Springtime breeding season (represented in the supplemental material of this question) imposes a winter birth concentration, and thus, a winter lactation, that will depress cows’ nutritional status during the breeding season. That is, when nutritional demands reach their peak, the pastures are at their lowest production.

Cows in moderate to depressed nutrition status at the time of their first postpartum ovulation will have problems with follicular maturation, according to Jolly et al. (1995), among others. This indicates the urgency in conserving at postpartum the good body condition at calving. So, nutritional supplementation must be taken in this springtime breeding season, firstly to achieve good body condition at calving, and after, to maintain the nutritional status.

Continuing with this analysis, it will be important understanding why body condition at calving is so relevant. In 1962, Wiltbank and collaborators, in a masterful work, have demonstrated that a good body condition at calving was the guarantee of fast emergence postpartum estrus, and the maintenance of this condition in the postpartum period was the guarantee of fertility of this first postpartum estrus. Several hundreds of other studies have proved the veracity of such conclusion, and today this is a fundamental clause.

The work of Wiltbank et al. (1962) brought additional information linked with the understanding of the inefficiency of thin cows at parturition. When parturition happens at a low body condition, recovery of fertility is possible, but at a high nutritional level (base of 7kg TDN/day) without total restoration. That is, the next parturition will be at a later moment in the subsequent birth season, due to a more prolonged interval between parturition to first fertile estrus (despite the great consumption of TDN), and then, will no longer conceive by 75 days after calving to maintain a 365 day calving interval. So, in general, it is a questionable financial procedure. And under C4 herbage consumption it is a more difficult situation. All the more, beef cattle systems (suckled beef cows) exacerbate the problem with opioid inhibition of LH secretion, which will long remain in the postpartum period of thin cows.

The opportunity to reach a good body condition at parturition can never be lost. Cows with a good body condition at calving have the most favorable hormonal balance to prompt the resumption of the postpartum cyclicality. Why? Simply because the pituitary gland of thin cows at parturition will be more refractory due to an elevated estrogen level produced by a big placenta. The large placenta of thin cows is a way to protect the fetus against sub nutrition, and the placenta is the major source of estrogen during gestation. This abnormally high circulatory estrogen depletes the pituitary reserves of LH, in a superior way relative to cows in good body condition at calving.

Even more, the low body condition amplifies the effect of the negative feedback of estrogen in the hypothalamus, and this situation is similar to that which currently prevails in prepubertal heifers. Therefore, the hormone blockade is more intense and more prolonged in cows with a low body condition at calving, and even high postpartum nutritional levels little can do about it.

Moreover, leptin (a proteinaceous hormone secreted by fat cells and implicated on the regulation of energy balance and the neuroendocrine axis) is responsible for another deleterious effect in the resumption of cyclicality in thin cows. That is, the low adiposity decreases leptin secretion, which results in a low pituitary stimulation of LH production.

In summary, cows at low body condition at calving remain with a more blocked hypothalamus by estrogen, and will also have a low LH pituitary reserve as a consequence of both the synthesis absence of LH, due to negative estrogen feedback in the pituitary at the end of gestation, and also the lack of leptin stimuli in LH secretory cells. The results of this are a scarcity of frequent LH pulses. Frequent LH pulses are indispensable for the conclusion of follicular maturation and to reduce the postpartum anestrus duration, which will enable early cyclicality resumption.

Good body condition at calving is therefore the challenge that should propel management changes that will really improve calf crop; reproduction is the most important economic trait in a beef cow herd. To face this challenge, a body condition score system must be assumed for cows’ evaluations.

As a former colleague of Dr. Rocha's, I can testify that the summer-autumn breeding season has been successful for the the herd at São Manuel Farm.

Firstly, it must be remembered that in Central Brazil 80-90% of the pasture growth occurs during the rainy season (roughly mid-October to mid-May). During the dry season, pastures hardly grow at all, and there is a severe dearth of digestible nutrients during that period, because both the quantity and quality of available dry matter decrease sharply.

If one uses the summer-autumn breeding season, it is easier to provide nutrients through grazing for at least the last two months of gestation and for suckling. It is also easier to have the dams calving with a good body condition score, which increases the chances of shortening the postpartum anestrus and of concentrating conceptions at the beginning of the breeding season.

There are two difficulties with this scheme, none of which are unsurmountable. The first one is intensive calf surveillance and care, due to the heavy rains during the birth season. It is true that this could pose a more serious problem for large breeding herds, but I am convinced it can be solved with proper organization. The second difficulty is that the calves are weaned at the beginning of the dry season, but this can be circumvented by means of a light supplementation of the calves until the start of the following rainy season.

The second scheme avoids the calf management problem and tends to produce calves with a somewhat higher weaning weight, but hinders the reproductive efficiency of the dams, unless they receive proper roughage and concentrate supplementation during the calving period. Supplementing dams is always more expensive than supplementing calves. It should also remembered that the difference in weaning weight between the two schemes will gradually vanish, due to compensatory growth of the calves in the first scheme.

The bottom line is that one should focus more on calf crop and overall weaning weight crop (kg of calf/dam/year), and not so much on weaning weght (kg/calf). Of course the latter is also important, but high reproductive efficiency is crucial.

This depends on the breed as well as the environment in which the breed lives