Archivos Latinoamericanos de Producción Animal. 2022. 30 (3)

Modelling the reproductive performance of tropical beef herds using long-term

experimental grazing data on Urochloa humidicola pastures

in the Llanos of Colombia

Received: 2021-10-06. Accepted: 2022-03-30

1Corresponding author: Consultant, rvi.2005@gmail.com; 2 Norte 443, Viña del Mar, Chile

2CR Eco-efficient Agriculture Consultancy (CREACTM), Queensland, Australia.

3Formerly International Center for Tropical Agriculture (CIAT), Valle del Cauca, Colombia.

225

Abstract. The long-term reproductive performance of beef breeding herds grazing sown pastures in the neotropical

savanna environment of Eastern Colombia (Llanos) and the lifelong reproductive consequences of raising

replacement heifers subject to different growth regimes are not well documented. The aim of the study was to

assess the effects of live weight (LW), age, and their interaction on conception probabilities of Bos indicus

replacement females by analyzing two seven- and eight-years lasting experiments that used commercial Brahman

heifers subjected initially to different growth rates on Urochloa humidicola pastures. The experiments included two

periods, namely the growth period from weaning until 270 kg of LW during which heifers were subjected to three

stocking rates (1.28; 1.71; 2.24 head/ha respectively) to attain different gain rates, and the subsequent reproduction

phase subject to common grazing for all growth treatments. The conception data fitted well logistic regressions in

which LW and age were the predictor variables during the first two conceptions, but the relationship disappeared

in subsequent conceptions. Prediction equations compared well with extensive data from northern Australia that

showed similar trends. The LW of adult cows only increased consistently during pregnancy but following weaning

their LWs returned to the initial low LWs attained at conception. Weaned calves as percentages of those born were

88 % for the two treatments with higher LW gains during the growth period and 81 % for animals with lower

weight gain. Calving intervals were 19-20 months across treatments. Liveweight and its interaction with age had a

close, logistical relationship to the occurrence of the first two conceptions but not thereafter.

Keywords: animal performance, reproduction, savanna, tropical pastures

Modelación del desempeño reproductivo de ganado de carne tropical considerando datos

experimentales de pastoreo de largo plazo en Urochloa humidicola en los Llanos de Colombia

Resumen. El desempeño reproductivo de largo plazo de hatos de carne pastoreando gramíneas sembradas en el

ambiente de las sabanas neotropicales del oriente de Colombia no ha sido bien documentado, así como el impacto

de ganancias de peso en novillas de reemplazo en su desempeño reproductivo posterior. Datos de dos experimentos

de siete y ocho años respectivamente utilizando novillas comerciales Brahman (Bos indicus) sometidas a diferentes

tasas de crecimiento (con cargas de 1.28; 1.71; 2.24 animales/ha) pastoreando Urochloa humidicola hasta alcanzar 270

kg de peso fueron reanalizados para determinar el impacto del peso, edad y su interacción sobre la probabilidad de

concepción durante la fase adulta bajo pastoreo común. Los datos se ajustaron bien a regresiones logísticas durante

las primeras dos concepciones, pero no posteriormente. Las ecuaciones de predicción mostraron tendencias

comparables con abundantes observaciones del norte de Australia. El peso de vacas adultas sólo aumentó debido a

la gestación, pero al destete regresó al peso inicial de concepción. En promedio, a lo largo de los experimentos, los

porcentajes de destete en relación a los terneros nacidos vivos fueron de 88 % para las novillas que experimentaron

las mayores ganancias de peso, y 81 % para las de menor ganancia. Los intervalos entre partos fueron de 19-20

meses sin diferencias entre tratamientos. El peso vivo y su interacción con la edad tuvieron una relación logística

estrecha con la ocurrencia de las dos primeras concepciones, lo que no se repitió en concepciones posteriores.

Palabras clave: desempeño animal, pasturas tropicales, reproducción, sabana

International Center for Tropical Agriculture, CIAT, Cali, Valle del Cauca, Colombia.

Raúl R Vera1,3 Carlos A. Ramírez Restrepo2,3

www.doi.org/10.53588/alpa.300307

226 Vera y Ramirez

The extensive Neotropical savannas (Llanos) of

Colombia's Orinoco River basin have long been

expected to contribute to food production, mainly

grain, vegetable oils, and beef production (Rodríguez

Borray and Cubillos, 2019). Nevertheless, during the

last decade and contrary to initial expectations, many

efforts to expand crop and plantation production had

limited success due to policies, input/output prices,

and soil constraints. Thus, most producers in the well-

drained, relatively flat, and tillable Llanos, located

south of the Meta River, have partially returned to

traditional beef breeding systems subject to a modest

degree of intensification (Rodríguez-Borray and

Cubillos, 2019). Surprisingly, despite the commercial

release of new productive and nutritive grass cultivars,

breeding herds continue grazing older Urochloa

humidicola [Uh; cultivars Tully (CIAT 679) and Llanero

(CIAT 6133; syn B. dictyoneura)], and U. decumbens (Ud;

syn. Brachiaria decumbens) germplasm, combined with

variable areas of native savanna depending upon

location, topography, and the proportion of tillable

soils in the farm (Romero et al., 2018; Rodríguez-Borray

et al., 2019; Enciso et al., 2022). In this context, although

the growth of replacement heifers and cows'

reproductive performance (RP) in native savannas has

been documented for decades (Levine et al., 1980;

