Productivity and efficiency of cow herds submitted to two weaning ages

It was aimed in this work to evaluate the productivity and efficiency of beef cows submitted to weaning of their calves at 76 days (early weaning) or at 148 days (weaning at conventional age) after calving during three consecutive years. It was evaluated pregnancy rate, birth and weaning, gain weight of the calves and cows at conventional weaning, weight at conventional weaning of calves and cows, production of weaned calves per cow and calf production index. Pregnancy rate in the herd submitted to early weaning (86.34%) was higher than in the conventional weaning (55.5%). Early weaning cows showed higher birth and weaning rates when compared to conventional weaning (83.6 and 83.6% vs. 47.5 and 44.5%, respectively) besides a higher production of calves (183 vs. 114). Early weaning cows produced 60.5% more calves than the conventional weaning cows. Conventional weaning calves gained more weight from birth to weaning (97.9 vs. 83.4 kg, respectively). Early weaning cows had more weight gain from calving to weaning (42.5 vs. 18.7 kg, respectively), and regarded to calf production index, they were more efficient (efficiency of the herds). At the average of the three years, the early weaning of the calves allows higher calving rates and weaning to the cows, in addition to a higher index of calf kilogram index produced in relation to weaning at conventional age.


Introduction
Breeding herd reproductive efficiency significantly influences productivity indexes (Beretta et al., 2002).Traditional beef production systems, with low reproductive rates and late-finishing steers present low productivity (Pötter et al., 2000).
Productive efficiency of beef cattle in breeding herds is directly related to the reproductive performance of cows, and it is associated to maternal ability of the dam and potential weight gain of their calves (Restle et al., 1984;Ribeiro et al., 2001).The productivity of breeding systems may be increased by using of technological processes (Costa et al., 1981;Vieira et al., 2005), aiming at improving R. Bras.Zootec., v.39, n.8, p.1849-1856, 2010 dam nutritional status and body condition throughout the year.
Nursing affects breeding herd performance (Simeone & Lobato, 1996;Restle et al., 2001).There are evidences of the effect of calf suckling on calving interval (Freetly, 1999;Lobato et al., 2000), suggesting that anestrus during lactation, as a consequence of cow nutritional status and a possible endocrine suppression due to lactation as well as the mere presence of the calf, may cause estrus inhibition (Short et al., 1990).
The practice of early weaning in production systems aims at allowing cows to recover their body weight and body condition score after being submitted to feed restriction, such as the case of high stocking rates (Simeone & Lobato, 1996;Fagundes et al., 2003), and to improve their reproductive efficiency (Moojen et al., 1994;Pascoal & Vaz, 1997;Restle et al., 2001;Fagundes et al., 2003;Almeida & Lobato, 2004).
The objective of the present study was to evaluate the influence of early weaning on the production and reproductive performance and on production efficiency of two beef cow herds.

