Secretaría de Desarrollo Territorial y Bienestar Social

Abstract

The reproductive performance of Holstein-Friesian cows from the heavy (H) and the light (L) mature live weight (LW) selection lines at Massey University, New Zealand was evaluated from 1 992 to 1 997. Differences between genetic lines were evaluated for age at first calving (AFC), calving interval (Cl) and the intervals from calving to first heat (CFH), calving to first service (CFS), first service to conception (FSCO), and calving to conception or days open (DO). Additionally, the proportion of induced calvings, calving dates, conception rate to the first service and two intervals unique to the New Zealand seasonal system of reproduction given by the start of mating to the first service (SMFS) and the start of mating to conception (SMCO) were also evaluated. Averaged across years, there were no differences between H and L cows for AFC, Cl, CFH, CFS or DO. However, cows from the H line calved on average 6 days later during the calving season, and had both longer FSCO ( 1 7 vs. 1 3 d; P < 0.05) and longer SMCO (29 vs. 24 d; P < 0.05) intervals than L cows. Cows from the H line had a lower conception rate to the first service (54 vs. 65%; P < 0.05) and a higher percentage of induced calvings ( 1 0.5 vs. 4.2%; P < 0.05) than the L

cows. The combination of later calving and lower conception rate at the first insemination extended the conception and calving pattern of H cows and increased their probability of an induced calving. Under the conditions of the present experiment, lighter mature LW cows had higher conception rates to the first service, achieved a more concentrated calving pattern and were less likely to be induced to calve than heavier mature live weight cows.

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Introduction

The profitability of the New Zealand seasonal dairy enterprise is directly related to the amount of milk solids produced per hectare of grazed grass (Deane, 1 993). High production per hectare when grazed grass is the main source of feed can only be achieved with high fertility high genetic merit cows that are capable of harvesting a high proportion of the grass grown on the farm and converting it efficiently into milksolids (McMeekan, 1 960), and with the skills of herd managers that make the most from both dairy cows and pastures (Holmes, 1 990). New Zealand dairy farmers address these components of their production system by artificially breeding their cows to sires with high genetic merit for farm profitability, which are proven under the environmental conditions in which their daughters are expected to perform (Harris, 1 998). Additional strategies are the use of relatively high stocking rates during the milking season (Holmes, 1 990), and the attainment of a highly concentrated calving pattern of their herds in late winter-early spring to match feed requirements with pasture growth (Macmillan, 1 974 and 1 979; Macmillan et al. 1 984a and 1 984b). Under this scenario, and because of the costs of meeting the cow's maintenance requirements, size of the lactating cow has been identified as a component affecting the final efficiency of the New Zealand dairy system (Ah1born and Dempfie, 1 992; Dempfie, 1 986; Holmes et al. 1 993). Therefore, live weight of the lactating cow is now given a negative weight in the new overall objective of increasing the value of milk solids produced per tonne of DM eaten (Livestock Improvement, 1 997).

A highly concentrated calving pattern is fundamental for the efficiency of the seasonal system, and will depend on the herd's reproductive performance during the previous season, particularly on the achievement of both a high submission rate and a high conception rate to a single service (Macmillan, 1 974; XU and Burton, 1 996). The detrimental effect of high genetic merit for milk yield on the reproductive performance of dairy cows is now well documented (Nebel and McGilliard, 1 993), but experimental evidence about the effect of cow size on fertility is scarce. Markusfeld and Ezra ( 1 993) reported lower pregnancy rates to the first service for heavier, taller compared to lighter, shorter Israeli Holstein-Friesian cows. After 20 years of divergent selection for live weight

(LW) in a research herd of USA Holsteins, Hansen et al. ( 1 998) reported an increased number of services per conception required by cows from the heavy live weight selection line.

The effects of genetic differences in live weight (and milk yield) on the reproductive performance of cows under the New Zealand seasonal system of reproduction have not been investigated previously. Parameters to evaluate the reproductive performance of spring calving herds in New Zealand have been proposed by Grosshans et al. ( 1 996), based on the cow's calving date and the planned start of mating (PSM) of the herd, (i.e. the date chosen by the farmer to start artificially breeding the cows). Thus, the objective of the present study was to compare the productive and reproductive performance of cows from the heavy (H) and light (L) mature live weight selection lines developed at Massey University, New Zealand. A preliminary analysis of the reproductive data has been given by Laborde et al. ( 1 998).

Materials and methods

Animals and management

New-born calves

Each year calves were born during July to October, with a peak in August, in a calving season of about 90 days. Cows calved on the paddock and were monitored for identification of dam and calving difficulty. Calves were identified with their mothers within the first 24 hours after calving. Scoring for calving difficulty was at the discretion of the herd manager and assistance was provided when required. Results on calving difficulty of primiparous and mUltiparous cows from the project have been given by Garcia-Muiiiz et al. ( 1 998a) and have been discussed in detail in Chapter 6.

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First calving heifers

Every year all heifer calves with an H or L bull as sire were reared as replacements. The heifers spent their first year of age at the home-farm and then moved to graze on a neighbouring farm until about two months before their first calving. While grazing away the heifers were naturally mated at 1 4 to 1 5 months of age to Holstein-Friesian ( 1 993-96) or Jersey (years 1 990-92) bulls, during a 90-day breeding season starting about one week before that of the adult cows. They were then brought back to the home farm in June each year or about two months before the start of calving, and grazed with the rest of the herd. Dates when bulls joined the maiden heifers for NM and when they finished mating were recorded every year. After calving they were submitted to the same reproductive management as the rest of the adult herd (Figure 7 . 1 ).

M ultiparous cows

Data were collected during the period 1 992 to 1 997 inclusive. The reproductive, grazing and feeding management of cows from the H and L lines is described in the following section.