Stonaker et al., 1984; Rivera, 1988; Vera and Seré, 1985;

Pérez et al., 2017), the long-term RP of Brahman (Bos

indicus) or Brahman crossbred replacement heifers and

cows grazing sown grasses are still poorly quantified.

A symposium (SSAESD, 2005) reviewed in detail

the physiological mechanisms underlying repro-

duction in adapted B. indicus breeds and crosses in the

southern USA, and it included a comprehensive

literature review by Randel (2005), who concluded that

there are subtle differences with the RP of B. taurus

that deserve special attention. Moreover, Ferrell et al.

(2005) did not find differences in feed efficiency

between B. indicus and taurus cattle in feedlot

conditions, but Forbes (2005) noted differences in the

size of the digestive tract and passage rates, and

grazing behavior was affected by perceived heat stress.

Overall, year-round grazing of tropical cattle is heavily

influenced by environmental conditions, since pasture

growth and quality vary greatly between seasons and

years (Durmic et al., 2017) and also in response to

continually changing grazing pressure as live weights

(LWs) vary with time (Stuth et al., 1996; Ramírez-

Restrepo and Vera, 2019; Ramírez-Restrepo et al.,

2020). In the extensive grazing conditions of northern

Australia, the above factors led to 22 % losses of

suckling calves (Schatz, 2011), a wide range (53 % to 76

%) of weaning rates (Fordyce et al., 2013), and low (25

%) re-conception rates in 3-year-old lactating animals

(Schatz, 2011). In the Llanos, Florez (2015) reported

calving intervals of un-supplemented B. indicus

females between the 2nd and 5th calving range of 326

and 596 open days at Carimagua Research Centre

(CRC). Under these conditions and lacking improved

feed strategies, the introduction of costly reproductive

technologies is futile, as shown by Jimenez-Rodriguez

and Manrique Perdomo (2018) in on-farm tests in the

Colombian Vichada Department. More recently, Pérez

et al. (2019) showed improved cows' performance and

reduced mortality at CRC when silage of annual crops

was fed to animals browsing and grazing silvopastoral

systems, at the cost of assumed larger GHG emissions.

Introduction

Modelagem do desempenho reprodutivo de bovinos de corte tropicais considerando dados

pastoreio experimental de longa duração em Urochloa humidicola nos Llanos da Colômbia

Resumo. O desempenho reprodutivo a longo prazo de bovinos de corte mantidos em pastejo direto em gramíneas

cultivadas na região da savana neotropical do leste da Colômbia foi pouco documentado, bem como o impacto do

ganho de peso de novilhas de reposição no seu desempenho reprodutivo posterior. Dados de dois experimentos de

sete e oito anos com novilhas Brahman comerciais (Bos indicus), submetidas a diferentes taxas de crescimento

(mediante lotações de 1.28; 1.71; 2.24 animais/ha) pastando Urochloa humidicola até atingir 270 kg de peso vivo

foram reanalisados para determinar o peso, a idade e a interação na probabilidade de concepção durante a fase

adulta mantida em sistema de pastejo direto. Os dados de concepção se ajustaram bem a regressões logísticas

durante as duas primeiras concepções. As equações de previsão mostraram tendências comparáveis às que foram

encontradas em observações registradas no norte da Austrália. O peso das vacas adultas aumentou de forma

sistemática com a gestação, mas ao desmame voltou ao peso inicial da concepção. Em média ao longo dos

experimentos, as porcentagens de desmame em relação aos bezerros nascidos vivos foram de 88 % para as novilhas

que tiveram os maiores ganhos de peso e de 81 % para aquelas com os menores ganhos. Os intervalos entre partos

foram de 19-20 meses no conjunto dos tratamentos. O peso vivo e a idade tiveram estreita relação logística com a

ocorrência das duas primeiras concepções, o que não se repetiu nas concepções posteriores.