Material and Methods
The experiment was carried out on Granja Itú, located in Itaqui, RS, Brazil, latitude 29° 12' south and longitude 55° 36' west.The relief consists of hills and valleys, with deep, naturally-acid soils with intermediate surface texture.The soil is classified as red podzolic latosol (EMBRAPA, 1999).The climate is subtropical, according to Köppen's classification (Moreno, 1961).
In this experiment, 141 Braford cows with first calving at 36 months of age, were distributed in two groups.In one group, calves were weaned at 76 days of age (early weaning), and in the other group, calves were weaned at 148 days of age (conventional weaning).
The number of replicates varied according to the observation year, as it follows: 71, 63, and 53 cows in the early-weaning group, and 70, 28, and 20 in the conventional-weaning group.Early-weaning cows calved 104 females and 81 males, whereas conventional-weaning cows calved 57 females and 81 males during the springs of 2004, 2005 and 2006, from September 7 th 2004 to October 15 th 2005, from October 5 th to December 1 st 2005, and from October 10 th 2006 to November 25 th 2006, respectively.These calves were calved by primiparous cows in the first year, and by the same cows in the following years.As the study started with primiparous cows at three years of age ( 2004), and with the same cows calving at 4 (2005) and 5 (2006) years of age, these effects are completely confounded and they were considered as the joint effect of calf birth year and cow age.Non-pregnant cows were culled every year.
Cows whose calves were submitted to the two weaning ages were managed as a single group; the only difference was the calf weaning age.Between the first (September/ 2004) and the second calving (September/2005) cows were maintained on natural pastures at a stocking rate of 320 kg live weight/ha, as well as during periods when they were not lactating.Between the second calving until conventional weaning of the second generation of calves (10 th /12/2005 to 2 nd /2/2006), cows were managed on Brachiaria pasture (Brachiaria brizantha, cv Marandu).After weaning, they returned to natural pastures until the beginning of the calving season (September/2006), when they were again grazed on another Brachiaria (Brachiaria humidicola (Rendle) schweick) pasture until the date of conventional weaning (15 th / 12 /2006 to 5 th /02/2007).
Early weaning occurred from December to February and it was performed when calves reached 60 to 70 days of age, except for those born in 2004, which were weaned together in a single lot on December, 26 th 2004 at an average age of 91 days, resulting in an average of 76 days for the three years of observation.Conventional weaning was performed on a single day in the beginning of March (5 th /03/2005, 3 rd /3/2006, and 6 th /3/2007), aiming at allowing the cows to recover their weight and to accumulate body reserves before winter (Lobato, 2003).
The following reproductive efficiency parameters were evaluated during three years (from calving in 2004 to pregnancy diagnosis in the 2006/2007 breeding season): pregnancy rate, birth rate, and weaning rate relative to the number of cows mated in the previous year.In addition, mortality rate took into account deaths that occurred from pregnancy diagnosis to birth and from birth to weaning.
Herd productivity and breeding-cow efficiency considered total cow and calf weight gains (kg) from calving up to conventional weaning.Calf production efficiency was measured as calf production rate adjusted to pregnancy rate in kilograms of weaned calf per cow (weight of calves submitted to conventional weaning * pregnancy rate/100) and in kg of weaned calf per 100 kg cow metabolic weight at weaning (kg calf weaned/100 kg cow 0.75 ).The obtained reproductive efficiency, as well as cow productivity and efficiency values express the average of three years of observation due to the different number of replicated per year resulting from the culling of cows with negative pregnancy diagnosis.
Herd productivity and breeding-cow efficiency considered conventionally weaned calf weight per cow and weaned calf weight per 100 kg of cow live weight.These parameters were calculated according to the numbers of cows exposed to mating during the breeding seasons of 2004/2005 and 2005/2006 and to the number of live calves at the time of conventional weaning in March, 2006 andMarch, 2007.Average cow weight at the beginning of the breeding season, obtained at the time bulls were placed with herd, was used.
A completely randomized experimental design in a 2 × 2 × 3 factorial arrangement (two weaning ages × two calf sexes × three years of observation) was applied.Results were submitted to analysis of variance and to the F test.The mathematical model used in the analyses was: where:Y ijkl = dependent variables; μ = mean of all observations; T i = effect of the i th weaning age, with i =1 (early weaning); 2 (conventional weaning); S j = effect of the j th calf sex, with j =1 (male); 2 = (female); T*S ij = effect of the interaction of the i th age at weaning with j th calf sex; R k (T*S) ij = effect of the k th animal within the interaction of the i th age at weaning with j th calf sex; A l = effect of l th year and cow age set; T*A il = effect of the interaction of i th age at weaning × effect l th year and cow age set; T*S*A ijl = effect of the interaction of i th age at weaning × j th calf sex × l th year and cow age set; Σ ijkl = residual error.
As the interactions (T*S) ij; (A*S) lj; (T*S*A) ijl were not significant, they were removed from the final statistical model.Analyses were carried out using the GLM procedures of SAS statistical package SAS, version 6.08 (SAS, 1997), at 5% significance level.Means were compared by the t-test.Pregnancy rate at different weaning ages was analyzed by the Chi-Square test at 5% significance level.

