REARING DAIRY REPLACEMENT HEIFERS

T J Dugmore
Cedara Agricultural Development Institute

A great deal of research work has been carried out in Australia, Europe and North America to determine the effect of the level of feeding and of age at first calving on subsequent production performance. Overfattening of heifers has been shown to inhibit milk production severely, while the milk production of undersized heifers is severely restricted by their incomplete development. Heifers mated to calve early (24 to 27 months) and fed to maintain growth rates of 0,6 to 0,7 kg/day for Holstein-Frieslands and 0,4 to 0,5 kg/day for Jerseys, usually produce higher total yields during their productive lives than do heifers mated to calve later.

The faster an animal grows, the more efficient it is in converting feed into livemass, because a smaller proportion of its feed is used for maintenance, and thus a greater proportion is available for growth. Although this suggests that heifers should grow as quickly as possible to attain the target livemass, there is an age below which they should not calve down. Rearing experiments have shown that heifers which calve very early have given disappointing milk yields, and suffered extra calving problems, in spite of having achieved adequate livemass.

It is therefore recommended that no Holstein-Friesland heifer should calve under 23 months of age, but as soon after as possible. Delaying age at first calving by a month resulted in an average yield increase of 73 litres of fat-corrected milk (FCM) for a 300 day first lactation in the Cedara Holstein-Friesland herd. This effect has been shown to fall away after 29 months of age (Figure 1), therefore calving at ages greater than 29 months carries no benefit due to age, and cannot be recommended.

Figure 1.  Effect of age at first calving on first lactation production

Lactation No.	Litres of milk advantage (per lactation) for every kg increase in livemass at first calving
	per kg	per 50 kg	Cumulative total
1 2 3	8,7 7,6 6,6	435 380 330	435 815 1 145

The size and condition of a heifer at calving influences her subsequent milk yield. Undersized heifers usually give low yields, particularly when high forage intakes are required. The effect of size at first calving is shown in Table 1.

An analysis of Cedara's first lactation records has shown that every increment of 50 kg bodymass before calving (pre-partum) produces 306 litres more FCM over a 300 day lactation, or 6,1 litres of FCM per additional kilogram livemass.

Larger responses, of 10 to 12 litres of milk per kg extra post-partum livemass, measured during first lactation, have been recorded in commercial dairy herds in KwaZulu-Natal (Figure 2). The correlation coefficient (r²) of these data is greater than 70, indicating that livemass accounts for more than 70% of the variation encountered in first lactation yields.

Figure 2.  First lactation production vs livemass for commercial herds in Natal (R I Jones)

Table 2.   Effect of growth rates (ADG) of Holstein-Friesland heifers, from 14 weeks to mating, on lifetime milk production (after Little & Kay, 1979)

Treatment	ADG 14 to 39 weeks of age	Mating		Production per lactation (kg FCM)
		mass (kg)	age (weeks)	1st	2nd	3rd	4th	Total yield
A B C	1,08 0,98 0,61	302 443 353	42,9 78,4 78,1	1959 2450 3863	2918 3216 4994	3545 3310 4813	3210 - -	11 632 8 976 13 370

The heavier an animal at calving (as long as she is not overfat), the higher her milk yield for that lactation. There are two reasons for the increase in milk production per lactation with increased livemass. First, the animals have greater body reserves, which can be utilized to produce more milk during the first three months of lactation, when energy intake does not equal energy output in the milk produced, i.e. the cows are milking "off their backs". Second, the heavier the animal at calving, the closer she is to her ultimate mature livemass. She therefore requires less energy for growth during her first lactation, to reach her mature weight. This allows more energy for milk production. Heifers that were well-grown at the time of calving have a production advantage not only during the first lactation, but that persists beyond the second and even into the third lactation, when compared with heifers that were smaller and lighter at first calving.

While additional livemass, and consequently increased body reserves, at calving improve both first lactation and lifetime milk production, excessive mass gains (more than 0,8 kg/day for large breeds and more than 0,45 kg/day for small breeds) prior to puberty have proved to be detrimental to milk production. This is particularly important for the heifer, during her first year of life. The effect of growth rates, from 14 weeks of age to mating, on milk production are shown in Table 2.

The data in Table 2 illustrate that, under similar growth rates (Treatment A & B), earlier calving produces greater lifetime yields, even though the first lactation yield may be much lower than that of the later calving animal. Excessive growth rates prior to mating are detrimental to future milk production, both first lactation and lifetime production (Treatments A & B compared with C). Conversely, calving at a very light mass causes diminished first and second lactation productions owing to incomplete development at calving. Undersized heifers experience even more calving problems than do overfat animals.

Diminished yields associated with fast growth are caused by excessive fat deposition in the developing udder tissue, at the expense of milk-secreting tissue. The most critical time, for secretory tissue development, is prior to mating, at puberty or even earlier. The mammary glands of cows which were reared as heifers never to exceed a livemass gain of 0,74 kg/day, weighed 39% more and contained 68% more secretory tissue than the glands of rapidly reared heifers, grown to gain 1,0 kg/day until 11 months of age.

In Table 3, the analyses of the mammary glands of pregnant heifers, reared at different rates of livemass gain, shows that high rates of livemass gain in heifers, prior to puberty, may enhance adipose tissue development in the mammary gland and limit subsequent development of milk secretory tissue during pregnancy. On the other hand low rates of gain during pregnancy may reduce cell division in the mammary gland and impair milk secretory tissue development. In practical terms what this means to the farmer is that impaired tissue development depresses milk yield. Figure 3 shows that high rates of gain, pre-puberty, impair milk production relative to the cows genetic potential. Higher rates of gain during pregnancy (as opposed to pre-puberty) result in enhanced milk production.

	Treatment
	LL*	LH#	HL
Gland mass             (kg) Parenchyma mass   (kg)                               (%) Gland mass as % of livemass ADG to 12 months (kg/day) ADG during pregnancy (kg/day)	6,4 4,0 63,7 1,53 0,58 0,68	9,0 5,4 61,0 1,82 0,58 0,84	6,8 3,2 49,6 1,39 1,03 0,58

* LL = low rate of gain for the first year followed by low rate of gain during pregnancy