Palavras-chave: desempenho animal, pastagens tropicais, reprodução, savana

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227

Beef cattle reproduction on Urochloa humidicola

Materials and Methods

Database

The dataset gathered includes two grazing

experiments conducted between 1985 and 1997 at CRC

(4°36'44.6"N, 74°08'42.2"W) on the Neotropical

savannas of the Colombian Meta Department. The

empirical strategy led to new data analyses and

interpretation to extend the boundaries of Vera (1991)

and Vera et al. (1993). The tactical use of existing long-

term data following newer analytical approaches is a

relevant procedure to overcome the high cost of large

grazing experiments (Tedeschi, 2019; Nizar et al., 2021).

This is particularly applicable when the studied

production system remains current (Rodríguez-Borray

and Cubillos, 2019; Enciso et al., 2022). Climatic

conditions at CRC over the selected experimental

period were reported previously by Vera and Ramírez-

Restrepo (2017).

Experiment 1

The trial lasted for 8 years and extended from the

time heifers weaned on savanna were introduced to

the trial on Uh, until soon after the 4th calving when it

was discontinued due to administrative reasons,

together with the second experiment. This first

experiment involved raising 50 heifers [165 ± 2 kg

(least square mean ± s.e.m) and 410 ± 17 days of initial

age] until reaching a target mating LW (270 kg)

subjected to low (L, 1.28; n = 16), medium (M, 1.71; n =

17), and high (H, 2.24; n = 17) stocking rates (SRs,

head/ha) to provide a comprehensible range of

growth rates.

A second low SR (L-2) with 15 heifers [174 ± 1.5 kg;

531 ± 4 days] was implemented one year later and

lasted for 7 years to provide a temporal replicate of the

SR with the highest expected daily LW gain (DLWG).

The SRs were chosen based on the results of a series of

three earlier experiments using yearling steers on Uh

(Tergas et al., 1982). Upon reaching the target LW, all

heifers were transferred to another Uh pasture under

common grazing and stocked at 1.28 head/ha.

Animals were mated (April-November; wet season)

with purebred Brahman bulls regularly rotated at 3: 42

female ratios while calves were weaned at a target age

of 270 days.

Experiment 2

The second experiment lasted for 5.9 years, starting

two years after the first one. During the growth phase

on Uh, it replicated the SRs of Experiment 1. Following

attainment of the 270 kg target, heifers were randomly

split to a single Uh pasture, or to a well-maintained Ud

pasture stocked at 1.2 head/ha and mated as above.

Thus, the experimental structure followed a classical

compensatory growth design (Wilson and Osbourn,

1960), where compensatory periods were identified as

high (h) for the Ud, and low (l) for the Uh pasture

treatments, respectively.

Animal and pasture management

In all cases, pastures were at least 3-years-old when

the experiments began, thereby avoiding the flush of

growth common to newly sown forages. During the

establishment phase, 20 P, 20 K, 48 Ca, 14 Mg, and 10 S

kg/ha were applied, while a third of that dosage was

used every 3 years for maintenance. The pastures were

subjected to continuous grazing and occasional

sampling complemented with a visual appraisal by

experienced technicians and researchers verified that

forage on offer stayed within the range of 1-3 t DM/ha

during the rainy season, depending upon the SR

implemented during the growth period. The

conditions of Uh pastures in the present experiments

have been thoroughly documented (Cajas et al., 1985;

Velásquez Valbuena, 1991; Ramírez-Restrepo and

Vera, 2019). The chemical composition of the forage on

offer conformed to the well documented low nutritive

value of Uh (Lascano, Hoyos, and Velasquez, 1982;

Pérez and Lascano, 1992) and the higher quality of Ud

(Lascano and Euclides, 1996) at CRC. Their nutritive

value varied little across the SRs used in the

experiments, regardless of forage availability (Cajas et

al., 1985). However, essential nutrients decreased

markedly during the dry season, leading to modest

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Nevertheless, Pérez et al. (2019) expressed reservations

about the long-term sustainability of the system.

The main hypothesis of this study was that the LWs

and/or age of Brahman heifers, determined by their

earlier growth rates, may serve as useful empirical

predictors of their conception probabilities and can

therefore be represented in a dynamic simulation

model. Furthermore, the initial growth rates have

consequences in their later mature reproductive life.

The aim of the study was to assess the effects of live

weight (LW), age, and their interaction on conception

probabilities of Bos indicus replacement females by

analyzing two seven- and eight-years lasting

experiments that used commercial Brahman heifers

subjected initially to different growth rates on Urochloa

humidicola pastures.

