marginal range plasma phosphorus (<4.0 mg%) enhanced ovarian function.

KEY-WORDS: Reproduction, Nellore cows, restricted suckling, phosphorus supplementation, ovarian activity.

The cattle population in Brazil exceeds 145 million (Cardelino and Castro 1987). Most of these animals are maintained under extensive management and graze native pastures without additional supplemental feeding, so that many show energy, protein and mineral deficiencies (De Brum et al 1987; Pott et al 1987; Santiago et al 1986). Current production efficiency is around 50-55% annual calf crop. If this could be increased to 70%, an additional 6 or 7 million calves would be born each year.

Most beef cattle in Sao Paulo State and throughout the country are Nellore, a breed that is adapted to survive in tropical climates, but demonstrates late maturity, short estrous periods and long intervals of postpartum acyclicity (Galina and Arthur 1989). Calving and weaning results collected over a prolonged period indicate that this genotype is more productive than several other locally popular breeds (Mereilles et al 1991).

Beef calves running with their dams contribute to delayed postpartum ovarian activity and reduce reproductive efficiency. A number of reviews emphasize the serious extent of this factor affecting postpartum acyclicity in suckled cattle. Weaning the calf would be an obvious technique to hasten oestrus (Carter et al 1980; Oxenreider 1968; Short et al 1972). Interrupted suckling by removing calves during a 48, 72 or 96 h period, in the second month postpartum, is one reported method which provides stimulation of the ovarian activity (Forrest et al 1979). However, 72 h calf removal does not always improve postpartum reproductive performance (Bonavera et al 1990).

The aims of this study were: i) To observe the effects of temporary calf removal from acyclic cows on the onset of ovarian activity and subsequent conception rate; ii) To observe the effects of pre- and post-partum phosphorus supplementation on the onset of ovarian activity and conception rate: and iii) to monitor body condition score and body weight changes during the postpartum and rebreeding period.

This study was carried out during the period from November 1991 to March 1992 at a private 600 ha ranch situated in the central area of the Sao Paulo State. Sequential plasma progesterone profiles, compiled during ten weeks immediately after parturition, were used to select 47 acyclic Nellore cows with suckling calves ranging in age from 55 to 70 days. Cows were divided into two groups: T=Treated group (n=22): the calves were temporarily removed from their dams for 48 h at the beginning of the trial. C=Control group (n=25): the calves stayed with the dams throughout the trial.

Cows in both groups were exposed continuously to bulls, previously evaluated for breeding soundness over a 60 day period (bull:cow ratio was 1:15). Animals grazed an 80 ha grass pasture (Brachiaria decumbens) with continuous access to water and mineral supplement in feeders without rain protection.

Animals were evaluated over 4 months for live weight change, plasma inorganic phosphorus (Fiske and Subbarow 1925) and glucose (Miller 1959). Ovarian activity was monitored by determining plasma progesterone concentration in blood samples collected twice per week using the RIA kit from the IAEA. Blood sampling continued throughout the breeding season. Pregnancy diagnosis was confirmed by rectal palpation 60 days after the end of the breeding period.

The chemical composition of the pastures (AOAC 1976) and digestibility (Orskov and McDonald 1979) were evaluated monthly and P content in mineral salt was also analyzed.

Continuous data were analyzed by ANOVA and multivariate analysis of variance. Chi square analysis was used to evaluate the conception rate (Freund et al 1986).

In the same private ranch, 66 Nellore cows, averaging 60 days postpartum and diagnosed as acyclic by sequential plasma progesterone concentrations, were selected from a group of 100 animals. Blood samples were collected twice a week through the breeding season for determination of plasma progesterone concentration using the FAO/IAEA kit. Resulting values were plotted to provide sequential progesterone profiles which indicated the presence or absence of ovarian cyclicity. Only cows with critical plasma phosphorus concentrations (<4.0 mg%) were included in this trial.

A: calves temporarily removed from their dams for 48 h on the first day of the breeding season (n=22).

The three groups were maintained on the same grass pasture (Brachiaria humidicula) separated by fencing. Glucose (AOAC 1976) and P (Fiske and Subbarow 1925) were determined at 60, 90, 120 and 150 days postpartum. Body condition score (BCS) on a 1 to 5 scale and live weight (BW) were measured at 60, 90, 120 and 150 days postpartum.

In the same ranch, 75 pregnant cows were selected in their final trimester of gestation. Cows were allocated at random in four groups.

A: the cows (n=19) received a free choice mineral mixture with 12%P during three months before and three months after the calving season. Calves were temporarily removed from their dams on the first day of the breeding season for a period of 96 h.

Cows in all treatments were bled two consecutive weeks before and after the breeding season (4 months) to determine plasma progesterone concentration using the FAO/IAEA kit. Blood samples were collected monthly for glucose (Miller 1959), P (Fiske and Subbarow 1925) and total plasma protein (Gornal and Bardiwill 1949) analyses.

Chemical composition of the mineral salt, digestibility, determined by the nylon bag technique and crude protein content of pasture were evaluated at the beginning and end of the breeding season. Dams were weighed and body condition scored (scale 1 to 5) at monthly intervals during the study.

Multivariate repeated-measures analysis of variance was used to test the differences between reproductive and metabolic parameters. The differences between cyclic and acyclic cows were analyzed by Chi square test.

Table 1: Chemical composition and digestibility (% DM basis) of pasture during the breeding season (Trial 1).

