Bovine biology series
Part - 43 Life cycle (4/8)
This is our fourth lesson of the life cycle. This month we examine heifer growth from weaning to calving, a period of 22-23 months.
As we know already, this period of an animal’s life is likened to the vegetative growth period for plants. During this time, the animal (and plant) is firstly adding mass, or weight, as cells in almost all parts of the body, including the structural portion, the muscular and organelle portion, and the brain itself is undergoing tremendous cell differentiation, division, and then accumulation.
Plants are like this too, albeit simpler. During their vegetative growth cycle they are laying down long strands of glucose molecules bound by the glue like lignin, with some vacuole cells necessary for water and nutrient transfer from root tissue upwards.
Animals require nutritive support, of course, and instead of absorbing it from root tissue as a means of growth, animals use the oxidation of stored sunlight in the form of plants as their fuel. And we know that when food is eaten, then the blood system takes the absorbable nutrients to all parts of the body so those cells can be fed.
What characterizes the growth period from any other period of life is simply the determination as to the degree at which an animal will increase cell numbers. That is, how large will the heart be, or the brain, or the length of the femur bone in the leg? The pre-secretorial udder tissues are growing too, laying down the basal cells as part of a secretory organ soon to be made productive.
If we remember nothing else about growth, let it be that within the DNA framework of cells, there exists a code, or series of codes, that signal further cell differentiation, division, and accumulation. The final one is the key.....at what point have enough brain cells been added to the cranium shell; at what point is the bone long enough, and at what point are enough liver cells in place to filter and screen incoming absorbed nutrients?
We know, for instance, that cells reach a point when instead of accumulating, they shift energy allocation unto merely replacing themselves. In fact one of the balances of energy allocation in the female is the relationship between accumulated cells and the onset of puberty. That is, as the heifer reaches enough body weight, she begins to allocate some energy towards the onset of puberty, somewhere about 450 - 500 pounds for a Holstein.
Holstein heifers are bred at 800 - 850 pounds, so during this interval between puberty and first breeding, the heifer is allocating more and more of her energy towards reproduction. She continues to grow, adding body weight, and since there is an additional demand for nutrients (reproduction), she eats more.
We have already discussed the relationship between adequate energy intake and reproductive ability. Simply, if caloric intake is lower than requirements, then the onset of puberty is put off, and if puberty is reached and the animal is underfed, she will not have the hormonally-driven estrus cycle....and she will not be capable of becoming pregnant.
At the time of first breeding, the Holstein heifer will be consuming 19 -20 pounds of dry matter. She will, over the course of nine months, gain another 200 - 250 pounds of body weight and about 100 pounds of fetal weight. Her daily DM intake will increase until at the body weight of 1,100 pounds, she will be consuming 27 - 28 DM pounds.
Interestingly enough, this is just over half what she will consume when she calves. For the simple reason another energy demand has been placed upon her.....lactation. The demand is for milk secretory function, especially given the primary effort towards its optimization via selective genetics (read: additive genes that allocate energy towards lactogenesis).
The daily rate of gain for Holstein heifers varies but is in the range of 1.5 - 2.2 pounds. At 2.0 pounds, this amounts to 7,000 calories, or 7.0 Mcal that feed the body cells that are differentiating, dividing and accumulating. A portion of this energy is devoted to maintenance, some for daily gain, and some for reproductive demand. For a 1,000 pound Holstein heifer, the net energy requirement is 11.0 Mcal of energy, and for net energy gain, 4.9 Mcal of energy is required. In order to gain weight efficiently, the heifer must be fed the correct ration, be in a good environment (housing, bedding comfort, fresh water, etc.).
The better-bred heifer (read: better genes) can eat more and commit a higher percentage of energy towards gain. This is a significant and often overlooked part of raising heifers.
Simply stated, the maintenance of any animal is allocated energy in the first order, then comes gain, and then comes reproductive demands. That is why in any female of any age that does not have sufficient coloric intake, the cascading of negative effects begins with reproduction first, then gain, and finally the body feeds upon itself in the case of extreme and catastrophic coloric failure.
The heifer is bred and thus begins some real changes in the body, the obvious one of course the changing reproductive tissues associated with fetal growth, but the related organ, the udder tissue, is undergoing a huge influx of hormonally driven changes in tissues.....from cell buds and largely fatty like tissue, into the secretory lobules that make up the alveolar tissue (driven by higher levels of progesterone because the heifer is pregnant), and the duct system (the result of estrogen).
The making of alveolar tissue is additive but progressively increased in the third trimester of pregnancy, until the seventh month or so, when visible signs of udder development cause us to say that a heifer is springing. This building of secretory of tissue continues right up until calving.
Mammary glands can begin secreting about seven months into a pregnancy as enough secretory tissue has been made. However, secretion volume is just a fraction of what it will be if the calf is carried to term.
There is an interplay of hormonal action that prevents lactation during pregnancy in the bred heifer. Secretion of milk is dependent upon a lactogenic hormone called prolactin produced by the anterior pituitary gland. Progesterone inhibits its secretion sufficiently, however, very near calving, progesterone levels are significantly reduced as circulating estrogen increases, and it is estrogen that takes over in signaling the pituitary into secreting prolactin, and secretion in the newly formed udder tissue begins.
Next month: calving.
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