228

Results

Vera y Ramirez

Growth period

Detailed analyses of the growth period for Ex-

periments 1 and 2 were previously reported by Vera

(1991) and Vera et al. (2002), respectively. Briefly, in

Experiment 1, the H SR was discontinued due to

stagnant LW that made it impossible to attain the

target LW (270 kg), and the animals were transferred

to the common Uh paddock simultaneously with

animals from the M SR treatment. During the growth

phase, DLWG were 0.259, 0.232, 0.215, and 0.097

kg/head, for L, L2, M, and H SRs, respectively (Vera,

1991). Daily LWGs during the growth phase of

Experiment 2 were 0.197, 0.192, and 0.096 kg/day, for

the L, M, and H SRs, respectively (Vera et al., 2002).

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weight losses during that season (Ramírez-Restrepo

and Vera, 2019). Additionally, paddocks of Uh were

found to be remarkably similar across several hundred

hectares monitored and sampled on numerous farms

over 17 years (Vera-Infanzón and Hoyos, 2019). In

parallel, soils under the experimental pastures at CRC

have been characterized in depth by Rao (2021).

During the initial growth period, animals were

weighed every 14 days and were carefully observed

daily for signs of estrus only in Experiment 1 (Cajas et

al., 1985). Beginning with the initial mating season,

pregnancy was diagnosed by rectal palpation, and

parasite diagnosis and LWs records were performed

approximately every 3 months. Calves' birth dates,

LWs, and sequential numerical identification were

recorded within 24 h of calving, thus allowing

subsequent calculation of ages at the different

reproductive events. Dates of conception and age were

calculated by subtracting 285 days from the recorded

calving date. Liveweight at conception (CONCEP) was

that of the nearest conception date, and LW at calving

was also the nearest LW before calving (PRECALV).

Cattle had free access to fresh water and a complete

mineral commercial product (80 g P plus macro and

micro minerals/kg of commercial product). Welfare

regulations across all animal manipulations in the field

were ensured by registered Colombian Doctor of

Veterinary Medicine.

Statistical analysis

Numerous graphical analyses for the two ex-

periments were carried out initially in EXCEL® to

identify recording errors if any, possible outliers,

ranges of values, and trends over time. Subsequently,

the distribution properties of all animal parameters

were examined using SAS® Univariate procedure.

The analyses of growth rates, LWs at or near

reproductive events, and the corresponding ages were

conducted using the Statistical Analysis System (SAS®

2016; version 3.5) GLIMMIX procedure with treatment

as fixed and animals as random effects. When

required, repeated measures of LW and ages were

specified in the model as in Gbur et al. (2012), further

discussed by Bello and Renter (2018) and Reuter and

Moffet (2016).

The occurrence of conceptions was fitted to a lo-

gistic model using SAS® LOGISTIC procedure. The

logistic regression was fitted to the data with the

response regarding conception assumed to be binary

(0,1), following the Bernoulli distribution. Given its

exponential nature, the coefficients of the regressions

could be interpreted as odds ratios. Independent

variables included treatment (TRT), CONCEP number,

age in days (Age), and LW (kg) and the respective

interactions using a forward stepwise method. The

suitability of the final model was judged from the

Akaike information and Schwarz criteria, while the

model and the independent variables were also

evaluated with the Ward Chi-square. Lack of fit was

estimated with the Hosmer-Lemeshow test, where

non-significant values were interpreted as no evidence

of lack of fit.

As is common in this type of research, full

contemporary field replication of TRTs was not

available due to the magnitude of the pasture areas

and the number of animals required, with implications

discussed at length by Bello and Rent (2018), but

treatment L-2 constituted a temporal replicate of L,

and Experiment 2 a full temporal replicate of the

growth phase of Experiment 1.

Simple simulation model

To illustrate the use of the prediction equations of

CONCEPT rates, an exemplary, very simple model

was developed in VENSIM PLE® using LWs, growth

rates, mortality, and other parameters recorded in

Experiment 1 to illustrate one possible approach to the

prediction of the first two conceptions (Supplementary

figure).

Table 1.Summary of reproductive performance in Experiment 1, in relation to initial stocking rates during the growth period on

Uh (Low = 1.28; Low-2 = 1.28; Medium 1.71, High = 2.24 head/ha), followed by grazing a common paddock of Uh at 1.71

head/ha during the breeding phase. Low-2 is a replicate of Low implemented one year later. Data are least squares means ±

s.e.m. The numbers of available records for calving intervals are in parentheses.