Month	Ash	N x 6.25	P	DM digest


December	7.12	8.80	0.19	48.9

January	7.65	7.04	0.17	53.3

February	7.41	4.22	0.09	42.3

March	7.10	4.10	0.05	43.8

The chemical composition of pasture throughout the observation period is shown in Table 1. Nitrogen and P content declined about 50% and digestibility went down about 10 units suggesting that the pasture quality decreased as the trial progressed.

A cow of 450 kg bodyweight requires 0.23% P in dietary dry matter (about 18 g P/day). Both the mineral mixture and pasture showed low P level. In this study the P level in pasture dry matter was about 0.12% which represents only 67% of the P requirement. Considering an intake of 10 kg DM/day, the animal would be supplied with only 12 g P/day. Since the P content of the mineral salt was only 1.53%, it is unlikely that any of the cows would consume sufficient to satisfy their requirements. This is a common situation in many areas of Brazil where producers often fail to provide an adequate mineral supplement (De Brun et al 1987; Pott et al 1987). Phosphorus concentration in plasma of both groups was 3.82 mg% in December at the beginning of the observation period, but the calf removal group showed higher mean values at the end (3.99"0.2 vs. 3.11"0.2 mg%; P<0.05). However, values in both groups were in the marginal range (Thompson 1978). This may be reflecting the low P content in the pasture during the last two months.

In cattle grazing low quality forage, supplementation with P did not improve reproductive performance or weight gains Sasser et al 1988; Teleni et al 1977) unless the cattle received sufficient protein (Holroyd et al 1977). However, in a borderline P deficient area in Australia, P supplementation improved reproductive performance (NRC 1984).

Data for blood glucose did not show significant differences between the treatments. However, the concentration was low in both groups during the 60 days breeding season (59.9"1.1 mg%) but increased to 103"7.2 mg% during the next five months. The absence of feed supplementation probably increased the period of negative energy balance as revealed by the glucose profile. However, live weight changes during the 4 months breeding period were 11.6 and 10 kg for the control and the treated group respectively.

The proportion of cows that cycled during the breeding season was 5 out of 25 (20%) in the control and 15 out of 22 (68%) in the treated group (Chi-2=11.1, P<.001). True pregnancy rate was 20% in the control and 55% in the calf removal groups (Chi-2=4.66, P<.05). Calf removal substantially increased the reproductive performance in this trial but the results must be interpreted cautiously because the number of cows per treatment was small.

Body weight and condition changed somewhat during the observation period. However, analysis of variance using square root transformation of bodyweight and body condition scores indicated that the changes during the 150 day post partum period were not different (P>.05).

In this trial, calf removal for 48 or 72h did not improve the postpartum reproductive performance. The percentage of pregnant cows at the end of the breeding season was 54.6 in the control cows compared with 50.0 in the 48h removal group and 63.6 in the 72h removal group. The relatively small number of cows per treatment group may have prevented the demonstration that the pregnancy rate recorded after 72 h was actually higher than the control value.

Table 2: Digestibility, dry matter and phosphorus content of the pasture in Trial 3

Period of
collection	Phosphorus	DM digest

Before calving	0.1±0.1	43.1±3.7

After calving	0.1±0.1	32.9±3.0

Data on chemical composition and digestibility of the pasture (Table 2) show that P was low and digestibility declined throughout the experimental period. There was no treatment effect on Body Condition Score (Table 3), the metabolic profile was not affected and plasma inorganic P in all groups was within the normal range according to Thompson (1978).

Although there was no significant difference in plasma P values detected in this trial, animals with more P in their diet showed higher concentrations. In non-lactating, feed-restricted beef cows, estrous cycles ceased when the condition score fell below 3.5 on a 1 to 9 assessment scale (Richards et al 1989). In other trials similar severe reduction in bodyweight in cattle caused quiescent ovaries and cessation of estrous cycles (Fordyce et al 1990).

Table 3: Least square means and standard error for plasma protein, plasma P and body condition score (BCS) at the end of the breeding season and body weight (BW) (Trial 3).

Calf removal	Yes	No	Yes	No

P in mineral mix (%)	12	12	8.8	8.8
Plasma protein (g %)	7.55±0.27	7.27±0.23	7.65±0.27	7.41±0.29

Plasma P (mg %)	5.13±0.14	5.04±0.20	4.64±0.23	4.94±0.16

BCS (1-5)	2.7±0.05	2.7±0.05	2.7±0.05	2.6

Bodyweight (kg)
Start breeding season	377±12	388±13	381±15	361±11
At 120 days	378±11	379±9.8	375±13	368±8.5

Significant differences were detected between treatments A, B, C and D for the percentage of cows cycling by 30, 60 and 90 days in the breeding season and between groups that received 8.8% or 12% P. However, after the 120d breeding season there were no differences in pregnancy rate among the groups A, B, C, and D (Table 4). It is concluded that a higher level of P in the mineral mixture increases the pregnancy rate and plasma P values, showing the importance of this element (Read et al 1986).

Table 4: Acyclic and cyclic cows at 30 days into the breeding season and actual pregnancy rate.

Calf removal	Yes	No	Yes	No

P in mineral mix (%)	12	12	8.8	8.8

--	----------	-%------	---------	--
Acyclic cows*	42	50	76	79
(n)	8	10	13	15

Cyclic cows*	58	50	24	21
(n)	11	10	4	4

Pregnancy rate**	84	70	59	79
(n)	16	14	10	15

AOAC 1976 Association of Official Analytical Chemists, Edn 12, Washington DC

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