Results and Discussion
From September 2004 to March 2007, it was evaluated the reproductive and productive performance of 141 cows in the beginning of the experiment and from those that got pregnant along three years of observation, totaling 305 replicates.As cows were initially distributed at first calving, there are no records of birth rates from the first breeding season (Table 1), and only weaning rates, considering the initial number of first-calf cows.
When observation years were compared, significant differences between rearing systems only during the first year, when cows were primiparous (Table1), were observed.Costa et al. (1981) and Restle et al. (2001) also found that primiparous cows were more affected by nursing on subsequent reproductive performance.
The obtained average birth and weaning rates of 68.5 and 66.8%, respectively, do not support economically efficient beef production.Beretta et al. (2002) evaluated the physical productivity and the biological efficiency of beefcattle rearing systems and observed increasing birth rates of 73.4%, 80.4%,and 84.4% as the age of the cows at first calving increased from 24, 36, and 48 months, respectively.Those authors concluded that this increase in birth rate determined the higher productivity of breeding-to fattening or traditional beef production systems.
Birth rates and weaning rates of breeding cows in both management systems were, in average, 83.8 and 83.6%, and 47.5 and 44.4% for cows whose calves were submitted to Table1 -Reproductive performance and mortality of cows during the evaluated years early or conventional weaning, respectively, relative to all cows exposed to bulls.Considering only those cows maintained in the herd after open cows were removed from the analysis, birth and weaning rates were 96.5 and 95.7%, and 95.7 and 90.0% for cows submitted to early and conventional weaning, respectively.Vieira et al. (2005) evaluating cows of different ages in a Nelore herd in the central region of Brazil for four consecutive years, observed average birth and weaning rates of 81.7 and 77.2%, when all cows exposed to bulls were considered, and higher rates, of 93.0 and 87.0%, when open cows were not taken into account.
When weanings are separately evaluated, a considerable advantage of early weaning is observed in term of production indexes, which are very close to those considered optimal.Several studies have shown that early weaning promotes higher pregnancy, birth, and weaning rates when different weaning ages are compared (Simeone & Lobato, 1996;Lobato et al., 2000;Restle et al., 2001;Fagundes et al., 2003).
When conventional weaning were individually analyzed, birth and weaning rates increased, probably because cows aged and were no longer primiparous cows (Vieira et al., 2005) thereby, had lower nutritional requirements (Freetly, 1999;Restle et al., 2001).Moreover, post-calving cows were offered better feeding conditions at the end of the second breeding season, when they were managed on Brachiaria brizanta, with 2,450 kg/ha available dry matter, contrary to the first year, when they grazed on natural pastures with only 1,640 kg/ha availability due to the high stocking rate of 320 kg/ha/year.Excessive stocking rates impair beef cattle reproduction (Simeone & Lobato, 1996;Fagundes et al., 2003).It must also be noted that, as open cows were culled at pregnancy diagnosis, cows whose genetics are better adapted to the environment remain in the herd (Vaz & Lobato, 2010b).
Calf mortality rates ranged from 3 to 5% between pregnancy diagnosis and calving and between calving and conventional weaning age.Rovira (1974) reported 8% mortality between pregnancy diagnosis and conventional weaning age.Corrêa et al. (2000), when evaluating the reproductive performance of cows with different genotypes, verified higher calf mortality as compared to the present study.Those authors concluded that, as primiparous cows are more sensitive, more care is required to prevent losses that influence the productive and economic performance of the herd.
When weaning ages were compared, no differences in mortality rates (P>0.05)were observed, even though there were differences of 100% in absolute numbers.However, due to the lower number of calves submitted to conventional weaning, different number of cows in each weaning age group in the same year, and the higher number of cows in the early weaning group due to their higher pregnancy rates, no statistical differences on calf mortality were detected.
During the three years of observation, 58.8, 28.0, and 11.7%, and 8.4, 16.1 and 18% cows of the conventional weaning and early weaning groups, respectively, were culled.This resulted in 70.4% and 25.4% total culling of cows in the conventional and early weaning groups, respectively, considering the initial number of animals.It must be noted that both cow groups were managed as a single group, and that stocking rate adjustments were made when open cows were culled to maintain a stocking rate of 320 kg live weight/ha.
Keeping open cows in commercial herds results in lower productivity in calving-to-fattening systems (Pötter et al., 2000;Beretta et al., 2002).Had culling continued in the observed herd, in two or three years, the number of animals would have matched the stocking rate, with consequent higher forage availability and better reproductive responses (Simeone & Lobato, 1996;Fagundes et al., 2003).In commercial production systems, as the one evaluated in the present study, such procedure may be used as an alternative; however, it is better to adjust stocking rates before emergence of "problems".
Another factor that must be considered when evaluating beef-cattle system profitability is adding value to market animals.Culled cows are often sold when they are thin and with low body condition score, obtaining lower prices as compared to culled cows and heifers with excellent body condition score (Vaz et al., 2000).These thin cows reduce herd profitability (Pötter et al., 2000;Beretta et al., 2002).
During the three years of observation, the practice of early weaning produced more calves (P<0.05;183 vs. 114 calves).The 71 and 70 cows in the beginning of the observation produced during the three years, in average, 61.3 and 38.0 early weaned and conventionally weaned calves per year, respectively (Table 2).Although many studies have not followed up subsequent calving of cows submitted to early weaning, the results consistently show that the pregnancy rates of these cows were higher than those submitted to conventional weaning (Lobato & Barcellos, 1992;Moojen et al., 1994;Lobato et al., 2000;Restle et al., 2001;Almeida et al., 2002;Fagundes et al., 2003).
These means indicate that each cow managed under the early weaning regime produced 2.6 calves in three years, whereas those maintained under conventional weaning produced 1.6 calves during the same period.According to these data, breeding systems with weaning age of 148 days require two cows to produce one calf/year, making these systems expensive and inefficient since these productivity levels is not sufficient to cover fixed costs, such as land, labor, and input costs used by cows that did not conceive (Pötter et al., 2000;Beretta et al., 2001).
Although the introduction of the practice of early weaning in a herd requires financial and labor investments, this expense must be evaluated in the context of the production system, and not merely considering that parameter.According to Vaz et al. (2000), economic assessments usually do not analyze the effect of technology on beef cattle production systems as a whole, rather using a simplistic and limited analysis that does not take into consideration the indirect benefits for the entire system.
At the time of conventional weaning, calf weight was similar between weaning-age groups (Table 3).However, weight gain from birth to weaning was higher in calves weaned at 148 days of age (P<0.05) as compared to those weaned at 76 days of age, with weight gains of 97.9 and 83.4 kg, respectively.This is explained by the better feeding conditions of calves weaned later than those submitted to early weaning, as the latter suffered the stress of weaning during the evaluation period (Restle et al., 2001), being fed feedstuffs to which their digestive tract was not yet completely adapted (Pascoal & Vaz, 1997).
Lower weaning weight of early weaned calves at the time of conventional weaning were reported by Restle et al., 1999, Almeida & Lobato, 2004;Lobato et al., 2007.However, the evaluation of the production system or the use of different techniques within a system have to regard other benefits promoted by these techniques.For instance, cow weight gain (kg) between calving and conventional weaning time (Table 3) was 172% (P<0.05)higher in cows in the early weaning group as compared to those in the conventional weaning group (42.5 and 18.7 kg, respectively).This difference in weight gain occurred in the period immediately after early weaning, when cows stop nursing, redirecting the consumed nutrients used for milk production to weight gain (Simeone & Lobato, 1996;Almeida et al., 2002;Vaz & Lobato, 2010b).
The weight gain of the cow-calf set between calving and time of conventional weaning was 126.0 vs. 116.6kg in the early weaning and conventional weaning systems, respectively.As calf development is not influenced by the use of early weaning (Restle et al., 1999;Lobato et al., 2007;Vaz & Lobato 2010a), the higher weight gain results in higher pregnancy rates, reduced number of thin cows and risks during the winter, and higher accumulation of body reserves, which are reflected in higher milk production during the following lactation, and consequent higher calf weight gain.In cows with their last calves, this higher weight gain allows marketing fatter cows at a lower production cost (Pascoal & Vaz, 1997).
In studies on herd productive efficiency, herds are submitted to the same management conditions (Barcellos et al., 1996;Ribeiro et al., 2001;Vieira et al., 2005).Most authors state that the ratio between calf weight and cow weight within a same production cycle is adequate to measure efficiency at individual or herd level (Barcellos et al., 1996;Ribeiro et al., 2001)   with Abeerden Angus cows and crossbred calves, and showed that smaller and lighter cows presented better production efficiency, when calf weight was adjusted to 205 days.Barcellos et al. (1996) obtained an average productivity of 117.9 kg (kg calf/cow), and 29.4 kg (kg calf/100 kg cow) efficiency when working with primiparous cows of the following genotypes: ½ Nelore ½ Hereford, ¼ Nelore ¾ Hereford, and purebred Hereford.These results are similar to those obtained by Vieira et al. (2005) in Nelore herds in central Brazil.It must be noted that primiparous cows have lower body weight than multiparous cows, which may affect efficiency results, although primiparous cows tend to produce lighter calves at weaning.