Stocking rate Low Low-2 Medium High

Number of animals 17 15 17 16

Growth phase

Initial liveweight (LW), kg 173 ± 4.4 174 ± 4.1 171 ± 4.0 170 ± 5.3

Initial age; days 442 ± 4.7 531 ± 4.1 447 ± 3.5 435 ± 4.2

LW at 1st detected heat, kg 241 ± 6.3 n.d. 231 ± 6.3 214 ± 6.5

Age at 1st detected heat, days 856 ± 31.5 n.d. 829 ± 31.5 846 ± 32.7

Age at 270 kg, months 26.8 30.1 29.2 38.4

Breeding phase under common grazing

Age at 1st calving, months 44.9 ± 1.9 46.0 ±1.1 44.8 ± 1.6 49.1 ± 1.5

Age at 2nd calving, months 64.3 ± 2.1 66.0 ± 1.8 63.6 ± 1.4 69.4 ±1.7

Age at 3rd calving, months 77.7 ± 3.2 76.7 ± 2.0 80.6 ± 3.5 85.6 ± 2.4

† Experiment discontinued. n.d.: not determined. .† Stocking rate discontinued due to inability to reach the target

breeding liveweight.

Table 2. Summary of least squares means (live weight, kg) ±

s.e.m. in Experiment 1 at key ages in relation to stocking rate

(Low = 1.28; Medium = 1.71, High = 2.24 head/ha) during the

growth period of heifers (weaning to target live weight of 270

kg)1

Stocking rates

Animal age (months) Low Medium High

18 186 ± 5.7 186 ± 5.7 186 ± 527

24 226 ± 6.9 204 ± 4.8 186 ± 4.8

36 271 ± 9.7 275 ± 4.7 252 ± 4.7 †

† Stocking rate discontinued due to inability to reach the target breeding

liveweight.

1As expected, differences between ages differ from each other (p < 0.001) for all

stocking rates

Least square means (LSMEANS) of LW and age for

the first two conceptions are listed in Table 3. Data

show that conception number had a significant effect on

Table 3. Age (days) and liveweight (kg) at the first two conceptions in Experiment 1 in relation to stocking rate during the

growth phase (Low = 1.28; Low-2 = 1.28; Medium = 1.71, High = 2.24 head/ha). Liveweights are those recorded closest to

calculated conception dates.

Age1Liveweight2

1st conception 2nd conception 1st conception 2nd conception

Stocking rate

Low 1.051 ± 31.4a1.845 ± 66.9a287 ± 6.1a310 ± 9.9a

Medium 1.057 ± 27.0a1.716 ± 43.0a285 ± 6.9a298 ± 7.9a

High 1.250 ± 51.1b1.839 ± 54.3a278 ± 11.8a325 ± 12.5b

Low-2 1.072 ± 24.3a1.793 ± 70.7a298 ± 4.3a311 ± 11.7a

229

Vera y Ramirez

1Age and liveweight means followed by different subscripts within each conception differ at p < 0.01.

2Differences in liveweight between conceptions within stocking rates differed at p < 0.05.

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Breeding phase

Experiment 1

Descriptive statistics for RP are shown in Table 1,

while Table 2 lists LWs at biologically and practically

significant ages frequently used to benchmark beef

production systems.

age and LW (p = 0.0001 and 0.002 respectively),

whereas the interaction between TRT x CONCEP was

non-significant. However, a slightly different picture

emerged from the more correct (given the distribution

properties of the two parameters) logistic analysis that

followed. In effect, the distribution of LW at the 1st

conception was well fitted by the normal distribution

(mean = 296.2, s = 33.89), whereas that of age was

lognormal (= normal distribution of the logs) skewed to

the right with a long tail, and with parameters Theta,

(threshold) = 0, n = 93; sigma (form) = 0.26, and Zeta

(scale) = 7.16. Similar distributions applied to the 2nd

conception, but trends disappeared in the subsequent

parities.

230

With two exceptions, re-conceptions during the 1st

lactation did not occur, but most animals re-conceived

within 60 days or less after weaning.

The amount of data regarding conceptions was

much less in Experiment 2, and the most reliable data

based on the numbers of observations available refer

only to the 1st conception. Fitting of the logistic model

as above, using LW as the main effect, resulted in

marginally different parameters (Table 4), but the

location and shape of the curve largely overlap that of

Experiment 1 (Figure 1).

Table 4. Parameters of the logistic regression for the 1st and 2nd conceptions with grouped animals from the Low (l) and Medium

(M) stocking rates. Liveweight (LW) and Age (days) effects were non-significant for the high stocking rate. The effect of LW on

later conceptions was non-significant.

Reproductive event n Intercept Coefficient 2P value

LW as predictor variable

1st conception, Experiment 1 93 -6.672541 0.0208803 6.04 < 0.014

1st conception, Experiment 2 64 -8.491598 0.00269403 7.85 < 0.0051

2nd conception, Experiment 1 72 -5.22117 0.011661 5.14 < 0.023

(LW*age/1000) as predictor variable

1st conception, Experiment 1 93 -7.21049 0.0229 17.66 < 0.0001

2nd conception, Experiment 1 72 -3.13323 0.0055 5.10 < 0.024

n: number of observations.

Figure 1. Logistic regression of the probability of conception on liveweight.

