Yves Gallard

Dairy cows' reproductive response to feeding level differs according to the reproductive stage and the breed.

Reproductive performance has decreased over the last decades in many dairy systems. This study aimed at comparing the effects of a high- and a low-feeding level on reproductive stages (cyclicity, oestrus and fertility) of Holstein and Normande cows in a grass-based winter compact calving system. High-fed cows received a total mixed ration composed of 55% maize silage, 15% dehydrated alfalfa pellets and 30% concentrate in winter and 4 kg/day concentrate supply at grazing. Low-fed cows only received 50% grass silage and 50% haylage in winter and no concentrate at grazing. Low-fed cows produced less milk over 44 weeks, but lost more body condition (BC) than high-fed ones (5207 v. 7457 kg, -1.28 v. -0.96 unit, P < 0.001). Normande cows produced less milk and lost less BC than Holstein ones (5596 v. 7068 kg, -0.89 v. -1.36 unit, P < 0.001). Post-partum ovarian activity was little affected by the feeding level. In both breeds, ovulation detection rate was higher in low-fed cows (74% v. 59%, P < 0.001) and detection relied more frequently on standing behaviour (67% v. 55%, P = 0.022). In both breeds, the recalving rate following the first and second inseminations was not significantly affected by the feeding level, although more non-fertilisations or early embryo mortalities occurred in low-fed cows (43% v. 26%, P = 0.004). For the Holstein breed, this was clearly explained by more late embryo mortalities in high-fed cows (30% v. 9%, P = 0.004). Finally, pregnancy rate by the end of the 13-week breeding period was similar between feeding groups. Conversely, the Normande dual-purpose cows had a higher pregnancy rate by the end of the breeding period than the Holstein dairy cows (72% v. 54%, P = 0.007), owing to a better ovarian activity (79% v. 54% normal, P < 0.001) and a higher recalving rate following insemination (53% v. 37%, P = 0.007). In conclusion, this study demonstrates that feeding levels with converse effects on milk yield and BC score also have converse effects on reproductive stages and lead to quite similar final reproductive performance in compact calving systems. Normande cows benefit from an overall better reproductive performance, but do not suit high feeding levels for very compact calving systems owing to depressed oestrous behaviour.

Towards a better understanding of the respective effects of milk yield and body condition dynamics on reproduction in Holstein dairy cows.

The overall reproductive performance has decreased over the last decades, involving changes in cyclicity, oestrous behaviour and fertility. High milk yield (MY), low body condition score (BCS) and large body condition (BC) loss have been identified as risk factors. However, these effects are often confounded, as high MY and body lipid reserve mobilization are correlated. The aim of this study was to evaluate the respective effects of MY and BC on post-partum ovarian cyclicity, oestrus and fertility of Holstein cows. This study provides novel information, as MY and BC change were uncoupled in the overall dataset that included 98 lactations and milk progesterone profiles. Cows were assigned to two feeding-level groups: high feed, which achieved high MY and moderate BC loss throughout lactation (8410 kg, -1.17 unit from calving to nadir BCS), and low feed, which limited MY and triggered a large BC loss (5719 kg, -1.54 unit). MY and BC had different effects at different stages of the reproductive process. Cyclicity as well as non-fertilization and early embryo mortality were mainly driven by body lipid reserves, whereas oestrous behaviour and late embryo mortality were related to MY. The results point to possible uncoupling between cyclicity, oestrus and early and late embryo survival allowing compensation along the reproductive process and leading to similar final reproductive performance. In compact calving systems, which require high pregnancy rates within a short period, higher MY strategies appear unsuitable even where BCS is maintained, owing to depressed oestrous behaviour and probably increased late embryo mortality, which delays rebreeding. Similarly, strategies that compromise cyclicity and fertility by excessively low BCS are unsuitable.

