The effect of feeding level of pre- and postpubertal heifers on their reproductive function and later milk production

Thirty-three heifers were assigned to 3 analogous groups of 11 animals each in which three different levels of feeding were used from 6.5 to 10 months (first period) and from 11 to 12 months of age (second period): the feeding level recommended by the IZ-INRA system (1997) (group C), lower and then 20% higher in subsequent periods in relation to group C (group D), and 20% higher in both periods in relation to group C (group H). Body weight gains in different rearing periods were assumed as follows (g/day): 700 (C), 560 and 840 (D), 840 (H), and from 13 months of age to 3 weeks before calving 700 g/day in all the groups. Next the heifers were fed the rations specified for primiparous cows assuming maximum production at peak of lactation to be 35 kg milk per day. The varying level (group D) of feeding prepubertal and postpubertal heifers was found to have no adverse effect on ovarian size and status at 12 months of age and on subsequent growth and development of these animals, while having a beneficial effect on their milk performance during the first lactation. The high feeding level (group H) from 6.5 to 12 months of age led to increased daily weight gains and accelerated the date of reproductive performance of heifers compared with their age-mates from group C.


INTRODUCTION
Studies on Holstein-Friesian cattle have shown that the feeding level of heifers of dairy breeds during their growth affects the development of the mammary gland and consequently, the cow's subsequent milk production (Sejrsen and Purup, 1997).The most intensive development of the udder parenchyma takes place in the period prior to the attainment of sexual maturity (Choi et al., 1997).An excessive level of feeding energy during this period and daily weight gains that are different from the optimum value specified for a given breed may result in excessive fattness of the mammary gland, a process that inhibits the growth of parenchymal tissue that produces milk (Foldager and Sejrsen, 1991).At present, most of the high-producing cows in Poland are Black-and-White breed with a higher proportion of Holstein-Friesian genes.There are no uniform recommendations in Poland yet for the feeding level of prepubertal and postpubertal heifers derived from these crossbreeds.
The aim of the present work was to determine the effect of various feeding levels of Black-and-White heifers with a high proportion of HF genes before and after the attainment of sexual maturity on their growth, on ovarian size and status at 12 months of age, and on their reproductive and milk performance.

MATERIAL AND METHODS
Thirty-three Black-and-White heifers with a high (>75%) proportion of HF genes were investigated from 6.5 months of age to 100 days of lactation.The animals were assigned to 3 analogous groups of 11 animals each in which three different levels of energy feeding were used from 6.5 to 10 months and from 11 to 12 months of age: that recommended by the IZ-INRA system (1997) in group C, lower and then 20% higher in subsequent periods in relation to group C (group D), and 20% higher in both periods in relation to group C in group H (Table 1).In all of the groups, the protein to energy ratio ranged from 95-100 g PDI/UFL.From 13 months of age to 3 weeks before calving, all of the heifers received a Experimental design ration to achieve 700 g daily weight gain.Next, the heifers were fed rations specified by the IZ-INRA system for primiparous cows assuming 35 kg milk production at the peak of lactation.At 12 months of age, 21 randomly chosen heifers (7 from each group) were examined ultrasonographically to determine ovarian size and function.For this purpose we used the C 9100 Oculus apparatus (Picker International GmbH 32 339 Esplecamp, Germany) equipped with a convex 6.5 MHz probe enabling a resolution of about 2 mm.Chemical feed analyses were performed according to AOAC (1990).Statistical calculations were made using one-factor analysis of variance, and the results of milk performance were analyzed with one-factor analysis of covariance with two accompanying variables (age at first calving and calving season), using the SAS statistical package (1989) and the GLH procedure.The results of calving percentage of heifers and cows were analyzed with the % 2 test.

RESULTS
The right ovary of heifers from group H was larger than that of their age-mates from the other groups (P<0.01 and P<0.05) (Table 2).There were no significant differences among the groups for the size of left ovary and for the size and number of ovarian follicles (P>0.05).One to three large follicles 9-15 mm in diameter and from several (3-7) to more than a dozen (12-16) small follicles 3-8 mm in diameter were found on the ovaries of the majority of the heifers.The corpus luteum was present in 4 heifers from group D, in 5 from group C and in 6 from group H.The low level of energy feeding (group D) of heifers from 6.5 to 10 months of age caused daily weight gains to decrease by about 23 and 33% and body weights at 10 months of age to decrease by about 6 and 10% compared with groups C and H, respectively.The highest daily weight gains of heifers from 6.5 to 10 months of age were found in group H (P<0.01 and P<0.05), and the lowest in group D (Table 3).From 11 to 12 months of age, heifers from group C had lower weight gains (P<0.05)than those from groups D and H, with no significant differences being found among them.During the next period (13-18 months of age), daily tive performance and the date of first calving.A similar relationship was found for HF heifers (Vicini et al., 1995;Hinders, 1997;Pirlo et al., 1997).It seems that Black-and-White dairy heifers (with a high HF blood share) can tolerate higher feeding levels than previously recommended for the period of sexual maturation without negative consequences for their subsequent reproductive and milk performance.

CONCLUSIONS
Varying the level of energy feeding of Black-and-White heifers with a high proportion of HF genes from 6.5 to 12 months of age in relation to the requirement specified by the IZ-INRA system (1997) for a weight gain of 700 g/day has a beneficial effect on milk production during the first lactation and on the conversion of energy (UFL) and protein (PDI) per kg milk production.
Advancing the growth of heifers prior to and after the attainment of sexual maturity does not adversely affect the parameters of reproductive performance or the milk yield of these animals during the first 100 days of lactation, making it possible to lower the age of first calving.1751-1758Peri J., Getler A., Bruchental J., Barash H., 1993.The effect of manipulation in energy allowance during the rearing period of heifers on hormone concentrations and milk production in first lactation cows.J. Dairy Sci. 39, 742-751 Pirlo G., Capelleti M., Marchetto G., 1997.Effects of energy and protein allowances on the diets of prepubertal heifers on growth and milk production.J. Dairy Sci.