Conclusions
Cows whose calves are weaned at 76 days of age present higher pregnancy, birth, and weaning rates as compared to cows whose calves are weaned at 148 days.Calf weight gain from birth to weaning is higher when they are weaned at 148 days of age, but these calves produce less calf weight per kilogram cow until weaning.Early weaning produces more calves (in kg live weight).The higher pregnancy and weaning rates of cows in the early weaning system determine higher production of kg calf/kg cow maintained in the herd, higher productivity, and higher efficiency in kg weaned calf per cow mated in the previous breeding season. .

Table 2 -
Calf production in production systems using early weaning or conventional weaning A, B Means followed by capital letters in the same row are different (P<0.05) by Chi-Square test.Production is related to the total initial number of cows during the three years of observation.

Table 3 -
Reproductive performance, cow and calf weight gains from birth to conventional weaning time, and herd productivity and efficiency according to weaning system A, B -Means followed by capital letters in the same column are different (P<0.05) by Chi-Square test.a,b-Means followed by small letters in the same row are different (P<0.05) by F test.1Calf weight at conventional weaning/cow metabolic weight at conventional weaning *100. 2 Calf weight at conventional weaning adjusted to pregnancy rate/cow metabolic weight at conventional weaning *100.
3Calf weight at conventional weaning × pregnancy rate/100 = kg weaned calf/ cow maintained in the herd.