See details in Table 4. Experiment 1: solid line; Experiment 2: dotted line.

Figure 2. Isolines depicting the interaction between age

and liveweight for two exemplary probabilities (0.50 and

0.75 respectively) of conception on two consecutive

parities. Lines are derived by solving equations shown in

Table 4.

Vera y Ramirez

Conception probabilities

As indicated earlier, the occurrence of conceptions

was fitted to logistic models, and the variables selected

by the stepwise method included LW, Age, CONCEP,

and the interaction between LW and Age. Separate

analyses were carried out for each conception. The fit

of the model was high during the 1st conception,

somewhat less for the 2nd conception, and not

significant thereafter (Table 4). Given that precise age

is seldom available for commercial cattle in extensive

systems, the simpler model using LW only as a

predictor variable is also included in the results

(Figure 1) since it may be more easily applicable to the

majority of practical applications. Nevertheless, the

full logistic regressions equations were used to show

the large interaction of LW and Age for two

predetermined and illustrative conception

probabilities (0.50 and 0.75, respectively) in the first

two parities (Figure 2).

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Logistic regressions regarding probabilities of

of conception in relation to LW can be used to

develop a simple exemplary simulation model to

simulate the dynamics of conceptions over a period

of 12 months, based on the monthly evolution of

LWGs shown as a horizontal table in the

supplementary material. The latter includes the

231

Table 5. Liveweights (kg, least squares means ± s.e.m.) near conception (CONCEP), calving (PRECALV), and at weaning

(WEANING) in relation to reproductive cycle. The effect of initial treatment became non-significant after the second calving.

D'Occhio et al. (2018) recently reviewed the

influence of nutrition operating via LW and body

condition on heifers' fertility and in postpartum cows.

They noted that heifers' weaning weight and their

subsequent weight gains determine Age and LW at

puberty, as well as lifetime RP. The present results

coincide with that assertion, and apply to commercial

Brahman heifers under the extensive management

conditions of the Colombian Llanos when grazing low

quality Uh pastures up until the fifth parity. Average

calf weaning LWs, number of conceptions observed,

and the yearly output of LW per cow were reported by

Vera et al. (1993) and showed a distinctive difference

between L and M relative to H in both experiments

(Vera et al. 2002).

The present findings allowed the development of

predictive logistic equations using LW and age as

effector variables, for the first two conceptions, both of

which were determined by earlier LWGs. It was

further shown that the interaction of LW with Age

influences conception probabilities, but that LW alone

may be sufficiently accurate under commercial

conditions that most frequently lack precise age

information and also body condition scores. Similarly,

mean Ages and their respective experimental errors

allow predictions of subsequent conceptions in the

third to fifth parities that complete lifetime

performance under extensive tropical ranching

conditions (Kleinheisterkamp and Habich (1985). In an

earlier analysis, Vera (1991) analyzed the cumulative

proportion of the first conception in Experiment 1 in

relation to LW nearest that conception, whereas the

more comprehensive and correct present analyses

fitted logistic models to conceptions in relation to LW,

Age, and their interaction from the first to fourth

parities. This was accomplished using a Bernouilli

distribution (using values of 0 and 1 for the absence

and presence of conception respectively), with the

result that starting on the third parity LW became non-

significant, and Age at conception became nearly

constant without a significant influence of the third

and later parities. Thus, the present results provide a

valid and fairly accurate long-term representation of

conceptions throughout the females' lifetime that

could be used in developing a comprehensive,

empirical simulation model of RP in Brahman herds

experiencing low lifetime LWGs in tropical extensive

systems.

Numerous initiatives have modelled reproductive

events as reviewed by Blanc et al. (2001) at various

organismic levels, ranging from endocrinology-driven

conceptions to highly empirical approaches as in the

present case. Nevertheless, Blanc et al. (2001) also

noted that the prediction of reproduction under a wide

Reproductive cycle CONCEP PRECALV WEANING

First 291 ± 5.4 347 ± 5.6 271 ± 4.9

Second 302 ± 5.3 381 ± 5.7 302 ± 4.8

Third 319 ± 5.6 380 ± 6.0 311 ± 5.1

Fourth 327 ± 6.9 357 ± 7.0 325 ± 7.5

Fifth 328 ± 15.0 379 ± 18.3 n.d.

P value < 0.001 < 0.001 < 0.001

n.d.: not determined.

Discussion

Beef cattle reproduction on Urochloa humidicola

Over the total length of the breeding phase of

Experiment 1 for the higher yielding treatments (6.68,

6.53, and 6.00 years for L, M, and L-2 respectively)

there were 151 calving events (sum of the 3 treatments)

that led to 133 weaned calves (88.1 %), versus 43

weaned calves out of 55 calvings (81.1 %) for the H

treatment. These values imply calf losses of 12-19 %

between birth and weaning.

