By Dr. Guillermo Nava
We understand that selection for reproductive characteristics, both the male and the female, It is not being considered as the goal of selection in the majority of cattle breeding in our country programs. This is truly worrying due to the enormous importance of economic characters associated with reproduction in our breeders systems. Systems breeders, on the other hand, they are increasingly relegated to poor fields, due to the advance that has had agriculture in our country. If a cow is not pregnant, and not do it in time, very few other things of its genetic composition import. If the pregnancy does not occur, or you need many inputs it is to, conditioned greatly the efficiency of transformation of pasture in calves, and why precisely fertility is so transcendent in the profitability of farming.
It is a fact that the reproductive characteristics of the cattle are not taking into account objective selection, in favour, in some cases, Search "trackers" animals, – belonging to those larger biotypes, you are that usually reward in the rural exhibition-, or a great emphasis on performance for rapid growth or features related to housing, in other situations. If we analyze the evolution of the different biotypes hunted in rural exposures in breeds such as Hereford and Angus since the beginning of the 20th century up to the present, We see that the herd of suckler cow has been the result of a continuous change, a kind of by-product of the kind of animal that was sought at that particular time on the slopes, What determines that hard this rodeo has been, -or can-, produce consistently and efficiently. Like this, the type of cow in herds of breeding in our fields has not been product of a programme of adaptation of the cattle to core from the premises or its production system, but that it varied according to that preference on the tracks.
The advent of the estimates of genetic merit of livestock in growth characteristics added a new component. Is that in recent years, There has been an effort by the cabañeros to produce more pounds of product in a short period of time more, looking to improve the growth in their rodeos, often with the use of extreme individuals at the race. This, a priori may think that it is beneficial, led, in practice, increase adult weight of the cow in Uruguay (see Figure 1 and 2) and dismissal of some relevant maternal characteristics such as fertility, the maternal ability, the longevity or suitability of cattle. It is that when you select by growth, the animals begin to transform your body composition, being ever higher, more skinny, closer. And also less fertile, because usually increase to delivery problems, is need more weight to reach her puberty and because need more resources food for return to impregnate is after the delivery.
The genetic trend of greater weight of the adult cow registered in the two majority races in Uruguay meat displayed in figures 1 and 2 It is also worrying because interactions between genotype and environment affecting the reproductive potential are described. Jenkins (2009) It says that under conditions of availability of material dry restrictive, -such as those that occur commonly in Uruguay-, biotypes with greatest potential for growth and increased production of milk produced less kilograms of calves per cow entorada. For example, two groups of cows producing in restrictive conditions, with 534 kg and 581 weight kg, He was weaned calves of 247 kg and 211 kg, respectively (Maddock and cabbage, 2013). In those cases, These biotypes with more performance potential experience longer periods of postpartum anoestrus (Nugent et to the, 1993), just as we usually see in many circumstances related to our work as field veterinarians. Postpartum anestrus tends to be the main limiting for which the national rodeo does not reach better pregnancy rates. In these populations with large animals, If I want to impregnate cows, We have to make a major adjustment of stocking, reducing staffing in the fields, or do we have to modify them the environment through supplementation to arrive in better condition to childbirth, or we need to massively resort to costly management practices such as early weaning.
It should be noted that all the days of existence of an animal on the grounds, There are certain fixed costs incurred only to keep that animal. Increases in the weight of the cow, also increases the amount of food that must be consumed to just keep. If we increase the level of production, We concomitantly increased production costs. Even, genetic increases in production levels can lead to decrease the longevity of cows.
A more long-lived cow not only means a female with less difficulty to impregnate it, but an animal to suffer fewer problems during his lifetime in the rodeo.
But there are other consequences of increasing adult size in cattle, and it is most necessary weight for heifers reach puberty. According to our own records obtained in the genital evaluation prior to insemination programs, it include some 71.259 checked heifers from the 2003 to the 2013, one of every three heifers is located in anestrus when the producer decides to serve them. This means that they are not suitable to be served. Many of these heifers are not preñaran in the reproductive season, or will belatedly, What brings consequences for chances to impregnate the next year and the number of kilos of calves weaned by this female. In Figure 3 shows a typical register of the kilos of weaned calves of cows, According to the time of calving; You can see that there is a big difference in weight of calves as cow head, medium or tail of parturition, so that even those bellies that finally can impregnate it, but do so late, they will be also noticeably less productive. At last, It should also be present another effect that has had the selection of livestock for higher growth, It is the increase in the weight of the calves associated with birth, a fact which is directly related to the major problems at birth.
We know that we need to reduce the incidence of dystocia because problems at birth, as well as increasing the chances of neonatal deaths and mothers, They also reduce female reproductive performance. In this sense, a study (Benton Glaze, 2011) reveals that Heifer that had a calving dystocia, He was weaned a 14% less than calves per cow entorada the following year, a consequence of anestrus pospartos longer and lower rates of conception in those bellies experiencing dystocia in the previous birth; which adds an additional interest to moderate adult sizes, in this case by their correlation with the weight of the calves at birth.
One of the reasons that we have heard by which stakeholders related to the bovine genetics in the Uruguay does not pay attention to the reproductive characteristics in their selection programs refers to the fact that with "a good reproductive management", and particularly with the massive use of early weaning, those cows could return to impregnate it. This argument ignores the point that not only is important to achieve a cow is preñe, but we must do so with the lowest cost that can be, improving the profitability of the system. With normal loads of fields, If instead of using early weaning as an effective practice to achieve pregnancies in climatically difficult years or to circumvent specific adverse conjunctures, massive and constantly use all the years, then we will increase production costs, reducing the net margin of breeders systems.