Effects of breed, feeding system, and lactation stage on milk fat characteristics and spontaneous lipolysis in dairy cows

Spontaneous lipolysis (SL) is an enzymatic reaction that leads to a release of fatty acids that can modify technological and sensory properties of milk and milk products. However, few studies have been done to assess the effect of feeding systems (FS) and breed on SL. Most of them were conducted in the 1980s and are not fully representative of cattle today. No previous study investigated the effect of cow breed at the whole-lactation scale. Thus, a trial was carried out to study the effects of 2 FS (high- and low-input FS) with 2 breeds [Holstein (HO) and Normande (NO)] during 1 entire lactation. Sixty-three cows were followed throughout 1 lactation. Cows were divided into 4 groups according to their breed and their FS. The high FS (HFS) consisted of a high-energy diet (in winter, corn silage with 30% concentrate; otherwise, pasture with 4 kg/d of concentrate) and the low FS (LFS) consisted of a low-energy diet (in winter, conserved grass with no concentrate; otherwise, pasture with no concentrate). The cows calved between January and March. Individual milk samples were collected every month from both morning and evening milkings for fat, protein, milk fat globule size, major fatty acids and proteins profiles, and SL determinations. Data were analyzed using the mixed procedure of SAS. The SL was higher in evening milks compared with morning milks. In early lactation, in evening milks, SL was higher in LFS than in HFS. No difference was shown according to the FS in mid and late lactation. Pasture was associated with low SL rate in mid lactation. The NO cows were less susceptible to SL during the entire lactation than HO cows. Finally, early and late lactation periods were identified as being more susceptible to SL, but this depended on breed and FS. During early lactation, HO cows and LFS were associated with higher levels of SL, particularly in evening milks, and, during late lactation, HO cows were associated with higher levels of SL. No intertreatment or intercow correlations (coefficient of determination <0.16) were found between SL, milk fat and protein contents, milk production, milk fat globule size, proportion of fatty acids and proteins, body condition, and weight during the entire lactation. Effects of breed, FS, and lactation stage were clearly identified and quantified. Causal mechanisms might involve energy balance and circadian secretion of milk fat globule components.

Effects of DGAT1 K232A polymorphism and milking frequency on milk composition and spontaneous lipolysis in dairy cows

Milk spontaneous lipolysis (SL) of milk triglycerides is induced by the lipoprotein lipase, a milk native enzyme, and leads to an accumulation of partial glycerides and free fatty acids that are responsible for the deterioration of the taste of milk products. The gene coding for diacylglycerol acyltransferase 1 (DGAT1), an enzyme implicated in triglycerides synthesis, has an important polymorphic site at the K232A locus. This gene is well known to modulate milk composition. No data are available on the effects of DGAT1 on SL. Thus, a trial was carried out to evaluate the effects of DGAT1 K232A polymorphism on milk SL upon milking frequency variations [once- (ODM) and twice-daily milking (TDM)]. Twenty-one cows were divided into 3 groups according their DGAT1 K232A genotype: 8 cows had the KK genotype of DGAT1 (KK cows), 8 had the KA genotype (KA cows), and 5 had the AA genotype (AA cows). The trial consisted in 3 successive periods: 3 wk of TDM, 3 of ODM, and 4 of TDM. Samples were collected for fat and protein contents, SL, fatty acid, and protein profiles determinations. The KK cows presented higher fat and protein contents, lower milk production, and higher κ-casein percentage. No significant difference in fatty acid composition was noted between groups. The SL was twice as high for KK cows in TDM situations (1.13 vs. 0.59 and 0.63mEq/100g of fat, respectively, for KK, KA, and AA cows during the first period of TDM, and 0.46 vs. 0.25 and 0.21mEq/100g of fat, respectively, for KK, KA, and AA during the second period of TDM). The SL remained lower in TDM2 than in TDM1. During ODM, no difference in SL was found between groups and SL remained below 0.2mEq/100g of fat. These results demonstrate the existence of a correlation between DGAT1 genotypes and spontaneous lipolysis, in interaction with an environmental factor, milking frequency, although it has not been possible to clarify the causal mechanism at this stage.