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model equations, parameters, and outputs using

actual and independent field data as input parameters.

Evolution of live weights in relation to parity

Table 5 shows cows' LWs nearest conception,

calving, and at weaning (adjusted to 240 days) for all

of the reproductive cycles across all treatments in

Experiment 1. Five-year average values for a large,

contemporaneous and contiguous experiment with

mature cows grazing conservatively managed native

savannas reported average LWs of 332, 344, and 299-

316 kg for CONCEP, PRECALV, and weaning,

respectively (Rivera, 1988). Despite significant

differences between parities, the only discernible trend

is that of LW at conception that increased slightly with

parity, and a non-significant effect of earlier

treatments on LWs, but of course, differences in age

persisted throughout the experiment (p < 0.01).

232 Vera y Ramirez

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e range of environmental conditions continues to be a

major challenge, and it can be inferred that models that

apply to more constrained, and less data-rich,

environments are useful, a situation that applies in the

current case. The exemplary simulation model

applicable to the first two parities and listed in the

supplementary material fits the above conditions

when LW is the only variable available, and it is

driven by monthly LWGs that are available for a wide

range of grazing situations in the Llanos (e.g.,

Depablos et al., 2009; Pérez et al., 2017; Rodriguez

Borray and Cubillos, 2019).

Attainment of puberty in cattle is generally related

to the body LW observed at maturity. Fordyce et al.

(2013) provided a detailed review of findings in

extensive systems of northern Australia and stated that

puberty in B. indicus reached about 70 % of mature

LW, as opposed to 60 % in B. taurus. In the present

case, LWs of 230-240 kg associated with the first

detected heat in the L and M SRs would be equivalent

to 344 kg mature size, well within the range of 325-350

kg estimated by Vera (1991) and Vera et al. (2002) for

animals that were bred while on Uh. Nevertheless, the

cows that were bred on Ud in Experiment 2 reached

asymptotic LWs of 380-390 kg (Vera et al., 2002),

implying conception at a proportion closer to 60-65 %

of mature size. It should be noted that the previous

mature sizes are those allowed for by the nutritional

regime used and cannot be taken as the potential LW

determined by the genotype.

As shown in the present experiments, as well as in

Pérez et al. (2019) for the Llanos of Colombia, Mora-

Luna et al. (2014) in Venezuela, and Australian data

(Schatz ., 2011), RP on extensive, low external input

systems based on low quality and un-supplemented

pastures is modest but it is postulated that it is

proportional to the low management demands of these

types of pastures and systems. Under these conditions,

on-farm data (Jimenez-Rodriguez and Manrique

Perdomo, 2018) clearly demonstrates the futility of

trying to incorporate sophisticated and expensive

breeding technologies into systems that rely on low-

quality forage resources. On the contrary, RP on

higher-quality Ud (Vera et al., 2002) shows the marked

difference made by better forage resources, and it is

hypothesized that the latter case may offer somewhat

better opportunities for the introduction of more

demanding breeding technologies. In fact, under the

extensive conditions of northern Australia, the on-farm

study of Bortolussi et al. (2005) found that "the results

suggest that producers associated increasing turn-off

weight or decreasing turn-off age more with pasture

improvement than with bulls of higher genetic merit

for growth". Advances in RP need therefore to be

paralleled by improved animal management, keeping

records of production, and closer scrutiny of the

breeding herd.

Ezenwa et al. (2006) studied the RP of Brahman x

British beef cows during 6 months of the warm season

at Ona, Florida, USA, grazing either Uh or Paspalum

notatum pastures during 4 consecutive years, but with

different cows every year. The cows were well fed as

attested by cows weaning LW of 517 kg on Uh, and

calf weaning LW of 250 kg (DLWG = 0.660 kg). During

the 90 days following weaning, cows LWs increased

very significantly, and exceeded the LW of those on P.

notatum suggesting that Uh may provide enhanced

LWG when grown on fertile soils. Ezenwa et al. (2006)

suggested that cattlemen in the region, as in the Llanos

(Vera and Ramírez-Restrepo, 2017; Vera and Hoyos,

2019) prefer to use these low input grasses for

breeding cows, rather than better quality but more

expensive alternative species. As the authors indicate,

differences with observations in South America may

be due to soil and fertility differences. Nevertheless, in

a transitional savanna in Venezuela based on more

fertile soils than in the present case, Depablos et al.