Others reason that offered for this lack of interest in the reproductive characteristics is that most of the studies reported internationally indicate a low heritability for these features, with a great influence of the environment. However, After more than 20 years working as a specialist in livestock breeding, We know that if satisfactory results and genetic progress can be achieved when these characteristics are taken into account in animal breeding programs selection objectives. If we focus exclusively on the category of Heifer, We have found that the rate of pregnancy of 22.237 animals evaluated over a period of four years, between the 2011 and the 2014, was of 94,3% and 85,2% for those who were in a rodeo with a program of selection taking into account reproductive characteristics and those whose selection not prioritized these features. Something similar, or an even more pronounced effect, as with the adult cows. It is likely that part of these differences were found in pregnancy are not only explained by genetic factors, but also reproductive management, nutrition or health. However, This finding is substantiated by some scientific works indicate that the breeding of reproductive characters of cows handled Uruguayan restrictive conditions is possible and economically justifiable (Urioste, 1996; Urioste and cabbage, 2007; Urioste, 2008). Even, Urioste and cabbage. (2007) they found a range of heritabilities rather higher than those reported internationally for days birth and successful delivery of 0,20 to 0,31 and 0,37 to 0,42, respectively, for the people of Angus animals from pedigree of Uruguay. In New Zealand pastoral conditions, Morris and cabbage. (1999) they found that Heifer with younger age at puberty have also improved reproductive rates in adulthood and that the selection using age at puberty in females and scrotal circumference on Bulls, You can improve rates of pregnancy through these genetic correlations. So in restrictive environments of pastoral production, There would be a greater response to selection for fertility than that which occurs in other more controlled environments, a concept that we had seen it will hold some scientists working in tropical environments.
All replacement sows breeding goal is to achieve that the heifers reach puberty early and have a very good fertility at the beginning of the reproductive season. For this reason, It is interesting to analyze the genetic factors involved influencing those objectives. For example, We know that we must increase scrotal circumference on the bulls, Since this is associated with an increase in the quality and quantity of semen. However, the improvement of the scrotal circumference of the bulls used has other beneficial correlations with reproductive characters of the female, as precisely the age at the first service (-0,10) and first calving (-0,35), the percentage of pregnancy (0.44) and interpartos intervals (-0,33) (Toelle and Robinson, 1985). Way to pay attention to the scrotal circumference of the bulls used in the rodeo is important in selection programs.
But the scrotal circumference is not the only factor. The concern of producers Australian at the loss of fertility of their cattle by the pursuit of animals with more growth and leaner, He took to his country researchers decided to study the effect of the genetic merit in fat in the reproduction. When two lines of heifers were grouped into an Australian site with genetic merit different fat, best pregnancy rates for the high line fat were found (95%) compared to that achieved in low line fat heifers (86%) (Pitchford and Graham, 2010). With these findings, evidence that the concern of the producers were contributed by the loss of fertility in cattle, by changes in the animal biotype (leaner animals) induced current selection programmes, had its foundations. In addition, It is appreciated to fat coverage as an important maternal characteristic in pastoral conditions, a concept that goes against the genetic trend of "modern" bovine populations be increasingly lean.
Then, If the size of the bulls is moderate, they are positive fat and his scrotal circumference is good, his daughters have more chances to impregnate early. So we believe that we must look for moderate cows, very fertile, that it remain in rodeos for a long time, that you can impregnate our restrictive terms of exploitation, at a cost as low as possible, and that they can still produce a calf that is attractive to the raisers and meat industry. That is the challenge. And that is the essence of a truly balanced selection programs.
Despite all the changes in them biotypes wanted in them tracks in the last 100 years and in the current trend of many sellers of semen, cabañeros and technicians of "better" associate with "large" and to promote and encourage extreme animals, the basic task of the breeding cow has not changed: He must impregnate at low cost and under restrictive conditions. That task will be fulfilled with efficiency, the cow must have the ability to get fat in times of plenty, It must mobilize fatty tissues in times of shortages, they must have the strength and hardiness to produce open-pit, impregnating easily and a moderately heavy calf weaning, and you should be doing this job for a long time at the rodeo. The majority of the selection programs that are being carried out at present, to put emphasis only on individual levels of production or the biotypes which tend to reward more often in exhibitions, are leading to a slow but progressive deterioration in the reproductive function of cattle.
For this reason, There are many reasons why the concept of "superior genetics" for the particular case of breeding, currently understood by a majority, Since producers commercial or cabañeros, passing by sellers of semen, agricultural technicians and even reporters, It should be strongly questioned. Aesthetics without substance is useless; the performance without adaptation to pastoral environments is harmful. Rather than obtaining animals "nice" or high-performance, our goal should be to obtain a genetic material that can improve the economic efficiency of commercial rodeos. For all that, We think that as fertility characters, the hardiness and adaptation to the environment, that not are being currently estimated by its genetic merit in Uruguay, and that they never were or will be evaluated on the tracks, are, However, a crucial factor to determine the differences in net income in establishments breeders. The efficiency of the cow is deeply affected by the reproduction, simply because cows are efficient are those that produce many calves regularly.
REFERENCES
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