(2009) provided three levels of supplementation to 166

beef heifers (636.0 ± 2.46 days of age and 219.6 ± 15.68

kg LW) grazing paddocks with a mixture of Urochloa

spp. and Cynodon dactylon. Even the un-supplemented

animals had much higher DLWG during the dry

season (0.483 kg) and RP than in the present

experiments, reaching 82 % pregnancy. Thus, LWGs,

LWs, and pregnancy success need to be referred to the

specific nutritional strategy used in growing and

breeding female cattle.

The present paper is predicated on the hypothesis

that the probability of conception during the first two

reproductive cycles by females (3-5-years-old) can be

predicted with reasonable accuracy from an easy-to-

measure parameter such as LW. Similarly, age, if

available, interacts with LW as shown in Figure 2 and

Tables 1 and 4 early in reproductive life, whereas later

reproduction performance during the 3rd to 5th parities

is associated with fairly constant LWs (Table 5 and

Vera et al., 1993) if the nutritional strategy persists. The

current results are supported by data from very

different, but still extensive, cattle systems of northern

Australia (Schatz et al., 2011; Schatz and Hearnden,

2017) that were derived from several thousands of

records pertaining to breeding heifers and cows in

different physiological conditions that were also

included for comparison in Figure 3. It should be

noted that the "maiden" heifers of Schatz et al. (2017)

were at least one year older than their yearling heifers

233

Beef cattle reproduction on Urochloa humidicola

Conclusions

The long-term data analyzed show the close

relationship of conceptions in extensive Brahman cattle

systems with LW and age in the initial two

reproductive cycles, whereas more advanced

conceptions relied largely on the age of the females.

The quantitative relationships between probability of

conception and both LW and Age were established,

and it is suggested that they can be an adjunct in

decision-making regarding mating and its success in

accomplishing pregnancy. Thus, using the proposed

equations can help establish thresholds for achieving

desired levels of conception, as exemplified in the

supplementary material. The production systems

investigated rely on low-quality extensive pastures

subject to minimal external and human-management

inputs and support low to medium rates of RP at the

cost of maintaining heifers and cows with low LWs.

Beef outputs of these systems are therefore largely due

to the production of weaners, complemented

eventually with the sale of cull cows.

Figure 3. Comparison of predicted probabilities of conception

in the present data (solid line) with that of Schatz et al. (2017;

dash lines).

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and showed much larger pregnancy rates at similar

LW, thus adding support to the effect of the LW by

Age interaction reported in the current results. The

coincidence between our estimates and that of Schatz

et al. (2017) with their yearling heifers is remarkable

(Figure 3).

As shown in Table 1, long calving intervals

characterize these extensive systems, a result that

coincides among others, with Burns et al. (2010,),

Sierra-Montoya et al. (2013), and Pérez et al. (2019).

Sierra-Montoya et al. (2013) reported on-farm data

showing that cows fed on enriched rice residues, not

dissimilar in quality to Uh, had a mean Age at 1st

calving of 41 months and subsequent calving intervals

of 17.5 months. It should be noted that calving

intervals of 18-20 months may inevitably lead to years

of high RP alternating with years of very low outputs,

as reported by Kleinheisterkamp and Habich (1985).

Our results show that loss of suckling calves,

particularly during the first pregnancy, can be

considerable. Schatz (2011) found that although

pregnancy rates in 2-year-old maiden heifers were

generally adequate (> 75 %), calf loss rates averaged 22

% in first-calf heifers, and re-conception rates were

also low (< 25 %).

Lastly, the remarkable behavior of LW during later

pregnancies reported in Table 5 should be noted, in

that cows' LWs only consistently increased due to

pregnancy, but returned to the initial CONCEP LW

following weaning. This observation coincides with

the observations made by Vera-Infanzón and Ramírez-

Restrepo (2020) in that the only meaningful weight

increase in these extensive systems is the weaning LW

of their calves since cows contribute very little to

systems' outputs other than when culled from the

herd. Although not documented in the Colombian case

(except by occasional observations by the authors),

cows older than 8 years of age may show an

accelerated rate of teeth erosion (Jones and Sadler,

2012), exposing them to severe undernutrition and

even death (Fordyce et al., 1990). Given that cull cows

constitute a large fraction of slaughter animals in

breeding systems (Florez, 2010; Ramírez-Restrepo et

al., 2020), earlier culling would therefore seem

advisable.

The implications of RP in extensive systems on

GHG emissions remain to be analyzed, but

Garnsworthy (2004) suggested that changes in RP

associated with changes in the proportion of different

animal categories in the breeding herd may have an

important influence on environmental efficiency, a

carbon footprint dimension proved on well-managed

B. decumbens pastures (Ramírez-Restrepo et al., 2020).

Blanc, F., G. B. Martin, and F. Bocquier. 2001.

Modelling reproduction in farm animals: a review.

Reproduction, Fertility and Development, 13: 337-

353. https://doi.org/10.1071/RD01038

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